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Impact of very low crop residues cover on wind erosion in the Sahel
Abstract
In the Sahel, with average annual precipitation in the order of 500 mm yr − 1 , wind erosion occurs mainly on cultivated millet fields whose surfaces are only partially covered by crop residues. The impact of these residues on wind erosion was not clearly established. The objective of this study is thus to quantify the actual amount of crop residues in traditional Sahelian fields and to determine their impacts on wind erosion by reference to a bare surface throughout the seasonal cycle over several years. At the beginning of the year during dry season, Sahelian farmers use to "clean" their fields, i.e. cut and lay flat on the soil surface any millet stalks still standing and yearly sprouts of shrubs. After this clearing, the crop residues cover (CRC) regularly decreases passing from 12% to less than 2% four months later. On traditional cultivated plot, crop residues efficiently prevent soil losses by wind erosion during the dry season and considerably reduce erosion fluxes at the beginning of the rainy season. However, for CRC lower than 2%, wind velocities were sufficient to produce important erosion even during dry season. A minimal cover rate of about 2% (100 kg ha − 1) thus appears as critical to reduce wind erosion. This reduction is driven by the higher aerodynamic roughness length which increases the wind erosion threshold velocity. If field clearing is made in January, as currently done, the CRC just after clearing should be about 800 kg ha − 1 to maintain CRC above 2% at beginning of the rainy season when wind velocities are the highest and wind erosion the most intense. Our results also demonstrate that during the second part of rainy season wind erosion is reduced, not so much due to vegetation development but rather to a decrease in the number and the intensity of high winds events.
Supplementary resources (2)
... Abdourhamane Touré et al., 2011). This period corresponds to the largest occurrence of high wind speeds and to the lowest vegetation coverage, that roughly coincide in these areas (Bergametti et al., 2020). ...
... We used the methodology proposed by Abdourhamane Touré et al. (2011) to assess the threshold wind speed u t required to compute the DUP. As u t stands for a dry and bare surface (see 2.4), we selected measurements from March 1st, 2020 to May 31st, 2020 that corresponds to a bare soil, before the onset of rainfall season and vegetation growth. ...
... As u t stands for a dry and bare surface (see 2.4), we selected measurements from March 1st, 2020 to May 31st, 2020 that corresponds to a bare soil, before the onset of rainfall season and vegetation growth. Following Abdourhamane Touré et al. (2011), we used Saltiphone and wind measurements acquired on the ENCAS-plot and estimated u t from the probability of saltation occurrence depending on wind speed class. Like these authors, we used wind speed classes of 0.2 m s − 1 (from 0 to 15 m s − 1 ). ...
... The most important method for preventing wind erosion from agricultural fields is the maintenance of a residue and vegetation cover [12][13][14][15][16][17][18]. The erodibility of a surface can furthermore be influenced by the presence of a soil crust that increases cohesion and thus in turn by the tillage operations that disturb the crust [19][20][21]. ...
... The efficiency of the BSNE correlates positively with the grainsize [44,45,49]. Since the efficiency of saltating grains is described to be near 100% [18,44], we will assume this efficiency for the horizontal mass flux samples. ...
... The mass data from the BSNEs is transformed to a horizontal sediment flux. For these calculations, we will use a power relationship as proposed by Zobeck and Fryrear [57] as well as others, Abdourhamane Toure et al. [18], Sharratt et al. [49], and Webb et al. [58]. Therefore, the sediment flux Q (g m −2 ) with height z (m) is described as: ...
Minimizing wind erosion on agricultural fields is of great interest to farmers. There is a general understanding that vegetation can greatly minimize the wind erosion taking place. However, after harvest, a low vegetation cover can be inevitable, whereby the amount of stubble that remains on a field is dependent on the crop type and land management. This study aims at quantifying the vulnerability to wind erosion of different crops, and the possibility to predict the vulnerability based on high precision aerial images. The study area was the semi-arid Free State, which holds large intensive agriculture on sandy soils. These croplands have been identified as the largest emitter of dust in South Africa. The main crop in the region is maize, but also sunflower, peanut and fallow fields are common land-use types. On these fields, the horizontal sediment flux, the saltation threshold, and aerodynamic roughness length were measured, and the soil cover was assessed using Unmanned Aerial Vehicle (UAV) imagery. The results showed a strong relationship between the soil cover and the sediment flux, whereby fallow and groundnut fields have the highest wind erosion risk. These results emphasize the great importance of soil cover management to prevent wind erosion.
... Goutorbe et al. 1997;Warren et al. 2003;De Rouw and Rajot 2004;Cappelaere et al. 2009;Hiernaux et al. 2009). Several field measurements were dedicated to the dynamics of wind erosion (Rajot 2001;Bielders et al. 2004), some of which included the monitoring of crop residue degradation and land use effects (Abdourhamane Touré et al. 2011). Measurements of meteorological data and dust concentration and deposition fluxes were also collected since 2006 (Marticorena et al. 2010(Marticorena et al. , 2017. ...
... Bergametti et al. 2017), a large proportion of the annual wind erosion occurs during this period (e.g. Abdourhamane Touré et al. 2011). ...
... The mean annual rainfall over the used time-series is 505 mm with a standard deviation of 144 mm. The proportion of high wind speeds (greater than 7 m s -1 , at 5-min resolution, 6.5 m height, corresponding to wind erosion threshold for the bare soil at the study site, see Abdourhamane Touré et al. 2011) was also highly variable through years with a mean of 1.35% and a standard deviation of 0.63%. ...
ContextWind erosion plays a major role in land degradation in semi-arid areas, especially in the Sahel. There, wind erosion is as sensitive to land use and land management as to climate factors. Future land use intensification may increase wind erosion and induce regional land degradation.Objective
We aimed to estimate wind erosion responses to changing land management in a Sahelian region.Methods
We defined land use intensification scenarios for a study site in southwestern Niger for two historical situations (1950s and 1990s), and two alternative prospective scenarios (2030s: extensive or intensive). We simulated vegetation growth and horizontal sediment flux of wind erosion for the corresponding landscapes.ResultsAnnual amounts of horizontal sediment flux increased with land management changes from 1950s (nil flux) to 1990s (176 kg m−1 yr−1) and 2030s (452 to 520 kg m−1 yr−1), mostly because of differences in land use, declining soil fertility, and practices decreasing the dry vegetation. For 2030s, intensive scenario exhibited larger vegetation yields than extensive conditions, but similar large values of horizontal sediment flux, thus questioning the sustainability of both scenarios. Realistic sets of practices had as large an influence as the largest theoretical range of practices on the variability of annual horizontal sediment flux. This variability was as large as that due to meteorological conditions.Conclusions
This study demonstrates that the environmental impact of land use and management practices, of which wind erosion is an aspect, must be assessed at the landscape scale to account for the variability in land cover and associated land management.
... In Southwest Niger, several studies (Abdourhamane Touré et al., 2011Touré et al., , 2013Touré et al., , 2018Bielders et al., 2002;Rajot, 2001) have highlighted the contribution of cultivated surfaces to wind erosion. In this region, these authors show that wind erosion is mainly due to the high surface wind velocities produced by convective systems associated with the onset of the monsoon. ...
... The horizontal flux, Fh, was computed as the integral of Eq. (1) from 0 to 40 cm for each period of sampling (Abdourhamane Touré et al., 2011;Bielders et al., 2002;Fryrear et al., 1991;Michels et al., 1995;Rajot, 2001;Sterk and Raats, 1996;Stout and Zobeck, 1996). ...
... The Harmattan season is a dry season known as a "dusty" season; the highest monthly mean dust concentrations are recorded in March-April as a consequence of dust transport from Saharan sources (Kaly et al., 2015;Marticorena et al., 2010). At Banizoumbou (70 km east of Niamey), the recorded wind speeds do not exceed the erosion thresholds, and no significant wind erosion (Abdourhamane Touré et al., 2011;Rajot, 2001) or local dust emissions (Marticorena et al., 2010) are observed at that period. On the other hand, short duration events associated with extremely high wind speeds are recorded at the beginning of the monsoon season, leading to the highest dust concentrations (Kaly et al., 2015;Marticorena et al., 2010). ...
In the Sahel, wind erosion increases the vulnerability of sandy soil through the removal of the finest and most fertile fraction of the soil, including nutrients, potentially limiting vegetation development. To provide experimental evidence of the role of different land uses on wind erosion, measurements of horizontal sand fluxes and associated meteorological parameters were taken for four years in Kilakina (southeast of Niger), where large rangeland surfaces coexist with cultivated surfaces and moving dunes in a typical Sahelian semiarid regime (average annual precipitation in the order of ± 300 mm yr −1). The horizontal sand fluxes were monitored using BSNE sediment traps on the three sandy surfaces representative of the main land uses in the region: a cultivated field, a pastured land and a mobile dune. In addition, a meteorological station was installed to monitor the wind speed and direction and rainfall. Saltation flux on the bare mobile dune appeared to be an excellent proxy for wind erosivity in this region. The mobile dune produced saltation fluxes 2 to 40 times higher than the cultivated field and the rangeland. The annual fluxes on the dune varied from 2000 to 6000 kg m −1 due to the variability of the winds from year to year. The sediment fluxes are higher in the millet field than those on the pastured land due to a larger fraction of bare surface compared to the relatively homogeneous grass cover that exists on pastured surfaces. The vegetation produced in a given year affects the wind erosion of the following year through the effect of dry vegetation and litter cover. These results highlight the sensitivity of wind erosion to dry vegetation, even for low cover rates. This outcome suggests that any agricultural practice that decreases dry vegetation cover, such as the uptake of vegetation residues from the fields and the pastured land, has a strong impact on wind erosion. Wind erosion in East Niger can be higher during the Harmattan season than during the monsoon season, while similar measurements in West Niger showed that saltation fluxes were higher during the monsoon season. This outcome suggests that the intensity of the Harmattan surface winds could decrease from east to west. In addition to land use changes, the regional pattern of the surface winds and its future evolution clearly deserve further investigation based on relevant long-term in situ measurements.
... Au Niger, la production de mil, principale culture vivrière, est supportée par des sols sableux particulièrement sensibles à l'érosion éolienne (J.L. RAJOT, 2001 ;C.L. BIELDERS et al., 2002 ;A. ABDOURHAMANE TOURÉ et al., 2011). ont montré, par exemple, qu'une perte de sol par hectare de 38 tonnes en seulement deux événements érosifs sur un champ de mil s'est accompagnée d'une perte de 57,1 kg de potassium, 79,6 kg de carbone, 18,3 kg d'azote et 6,1 kg de phosphore. Ces pertes équivalent approximativement, en termes de potassium et phosphore, aux quantités néc ...
... Entre 2006et 2008, A. ABDOU-RHAMANE TOURÉ et al. (2011 ont mis en évidence une baisse du recouvrement par les résidus de culture, les quantités présentes passant de près de 800 kg/ha en février-mars à 220 kg/ha environ en mai, puis à moins de 100 kg/ha en juillet (A. ABDOURHAMANE TOURÉ et al., 2011). La parcelle PI a été mise en jachère en 2010. ...
... YREAR, 1986). Huit mâts de BSNE, espacés de 30 m les uns des autres, sont disposés suivant une ligne est-ouest (N94° - Fig. 3). Cette orientation était censée correspondre à la direction dominante des vents convectifs (lignes de grains) qui provoquent les intenses érosions dans l'Ouest du Niger (K. MICHELS et al., 1995 ;C.L. BIELDERS et al., 2002 ;A. ABDOURHAMANE TOURÉ et al., 2011). Les mâts de BSNE ont permis de suivre le flux horizontal de manière quasi continue de 2006 à 2014 (excepté en 2011, où les observations ont été interrompues dans l'attente de nouveaux financements). Chaque mât supporte trois BSNE superposés, le centre des ouvertures se trouvant respectivement à 5, 15 et 30 cm du sol (Photo 2). ...
In the Sahel, under 500 mm of annual rainfall, wind erosion is an important factor leading to the degradation of the environment. It causes soil losses and reduces soil productivity. This work, conducted in the southwestern Niger, aims at quantifying wind erosion fluxes using BSNE sand traps. The study was conducted, from 2006 to 2014, on a plot first in cultivation, then set in fallow. It emerges that the cultivated areas are very sensitive to erosion, particularly at the beginning of the rainy season (May-July), when more than 90 % of the fluxes are recorded. Once the plot in fallow, the progressive densification of the vegetation cover caused a sharp reduction of fluxes. Compared to the results obtained during the cultivated period, they decreased by 28 % in the first year of fallowing and
by 94 % in the fifth.
... Ainsi, des impacts néfastes liés à l'érosion éolienne sont souvent enregistrés: le déchaussement de la végétation, et l'appauvrissement en nutriments des sols conduisant à la réduction de leur fertilité et de leur productivité. Les sols sableux sur lesquels repose la production du mil, principale culture de la région sahélienne, sont particulièrement sensibles à l'érosion éolienne [7][8][9][10][11]. Celle-ci, par processus sélectif, affecte spécifiquement l'horizon de surface et conduit à l'exportation des fines particules du sol où l'essentiel des réserves en nutriments pour les plantes est stocké [8]. ...
... Le flux horizontal (Fh) est calculé (équation 2) en intégrant la densité de flux en fonction de la hauteur (0 à 40 cm) [7][8][9][20][21][22][23]. ...
... Sur la parcelle cultivée Pc, le couvert végétal constitué principalement de résidus de récolte est la plus faible passant de 2,25 % (±0,66) en mai à 2,13 (±0,94) en juillet 2014. Ces taux de recouvrement par la litière sont du même ordre de grandeur que ceux déterminés, en juin (~2%), sur des surfaces cultivées du même terroir entre 2006 et 2008 [9]. La densité de recouvrement sur la jachère Pj est relativement plus importante variant entre 6,20 % (±4,4) en mai à 6,19 (±3,2) en juillet 2014. ...
Au Sahel, l'érosion éolienne cause des pertes en terres et en nutriments. Au Niger méridional, cette orme érosion est une cause majeure de la dégradation de l'environnement. Le paysage de ce milieu est dominé par une juxtaposition de surfaces exploitées sous culture pluviale, et de jachères d'âges différents. L'objectif de ce travail est de quantifier les flux d'érosion éolienne et les transferts de nutriments associés en fonction du type d'occupation du sol sur une saison entière. Il apparaît que l'érosion éolienne est faible dans la jachère ancienne (Pi) du fait d'un fort couvert végétal (80%). Cette occupation du sol a respectivement émis près de 7 et 3 fois moins de flux d'érosion que le champ (Pc) et la jachère d'une année (Pj). De même, la somme des bases échangeables sur la jachère ancienne (2,669 Mèq/100g) est supérieure à celles sur le champ (2,085 Mèq/100g) et la jachère jeune (2 Mèq/100g). Ceci est lié, en particulier, au piégeage des particules et des nutriments associés transportés depuis les surfaces très sensibles à l'érosion éolienne (champ et jachère jeune) par le couvert végétal de la jachère ancienne.
... 2. 1. Mesure de la vitesse du vent et de la pluviométrie L"érosion éolienne est un phénomène à seuil contrôlé par la rugosité de surface et la vitesse du vent. Au Sahel, la dynamique de l"érosion éolienne est intimement liée à celle de la vitesse du vent [12,13]. Ainsi, la vitesse du vent a été mesurée à l"aide d"un anémomètre installé à 250 cm de hauteur sur un champ cultivé situé à Banizoumbou (13°54"N ; 2°66"E ; 50 km à l"Est de Bangou Bi). ...
... Sur les sols sableux cultivés de la région de Niamey, la vitesse seuil d"érosion éolienne des sols est de 7,2 m s -1 pour des vents mesurés à 2,5 m du sol [12]. Relativement à cette valeur seuil, les vitesses journalières maximales de vent ont présenté une dynamique saisonnière marquée (figure 2) : pendant la saison des pluies (mai à septembre), les vitesses maximales ont été largement au-dessus de ce seuil : ces vents violents précèdent les systèmes convectifs qui produisent 100 m a b ...
... Les ordres de grandeur mesurés ici sont tout à fait comparables à ceux obtenus par [17], mais atteignent le double de ceux mesurés par [16,18]. Ces différentes périodes de dépôt de poussière sur Bangou Bi coïncident avec les périodes d"érosion observées sur les surfaces cultivées et nues de Banizoumbou [12,13]. Ces périodes sont en effet déterminées par la météorologie, particulièrement, par les vitesses de vent ( figure 2). ...
In Niamey region, ponds and lakes formed in the past six decades are being filled with a
sedimentation rate of 4 cm per year. The cultivated sandy soils covering their watershed are prone to wind and water erosions. This work aimed to estimate the proportion of aeolian deposits in the filling of one of these lakes, Bangou Bi, by the means of aeolian deposition flux measurements on the lake and wind erosion flux measurements on cultivated and bare soils.
It appeared that more than 70% of aeolian deposits on Bangou Bi were provided by locale wind erosion that is controlled by the annual cycle of cultivated and bare soils erosion. Anyway, these deposits only reached a maximum thickness of 500μm per year which appears negligible in the filling of the lake.
... Ainsi, des impacts néfastes liés à l'érosion éolienne sont souvent enregistrés: le déchaussement de la végétation, et l'appauvrissement en nutriments des sols conduisant à la réduction de leur fertilité et de leur productivité. Les sols sableux sur lesquels repose la production du mil, principale culture de la région sahélienne, sont particulièrement sensibles à l'érosion éolienne [7][8][9][10][11]. Celle-ci, par processus sélectif, affecte spécifiquement l'horizon de surface et conduit à l'exportation des fines particules du sol où l'essentiel des réserves en nutriments pour les plantes est stocké [8]. ...
... Le flux horizontal (Fh) est calculé (équation 2) en intégrant la densité de flux en fonction de la hauteur (0 à 40 cm) [7][8][9][20][21][22][23]. ...
... Sur la parcelle cultivée Pc, le couvert végétal constitué principalement de résidus de récolte est la plus faible passant de 2,25 % (±0,66) en mai à 2,13 (±0,94) en juillet 2014. Ces taux de recouvrement par la litière sont du même ordre de grandeur que ceux déterminés, en juin (~2%), sur des surfaces cultivées du même terroir entre 2006 et 2008 [9]. La densité de recouvrement sur la jachère Pj est relativement plus importante variant entre 6,20 % (±4,4) en mai à 6,19 (±3,2) en juillet 2014. ...
... It is of the same order as the annual saltation flux measured on traditionally cultivated millet fields by different authors using the same sand traps. In western Niger, from 1996 to 1998, Rajot [9] measured an annual saltation flux of 209 to 601 kg m −1 , and Abdourhamane Touré et al. [49] Table 7), which may be explained by the presence of olive trees which significantly reduces the shear stress at the surface [50]. This is the same mechanism as in the oasis, but as olive trees are taller and the space between them is larger, the reduction of shear stress at the surface is lower than in the oasis. ...
... Even if they cover a large area of the studied domain, especially compared to croplands (olive groves occupy about five times more area than croplands), fluxes of wind-eroded sediments from olive groves are lower than those from croplands (2 kg m −1 yr −1 , and 9 kg m −1 yr −1 , respectively- Table 8). This result highlights that particular attention must be paid to croplands, and that agricultural practices that limit wind erosion (e.g., using a mouldboard plough instead of a disc plough [20], leaving crop residues in the field after harvest [49,53,54]) must be encouraged. The Chott can be an active dust source but does not seem to be the major one in the area during the period of measurement, contrary to the observations reported by Prospero et al. [2]. ...
Wind erosion is a worldwide phenomenon for which several recent studies have shown that the intensity is expected to evolve because of climate and land use changes. Identifying the areas where wind erosion is the most active and its associated drivers may help to define efficient solutions to protect the environment from this hazard. The south of Tunisia is a region highly prone to wind erosion and presents a variety of landscapes and land uses in a relatively small area. Thus, from November 2012 to June 2016, six sites were instrumented to monitor wind erosion in the most extended land uses existing in southern Tunisia. The main results are that wind erosion: (i) is nil in the oasis, (ii) is weak (<120 kg m−1 yr−1) in the olive grove even if the surface is be ploughed up to eight times a year, (iii) is moderate (~1000 kg m−1 yr−1) in the barley field, for which the sowing date is a driving parameter of wind erosion seasonality, (iv) can occur in the Chott El Jerid except when it is flooded, (v) is the most intense in rangelands on sand (~2500 kg m−1 yr−1) and on flatbeds (>3000 kg m−1 yr−1).
... For experimental sites with MAT of <5 and 5-10 C, single cropping is dominant in most cases due to the temperature limitation (Dong et al., 2016). The biomass of the single cropping system is much less than the experimental sites that allow double cropping system when MAT is >10 C. The reduced residue amount may significantly decrease wind erosion control efficacy (Toure et al., 2011). Furthermore, the lower erosion control efficacy in sites with MAP of <400 mm may also be attributed to the limited growth of vegetation and thus the reduced biomass compared to the sites in humid regions. ...
... control. Actually, a previous field study suggested a minimal cover rate of 100 kg ha À1 was critical for wind erosion control (Toure et al., 2011). Therefore, in wind erosion areas where full mulching could not be guaranteed due to other uses of straw (e.g. ...
Wind erosion represents one of the major land degradation forms that significantly
threatens sustainable development across the globe. A series of conservation practices have been widely developed and tested in order to effectively reduce wind erosion rates in non-irrigated dry and semi-dry lands. However, a comprehensive
investigation regarding the efficacy of conservation practices and potential impacting factors on erosion control at the global scale has not been undertaken. In this study, a synthesis was conducted, based on data from global experimental plots, to comprehensively examine the efficacy of varying conservation practices, and their effectiveness at reducing erosion, in different geographical regions, under varying climatic conditions, and with different soil properties. The results showed that conservation practices were able to reduce wind erosion rates by over 52% globally. Straw use and soil stabilizer were the most effective measures in wind erosion control, which could reduce erosion rates by over 70%. Wind erosion control efficacy of conservation practices differed among climatic zones, continents, and countries. Notably, the lowest efficacy was observed in dry climatic zones. Erosion control efficacy highly depended on climatic conditions, such as mean annual precipitation (MAP), mean annual temperature (MAT), wind speed, and soil properties, such as water content and clay content. We further observed a positive correlation between wind erosion reduction rates and straw mulching rates, suggesting a straw mulching rate of at least 40% was necessary in order to reach a potential erosion reduction rate of 70%. To sum up, this study illustrates that the overall efficacy of conservation practices in wind erosion control is highly effective at the global scale, but the specific effects vary among different regions, and depend on conservation practice type, climatic condition, and soil property. The lower erosion reduction rate of dry regions at high erosion risk suggests the high erosion control pressure, and more effective measures should be considered and developed. The results of this study are, importantly, complementary to existing wind erosion control guidelines.
... Measured saltation flux (in kg m − 1 day − 1 ), DUP (in m 3 s − 3 ), and z 0 (in m) for 2 sets of selected periods before and after harvest. Rajot (2001) and Abdourhamane Touré et al. (2011) showed that wind erosion on fields cultivated with pearl millet reached its maximum in May and June, at the beginning of the wet season. This corresponds to the time of the year when the soil surface is almost bare because crop residues from the previous rainy season have disappeared and the pearl millet has not started to grow, and also to the period when the DUP is the highest . ...
... So, traditional practices to produce barley (date of tillage and sowing) allow to limit wind erosion. The only way to improve this strategy and further limit wind erosion would probably consist in maintaining a minimum coverage of the surface in the post-harvest period, in particular by leaving some of the vegetation residues on the surface (see for instance Abdourhamane Touré et al. (2011) or Pi et al. (2020)). In the use of these vegetation residues, it will then be a matter of finding the best balance between soil cover and feed for the herds. ...
Dust emission by wind erosion is a worldwide phenomenon that threatens sustainable development and population wellness in areas where anthropogenic activities develop. However, uncertainties on the current estimates of dust originating from agricultural activities remain high. This study aims at disentangling the respective roles of meteorology, surface properties, and human practices in the dynamics of wind erosion over croplands. Therefore, an experimental field campaign was conducted in a traditionally cultivated barley field in southern Tunisia during the agricultural year 2015-2016. Meteorological parameters (wind speed and direction, rainfall), surface characteristics (barley surface cover and height), and the horizontal flux of aeolian sediments were measured. Land management was also documented. 97% of the wind erosion fluxes occurred between mid-May and November 2016. This was explained by the seasonal cycle of barley crop, land management, and meteorological conditions: (i) in autumn, the soil surface sufficiently wet to allow barley growth is tilled, which creates clods that increase the soil surface roughness and inhibits wind erosion, (ii) late winter coincides with the period of the highest wind speed but the height of the barley, maximal in this period, prevents wind erosion, and (iii) the field surface left bare and trampled at the end of harvest in spring is totally prone to wind erosion. This study highlights the importance of accounting for the joint seasonality of the meteorological parameters, vegetation cover, and human practices when studying wind erosion. Neglecting one of these parameters can induce a net overestimation/underestimation of wind erosion by the models.
... However, under the climate condition of low temperature in northeast China, the corn cobs decompose slowly after returning to the field, which causes short-term soil acidification [7], affects crop root development, aggravates pests and diseases, and reduces the quality of spring sowing [8,9]. Therefore, in order to improve soil physical and chemical properties [10], combat soil erosion [11], promote crop growth, and increase crop yield [12,13], it is of great significance to explore the field decomposition characteristics of corn cobs under the conditions of corn kernel harvesting technology in northeast China. ...
... Agriculture 2021, 11, 556 ...
Returning corn cobs to the field during corn kernel harvesting is an effective way to improve soil properties and increase crop yield. However, seasonally frozen soil seriously hinders the field decomposition process of corn cobs. To explore the decomposition characteristics and promote field decomposition, in this study, the nylon mesh bag method was used to perform field decomposition tests for 150 days. Fiber composition analysis and microstructure observation were carried out. The results showed that the field decomposition of corn cob was influenced by temperature, precipitation, and frozen soil environment. The 150-day cumulative decomposition rates of the pith, woody ring, and glume were 40.0%, 24.2%, and 36.3%, respectively. Caused by the difference in fiber compositions, the decomposition speeds of pith and glume were much higher than that of the woody ring. The complex microstructures of the pith, woody ring, and glume led to differences in the accessibility of cellulose, which indirectly influenced the field decomposition characteristics. The homogeneous sponge-like structure of the pith and glume increased the accessibility of cellulose and ultimately accelerated the field decomposition, while the compact lignocellulosic structure of the woody ring hindered the decomposition process. Compared with corn stalk, corn cob had similar or even better field decomposition characteristics and excellent application prospects.
... Vegetation, whether native or exotic, has been widely reported to be effective in reducing dust storms (e.g. Goudie and Middleton 2006;Micklin 2007;Mendez and Maier 2008;Toure et al. 2011;Li and Sherman 2015;Ahmed, Al-Dousari, and Al-Dousari 2016;Dong 2017, 2019;Torshizi et al. 2020b;Al-Dousari et al. 2020). Native plants and green belts have also contributed to the reduction in the annual rates of mobile sand by 94% and 95.3%, and dust by 64.5% and 68.4%, respectively (Al-Dousari et al. 2019). ...
... Native plants and green belts have also contributed to the reduction in the annual rates of mobile sand by 94% and 95.3%, and dust by 64.5% and 68.4%, respectively (Al-Dousari et al. 2019). Even when vegetation cover is as low as 2%, the potential for wind erosion and SDS is reduced (Toure et al. 2011). In addition to the presence of vegetation, Li and Sherman (2015) showed that the efficiency of dust stabilization is related to the height of vegetation. ...
Sand and dust storms (SDS) are a natural phenomenon dominantly originating in
arid and semi-arid regions. The vastness and changing distribution of the SDS
hotspots, their inaccessibility, and financial limitations are the greatest challenges to
projects combating SDS. To boost project success, a prioritized site-selection
method which incorporates the physical and human variables interacting with SDS
is applied in this paper. This ensures that potential economic and health impacts of
SDS on human populations are also included. Six variables were selected for
analysis: within-region sand and dust hotspots, changing distribution of the
hotspots, residential areas, vegetation cover, soil texture, and the maximum
drought-inundation of the Hamoun wetland. SDS hotspot sites for possible
stabilization were identified and prioritized using the multi criteria evaluation
(MCE) method. Such prioritization assists the practical management of hotspots
under conditions of resource and budgetary constraints which are especially
common in developing countries. This paper thus provides a template for site
selection and prioritization of SDS hotspots for remediation. It also highlights the
importance of variables other than SDS source areas for selection of rehabilitation
sites. The proposed method, using the Sistan region as an example, is applicable to
other regions of the world as a means of reducing the negative effects of known
dust storm hotspots.
... By contrast, at a smaller spatial scale, field studies in the Sahel have repeatedly underlined the importance of land surface conditions, especially vegetation cover and soil moisture, in controlling wind erosion [Nickling and Gillies, 1993, Sterk, 2003, Maurer et al., 2009, Bielders et al., 2002, Ikazaki et al., 2011. Toure et al. [2011] suggested that a value as low as 10 g/m 2 of crop residues (~2% cover) may significantly inhibit wind erosion in southwestern Niger. The comparison of wind erosion over fallow fields and millet fields, with or without millet residues left over in the dry season, also points toward the importance of Sahelian vegetation to limit dust emission, especially in the late dry season (April-June). ...
... The strong persistence of STI anomalies until the next rainy season (Figures 2 and S1) implies that the drivers of vegetation decay in the dry season, namely, grazing, bush fires, residues management, decomposition, and oxidation are not fluctuating enough to generate noticeable interannual variability at the Sahel scale over 2002-2014. This persistence also supports the use of previous summer's NDVI as an indicator of dry-season vegetation variability for two reasons: First, the correlation of NDVI and dry-season STI with AOD are fairly close, and second, from a physical process point of view, the dry-season cover fraction identified here is close to plausible thresholds of inhibition of dust emission [Toure et al., 2011]. The shape of the persistence time series also calls for using a calendar starting in July and ending in June, rather than January to December, when studying at vegetation impact on dust emissions with annual data. ...
... Thus, from October to the next wet season, the remaining dry vegetation controls the percentage of cover of the surface: It continuously decreases due to agricultural practices, soil trampling, grazing, and consumption by termites and gradually uncovers the surface until the start of the next wet season (e.g., Pierre et al., 2015). As a result, dry vegetation plays a major role in protecting the surface against wind erosion during the dry season and the beginning of the following wet season: Abdourhamane Touré et al. (2011) estimated that a percentage of cover of agricultural fields by vegetation residues higher than 2% could be sufficient for significantly reduce wind erosion and dust emission. On an interannual time scale, Kergoat et al. (2017), using an indicator of the dry-season nonphotosynthetic vegetation cover in the Sahel based on Moderate Resolution Imaging Spectroradiometer (MODIS), suggested that 43% of the year-to-year variance in Sahelian-mean dry-season aerosol optical depth can be explained by the change in dry vegetation cover as a consequence of differences in precipitation patterns occurring during the previous wet season. ...
... In the cultivated field most of the surfaces remains bare due to the repeated weeding all along the first part of the rainy season maintaining the surface highly erodible (e.g., Klaij & Hoogmoed, 1996). Then, after the first phenological stage of vegetation, when the millet begin to grow it tends to increase the TWS despite the vegetation cover remain low (Abdourhamane Touré et al., 2011). This suggests that the impact of growing vegetation on dust emission could vary in time and space in the Sahel according to land uses. ...
Based on 10 years of continuous measurements of wind speed, rainfall and PM10 concentrations (i.e., concentrations of the particulate matter having a diameter lower than 10 μm) performed in two Sahelian stations, we examine how wind speed and vegetation interact during the wet season to control the dust concentration when it is due to local dust emissions. The results clearly show that the frequency of the high wind speeds is higher at the beginning of the wet season and is the main driver of the seasonal dust emission. During the second part of the wet season, the frequency of high wind speeds is much lower and, in addition, their efficiency for wind erosion and dust emission is strongly affected by the vegetation whose growth reduces progressively PM10 concentrations up to 80%.
... Ainsi, la région de Tillabéry, à laquelle appartient administrativement le site d'étude, a connu un quadruplement de sa population en 35 ans entre 1977 et 2012 [7]. Une telle croissance démographique implique l'exploitation quasi-régulière des ressources foncières et forestières [8] avec, dans certains cas, des pratiques peu conservatrices [9,10]. D'où le déclenchement des processus d'érosion hydrique et éolienne qui se manifestent par l'encroûtement [11 -13] et ou par le ravinement [14 -16]. ...
... A B Figure 10 : Impact de ré-encroûtement de la parcelle sous solée sur la production de ruissellement L'encroûtement de la parcelle sous-solée traduit aussi la nécessité d'un élément structurant empêchant la compaction du sol. Il est ainsi montré qu'une proportion des résidus de culture de l'ordre de 2 % est suffisante pour limiter la déflation et la compaction des sols [9] dans le secteur de cette étude. Donc la réalisation d'un sous-solage couplé à l'épandage de résidus permettrait de maintenir durablement les fonctions du sol. ...
... The sandy soil was almost bare: At the time of the experiment (mid-June), most of the plant residues from the previous year have been decomposed or grazed. Indeed, as observed by Abdourhamane Touré et al. (2011), the crop residue estimated at the end of the dry season was very low on this field, less than 100 kg/ha, covering about 1% of the surface. ...
... In the Sahel, most of the wind erosion events occur from May to mid-July when the protection of the soil surface by vegetation and crop residues is the lowest and episodes of high wind velocities the most frequent (e.g., Abdourhamane Touré et al., 2011;Bergametti et al., 2017;Bielders et al., 2004;Rajot, 2001). At that time, in West Africa, most of the humidity comes from the South Tropical Atlantic Ocean and is transported northward by the African monsoon. ...
Bulk and size-resolved particle concentrations were measured at 2.1 and 6.5 m above the soil surface during an intense dust deposition event that occurred in June 2006 in Niger. Bulk concentration measurements were performed using two tapered element oscillating microbalance instruments, and the size-resolved particle concentrations (from 0.3 to 20 μm) using two optical particle counters. The deposition fluxes derived from the bulk concentrations and those derived from the size-resolved ones are in very good agreement. The largest deposition fluxes are recorded when the dust concentrations are maximal. The temporal evolution of the dust deposition flux follows that of the dust concentration. This is not the case of the dry deposition velocities that are most of the time controlled by the wind friction velocity. The results also show that large particles are strongly sensible to rebound when the wind friction velocity is the highest. Size-resolved dry deposition velocities are compared with the rare measurements of dust deposition velocities over bare sandy soils and are confronted to existing parameterizations of the dry deposition velocity. The parameterization of Zhang and Shao (2014, https://doi.org/10.5194/acp-14-12429-2014) appears to be the only one able to reproduce satisfyingly the measured dry deposition velocities on sandy soils in the 1- to 10-μm particle size range. Indeed, unlike others, this scheme considers the desert sandy surfaces as rough surfaces, allowing the interception of dust particles by the sand grains and/or by the small roughness elements present on the surface.
... The wind has a seasonal regime typical of Sahelian environments; it is characterized by two wind types: the Harmattan, a dry wind blowing from November to March in a north-east to south-west direction; and the monsoon, a wet wind blowing from south-west to north-east between May and September. The average daily wind speed usually exceeds 5 m s -1 (Abdourhamane Touré et al., 2011). ...
This study examines the origin and distribution of clay minerals of the pedological horizons of Kori Ouallam watershed (south-western Niger). It is based on field sampling campaigns and a series of laboratory analyses. A total of 49 samples were analysed, 28 from surface horizons (0–10 cm depth) and 21 from pedological profiles (0–1 m depth). The samples were analysed by X-ray diffraction on bulk and clay (<2 μm) fractions, X-ray fluorescence spectrometry, laser granulometry, organic matter and calcium carbonate content, macroscopic observations (binocular loupe) and scanning electron microscopy equipped with an energy-dispersive spectrometry system. The pedological horizons are characterized by low organic matter contents (<1%) and no calcium carbonate. The particle-size distribution shows net textural differentiation, with a predominance of sandy loam to sandy clay loam textures in the upper horizons and clay loam to clay in the deep horizons. The main major oxides were SiO 2 (46.3–89.0%), Al 2 O 3 (5.0–24.2%) and Fe 2 O 3 (1.0–27.9%). Kaolinite (64–98%) is the predominant clay mineral at all horizons, associated with low to moderate proportions of illite (1–34%) and traces of chlorite. Kaolinite is essentially inherited from the parent rock, whereas illite results from chemical alteration by bisialitization of the primary minerals initially rich in potassium feldspar contained in the parent rock. However, soil texture and organic matter vary independently with clay mineralogy. An extended study of all of the pedological facies that make up south-western Niger, combined with supplementary analyses, would further improve our understanding of clay mineralogy in the Sahelian zone.
... Act as an agricultural resource to replace traditional organic fertilizer, the direct or indirect return of crop residue to soil can alleviate the shortage of fertilizer needed for agriculture and reduce the pollution of fertilizer on farmland environment [10,[18][19][20][21]. After crop harvest, crop residues can be removed from the field (to be used for livestock feed, for fuel, or for other off-field purposes), burned in the field (for pest control or for simple residue removal management), covered directly, or plowed to cover the field surface [22,23]. ...
China stands as one of the world’s largest agricultural powerhouses, boasting abundant crop resources. Nonetheless, there remains a lack of clarity regarding the extensive scale of crop residue return in the fields. Drawing from direct field measurements and comprehensive survey data, this paper pioneers the reporting of residues from the five primary crops, shedding light on the associated nutrient components, including carbon (C), nitrogen (N), and phosphorus (P) replenishment via crop roots, stubble, and straw in the Southwest China region for the year 2012. The results showed that the total amount of the main crop residue resources was 97.4 Mt, which was composed of 17.8 Mt, 12.6 Mt, and 67 Mt for crop root, stubble, and straw, respectively. After crops harvested, there were 7165.8 kilotonne nutrient C, 132.2 kilotonne nutrient N, and 9.8 kilotonne nutrient P of crop residues returned to the fields through crop root, respectively, accounting for 44.6%, 48.2%, and 43.4% of the total nutrient returned, which was the main part of crop nutrients return to fields. The amount of nutrient C, N, and P returned through stubbles were 5017.3 kilotonne, 75.9 kilotonne, and 6.8 kilotonne, respectively, accounting for 31.3%, 27.6%, and 30.6% of the total return of crops. From the composition proportion of residues nutrients return to field, the orders were all expressed as follows: root > stubble > straw. According to the optimum fertilization amount of the main crops in Southwest China, the returned of crop residues nutrient N in maize, rice, rapeseed, and wheat can replace approximately 5.6%, 18.4%, 11.2%, and 14.8% of nitrogen fertilizer, and 2.4%, 8.3%, 3%, and 9.2% of phosphate fertilizer, respectively. This conclusion is beneficial for regulating the practice of returning crop residues to the fields and the use of agricultural fertilizers, aiming to achieve sustainable development in agricultural production.
... However, due to the different methodologies used in the studies, their mitigation effects could not be accurately determined (Sterk 2003). To evaluate the effectiveness of this measure, Abdourhamane Toure et al. (2011) investigated the quantity of crop residue required to control wind erosion by monitoring the erosion flux over 2 years. The results showed that generally low crop residues of approximately 100 kg ha −1 or approximately 2% of the total cover are sufficient for reducing wind erosion by a factor of 4. ...
Sand and dust storms (SDS) are a major disruptor in both the source areas where they occur and at distant locations. This critical review aims to address the question of whether mitigation and adaptation measures have been or can be implemented and what is the optimal scale of their implementation to negate the impacts of SDS in Eastern Mediterranean Region (EMR)? Measures which differ in approach are also assessed by recording their successes, failures, and future challenges. We conclude that developing and implementing appropriate mitigation or adaptation measures for SDS at the local level is feasible but, at a wider scale, is a new challenge. This challenge is even more complex in areas like the EMR and the SDS sources affecting it, as it is a crossroad of air masses originating from three major SDS areas, which exhibit economic, political, and social diversity. This review also aims to identify successful mitigation strategies that have been used for similar environmental issues and to draw attention to the lack of adaptation measures in the region. This critical synthesis will serve as a guide for public stakeholders considering measures to mitigate or adapt to SDS based on their effectiveness and the area of implementation.
... Dunkerley, 2019;Toure et al., 2011). ...
Erosion and sediment in a basin can be estimated by calculating and forecasting using various methods. This study aims to assess erosion and sedimentation in the Krueng Jreu sub-basin in the province of Aceh, Indonesia, using the Gavrilović method. This research was carried out by analyzing secondary data for the last ten years, from 2012 to 2021. Data include geology, slope, land use, and river channel networks. The observed parameters include the coefficient of intensity of erosion, temperature coefficient, and sedimentation coefficient, which are used to analyze the erosion volume, spatial sediment rate, and total sediment rate in the Krueng Jreu sub-basin area. The results of calculations using the Gavrilović method show that four main parameters of the biophysical characteristics of the sub-basin, including (1) sensitivity of soil and local geological conditions to erosion, (2) land use, (3) erosion type, and (4) slope of land, have been shown to affect the occurrence of erosion and annual sediment rates. Geological conditions and land use provide a high level of sensitivity to the results of the coefficient of intensity of erosion. Temperature and rainfall are directly proportional to the annual erosion volume and the spatial sediment rate. The lowest yearly erosion volume and spatial sediment rate in 2019 were 64965.41 m3km-2year-1 and 58206.18 m3km-2year-1. Meanwhile, the highest annual erosion volume and spatial sediment rate will occur in 2021, 101500.71 m3km-2year-1 and 90940.21 m3km-2year-1. Fluctuations in the annual volume of erosion are caused by rainfall, affecting the spatial sediment rate and the total sediment rate.
... In tree crops, there is usually a larger contribution of plant remains in the soil and natural vegetation, favoring its aggregation. Moreover, soil loss from wind is reduced as windspeed is slowed down, increasing capture of transported material [6][7][8]. Nevertheless, cultivation causes loss of organic matter and reduces stability of the aggregates in dry medium-texture soils, which conditions an increase in the erodible fraction of the soil by wind [9][10][11]. Under the absence of conservationist strategies, which usually occur in secluded areas, grazing, abandonment of farmlands, and deforestation increase the effects of wind erosion, producing significant loss of soil material, mainly from the most fertile layers [12]. ...
We used wind-transported particle collectors of our own inhouse design to monitor the sediment flow in a citrus orchard in Southeast Spain. These collectors, which can differentiate sediment collected by direction of origin, are very efficient, economical, and easy to manufacture from thermoplastic filaments with an industrial 3D printer. Data were acquired from six vaned masts, each with four collectors at different heights, and on one of those masts, the collectors included load cells with one end attached to the collector floor and the other end to each oriented compartment in the collectors. The load cell values were interpreted in real time by a microcontroller and amplifier. The remote monitoring system was developed with an internet of things (IoT) platform. The results showed clear predominance of winds from the Northeast after dark, and from the South during the middle of the day. After analyzing the sediment transport rates and their balance, we found that those being deposited in the citrus orchard from the Northeast had a higher carbonate content (mainly calcite), which had an aggregating and therefore stabilizing effect against wind erosion of the soil. Furthermore, significant amounts of highly adhesive phyllosilicates were captured by the upper traps, which also contributed to reducing soil wind erodibility because of their adhesiveness. However, the sediments from the South with much more total transported mass were not deposited in the study zone, but leeward of it and contained a large amount of quartz, promoting abrasion and increasing wind erodibility of the soil.
... This clearly suggests that most of the high wind speed associated with such rainy events are the most efficient dust producers since they generally occur before the rain inhibits the erodibility of the surface. Moreover, the prevalence of the highest wind speed in May/June has large consequences in terms of dust emission since this period of the year is that during which the soil surface is the less protected by the vegetative residues from the previous year (e.g., Abdourhamane Touré et al., 2011;Bergametti et al., 2020;Pierre et al., 2015). Thus, it can be concluded that meteorological conditions and surface characteristics converge to favor the genesis of intense wind erosion and dust emission events during the premonsoon period and the beginning of the monsoon. ...
The Sahel is a dust source region where dust emission could be drastically modified in the future due to climatic and land use changes. Based on observations of meteorological parameters and dust concentration for about 1,000 rain events, we investigated the processes leading to dust emission during the rainy season when Mesoscale Convective Systems (MCSs) regularly cross the Sahel. We show that the highest wind speed is strongly linked to the MCS cold pool intensity, which is characterized by a drop in surface temperature. This is observed during the premonsoon period (∼May to June) when the midtroposphere is still sufficiently dry to allow intense evaporation of raindrops. Because this coincides with the time of the year that the surface is the least protected by the vegetative residue, the premonsoon wind speed leads to the highest observed dust concentration in our record. Most of the highest wind speed occur before or just at the beginning of a rainy event allowing a large part of the dust raised to be transported ahead the rain limiting dust removal by wet scavenging. Finally, we show that the number of 5‐min dust concentration higher than 5,000 μg m⁻³ is almost only occurring during the rainy season. These results suggest that until the dust models fail to correctly resolve MCS, it will be difficulty to obtain reliable estimates of dust emission from the Sahel for the present or future scenarios.
... As well, crop residues may form small dams which retard runoff and create localized puddles which absorb raindrop energy, reducing both the detachment and the transport of soil particles (Nalatwadmath et al. 2006). The frontline defence measures to reduce and stop soil erosion from cultivated fields are the reduction or elimination of tillage combined with the retention of residues and/or the application of mulches (Toure et al. 2011;Alliaume et al. 2014). Bhatt and Arora (2019) reported that treatments under ZT or NT combined with a weed mulch had only 3 Mg ha À1 of soil loss, whereas treatments under CT with the weed mulch or under ZT without mulch had 7 and 12 Mg ha À1 soil loss, respectively. ...
Agriculture in Bangladesh is subsistence-oriented, with traditional management practices still widespread. More recently, new management options have been introduced which have led to substantial improvements in national food and nutrition security as well as a decline in rural poverty. Globally, Bangladesh is the second largest consumer per capita of rice (about 200 kg year⁻¹). Between 77% and 80% of the country’s arable land is used for rice-based crop production. Depending on local edaphic and hydrologic conditions, rice may be grown over three key cropping periods: aman (grown in the wet season and rainfed from monsoon rains); boro (grown in winter and fully irrigated); and aus (grown in spring largely using pre-monsoon rainfall). To meet the increasing food and nutrition demands of Bangladesh’s increasing population, farmers apply high doses of agrochemicals (e.g. fertilizers, pesticides, and herbicides) without realizing the deleterious effect overapplication has in terms of depleting soil organic matter, increasing both macro- and micro-nutrient insufficiencies, increasing water-logging and/or poor drainage, and increases in soil salinity and acidity. In addition, intensive rice cultivation under irrigation is the greatest source of greenhouse gas emissions from cropland. In 2014, global greenhouse gas emissions from rice cultivation were 192 megatons. To mitigate the adverse effects on soil health of traditional intensive crop management, and also to reduce greenhouse gas emissions from food grain production, conservation agriculture has been proposed as a key tool to sustainably maintain or increase agricultural productivity and profitability while preserving or enhancing natural resources and the environment. Conservation agriculture is based on three principle strategies: minimal disturbance of soil; maintaining soil cover through the retention of crop residues and/or cover crops; and the use of crop rotations. This chapter explores how, in Bangladesh, conservation agriculture improves soil physical, biochemical and biological health, leading to improved cropping system productivity while minimizing environmental damage. We also examine key challenges and potential solutions to promote the wider expansion of conservation agriculture practices in the intensive rice-based cropping systems of South Asia, in particular in Bangladesh.
... In the absence of tillage, there is more natural vegetation and soil aggregation, which reduce soil loss from wind by slowing down wind speed, further increasing the capacity for capturing lost material [6,7]. Climate and the spatial and temporal variability in threshold wind velocity strongly influence the prediction of the amount and type of wind-blown particles [8,9]. ...
We tested an efficient, easily and economically manufactured wind-transported particle collector of our own design, called a multidirectional trap (MDt), on fine-tilled Anthrosols. Results from the logs of nine vaned masts, each with four MDt collectors at different heights, showed a clear predominance of northeast and south winds. After analyzing sediment transport rates and their balance, we found that sediments from the south were being deposited rather than lost. A large amount of phyllosilicates, which are highly adhesive sediments, and therefore, increase aggregation, decreasing erodibility, were captured in the upper traps. Moreover, they are rich in calcium carbonate, mainly calcite, which is a powerful aggregate, and therefore, also decreases their wind erodibility. Sediments from the northeast, however, with almost double the total mass transport, contained the largest amount of captured quartz, promoting abrasion and increasing soil erodibility. Nevertheless, large amounts of organic matter found in sediments from the NE led to some aggregation, which balances material lost.
... Standing residue can result in sediment accumulation, as observed by Jia et al. (2015) in corn fields of northern China. The required amount of crop residues to mitigate wind erosion can be relatively low, even for litter (Abdourhamane Touré et al., 2011). However, critical standing and prostrate residue cover levels required to control wind erosion have only been established for a few crop types, sowing densities and wind conditions. ...
... One of the primary goals of this study is to generate region-specific information that will help the growers in the desert southwest to select appropriate cover crops to protect the soil surface from wind erosion under deficit and full irrigation conditions. Most of the previous studies have evaluated the effects of crop residues on wind erosion control in agricultural lands (Unger et al., 1991;Michels et al., 1995aMichels et al., , 1995bToure et al., 2011). Soils are highly vulnerable to wind erosion in arid and semiarid regions because of limited or no residue cover during the wind erosion events. ...
Highlights
Four summer cover crops under two irrigation regimes were tested for soil wind erosion control.
Soil aggregate stability and wind erosion losses were improved with plant cover in the peak summer.
Sorghum sudan is a prominent summer species for wind erosion control in the arid southwest.
Abstract . Cover crops can have significant impacts on minimizing soil erosion by wind, which is a common problem in the arid southwest. A study was conducted at NMSU Leyendecker Plant Science Center, Las Cruces, New Mexico, to evaluate the impacts of selected summer cover crops on soil loss during wind erosion events. Four summer grass species [Japanese millet (Echinochloa esculenta), pearl millet (Pennisetum glaucum), brown top millet (Urochloa ramosa (L.) Nguyen), and sorghum sudangrass (Sorghum bicolor × S. bicolor var. Sudanese)] were randomly assigned to four blocks under two irrigation regimes (full-irrigation and deficit-irrigation). Results showed significant effect of grasses on horizontal soil flux compared to control under both irrigation regimes. When comparing the grasses, sorghum sudan grass performed better than the other tested summer cover crops for soil surface protection from wind erosion with its higher ground coverage, higher plant density, taller plant height and higher amount of biomass, while brown top millet was least effective in terms of plant and erosion control characteristics (P=0.05). Pearl millet and Japanese millet performed better than brown top millet, but was as effective as sorghum sudan as barriers against the wind erosion. Therefore, sorghum sudan is a prominent cover crop for summer in the arid zones of desert southwest. Producers of this region can be significantly benefited from the current research recommendations about cover crops choices for summer season. Keywords: Arid climates, Cover cropping, Summer grasses, Wind erosion control.
... Wind erosion influences soil drying and nutrient loss (Molchanov et al, 2015), both of which are conditioned by soil surface compaction. Vegetation protects the soil from wind erosion by reducing wind speed, it also reduces soil erodibility, and traps eroded material (Touré et al., 2011;Leenders et al., 2011;Asensio et al., 2015b). Plant cover acts like a windbreak, forcing air to flow through it more slowly and faster over the top (Molina-Aiz et al., 2006). ...
... Standing residue can result in sediment accumulation, as observed by Jia et al. (2015) in corn fields of northern China. The required amount of crop residues to mitigate wind erosion can be relatively low, even for litter (Abdourhamane Touré et al., 2011). However, critical standing and prostrate residue cover levels required to control wind erosion have only been established for a few crop types, sowing densities and wind conditions. ...
Human land uses and land management systems have created anthropogenic environments that influence rates of wind erosion and dust emission. This article begins by reviewing the sensitivity of aeolian processes to anthropogenic land use and management. It reviews the latest research addressing anthropogenic wind erosion, including impacts on agroecosystems, human health and built infrastructure. Conceptual frameworks underpinning assessments are critically evaluated relative to the state-of-the-science and support for wind erosion management. The article then explores how future research can connect anthropogenic wind erosion mitigation to broader land management and air quality objectives.
... Vegetation provides an effective agent in reducing dust storms (Toure et al., 2011;Miri et al., 2017Miri et al., , 2018Miri et al., , 2019Miri et al., , 2021Torshizi et al., 2020aTorshizi et al., , 2020b. Tan (2016) found a clear relationship between the Normalised Difference Vegetation Index (NDVI) and dust storms in China. ...
Dust storms cause a wide range of impacts on environment, economy and human health in the Sistan region of southeastern Iran. This paper investigates long-term variability of dust activity over 23 years (1997-2019) using the Dust Storm Index (DSI) and the frequency of dust-storm days (DSD, visibility <1000 m) and assesses the associated importance of various terrestrial and climatic drivers. A dust storm corridor was identified, based on the prevailing wind direction at Zabol, including parts of the Hamoun lakes and surrounding desert in order to study the effects of vegetation cover and lake water levels on dust activity. The results show maximum intensity of dust storms occurred at 10:30 a.m. and in the summer, consistent with the highest wind speeds – associated with the regionally important Levar wind – and highest air temperatures and lowest precipitation and relative humidity. Strong positive correlations were demonstrated between DSI and wind speed, particularly in summer. The 2000-2004 period saw severe dust-raising activity with a DSI of 530.6. Mean wind speeds were greater and precipitation, humidity, vegetation and water coverage were lower during this severe dust-activity period than in other periods. Comparing 2000-2004 with 1997-1999, DSI was five times higher and DSD eight times higher. The dust storms with the longest duration occurred in July 2001 and June 2008 (114 h and 78 h respectively). The July 2001 event, in which wind speed peaked at 25 m/s and visibility dropped to 100 m on several occasions, may be the longest continuous dust storm on record. The key role of water and vegetation cover in the Hamouns was highlighted, indicating the importance of protecting the Hamoun ecosystems and sustainably managing their water resources in efforts to mitigate dust storm hazards in the Sistan region
... Crop residue on the agricultural land generally protects the soil from wind erosion (Hagen, 1996;Sterk and Spaan, 1997;Toure et al., 2011;Gao et al., 2014;Jia et al., 2015). According to its character, crop residue can be divided into two types: standing residue and flat residue. ...
Atmospheric particulate matter and dust from agricultural land significantly affect air quality and human health in nearby residential areas. To quantify these air pollutants, it is important to estimate the vertical atmospheric particulate matter PM2.5, PM10, and dust flux using observations and validated models. To the authors´ knowledge, this has previously not been performed for semi-humid agricultural areas in China. For this purpose, we used wind tunnel tests together with field observations and the validated WEAM simulation model to assess vertical PM2.5, PM10, and dust flux from an experimental agricultural area. Four maize residue management procedures were included in the experiments, namely, conventional tillage (CT), remaining flat residue (FR), remaining standing stubble (MS), and a combination of flat residue and standing stubble (SR). Results showed that: (1) the modified WEAM model adequately simulated the vertical PM10 and dust flux from agricultural land with the four residue managements and the vertical PM2.5 flux from soils of CT and MS; (2) the vertical PM2.5, PM10, and dust flux decrease with increase of density and height of standing maize residue >10 cm, and increase with increase height of standing maize residue <10 cm; (3) the vertical PM2.5, PM10, and dust flux decrease with increase of flat residue coverage, and the threshold coverage of flat maize residue varies linearly with the wind speed; and (4) a combination of standing and flat maize residue is needed to prevent vertical PM2.5, PM10, and dust flux at wind speed 12−24 ms⁻¹. It can be concluded that flat or standing maize residue may reduce dust emission to the National Environment Air Quality Standard when the wind speed is <16 ms⁻¹. A combination of flat and standing maize residue (>30 cm, denser than 5 cm, and coverage >45 %) is needed for higher wind speed. The study provides an approach to control PM2.5, PM10, and dust emission from agricultural areas and ways to better manage of crop residue.
... Our results reaffirmed previous findings representing the potential of vegetation to reduce the rate of soil loss (Youssef et al., 2012), as indicated by the decreased soil loss of zigzag and random vegetation patterns. The results obtained from our experiment with the wind tunnel are consistent with the findings of Van De Ven, Fryrear, andSpaan (1989, Sterk, Jacobs, andVan Boxel (1998), Cornelis and Gabriels (2005), Udo and Takewaka (2007), Abdourhamane Toure et al. (2011), Leenders, Sterk, Van Boxel (2011), Munson, Belnap, and Okin (2011 and Youssef et al. (2012) who reported the capacity of vegetation to decrease the rate of soil loss by wind, through reducing the wind speed and wind erosivity and increased entrapment of eroded material. The greater coverage at upwind positions of the zigzag vegetation pattern was crucial in intercepting wind and delaying soil loss as compared to random and raw vegetation pattern. ...
Dust emission from wind erosion is a widespread phenomenon in arid and semi‐arid areas having considerable implications for ecosystems and human well‐being. However, few studies have examined the efficiency of biochar amended to soil on wind erosion control. Aimed at studying the effect of biochar on resistance of soils against wind erosion, a wind tunnel experiment was conducted. We tested (i) soils amended with hard waste walnut wood biochar and soft maize cob biochar, (ii) soils amended with powdery waste wood and powdery maize cob, and compared them with (iii) non‐treated soil, in their susceptibility to wind erosion and also the additional effect of various patterns of vegetation cover. Amending soil with biochar and powdery material did significantly increase their resilience to wind erosion because of increased soil aggregation. In comparison with the non‐treated control, the mass flux of un‐vegetated soil reduced from 4.42 to 1.86 g/m2/s for the waste walnut wood biochar, from 4.28 to 1.50 g/m2/s for maize cob biochar, from 4.11 to 1.44 g/m2/s for powdery maize cob and from 3.97 to 1.14 g/m2/s for powdery waste walnut wood. When combining amendments with vegetation, there was still a substantial improvement, though the soil treatments responded differently in terms of soil loss to different vegetation patterns. A single row vegetation pattern had the highest mass flux, while a zigzag vegetation pattern had the lowest. In conclusion, waste wood or maize cobs, whether applied as biochar or as powdery material are able to fix soil and reduce wind erosion.
... This research is supported by the extensive literature on momentum partitioning over different roughness arrays (e.g., Raupach et al., 1993;Crawley and Nickling, 2003;Pierre et al., 2014;Webb et al., 2014). Recent studies have sought to establish more integrative effects of residue management on wind erosion for different crop production systems from field measurements and mechanistic models (e.g., Touré et al., 2011;Funk and Engel, 2015;Pierre et al., 2018;Rakkar et al., 2019). Results from these studies can inform wind erosion management and provide references to managers to identify critical crop residue amounts for controlling wind erosion (Rakkar et al., 2019). ...
Crop residue is an important factor influencing wind erosion of cultivated soils. Establishing soil surface protection afforded by standing crop residue is critical for land managers seeking to reduce or prevent soil loss by wind erosion and the impacts of blowing dust from agricultural lands. The objectives of this study were to evaluate the effect of standing residue on soil wind erosion in the inland Pacific Northwest (iPNW), USA, and test the performance of the plant factor algorithm of the Agricultural Policy/Environmental eXtender (APEX) and Revised Wind Erosion Equation (RWEQ) models in influencing soil loss. The effect of standing winter wheat (Triticum aestivum L.), spring canola (Brassica napus L.), and chickpea (Cicer arietinum) residue on wind erosion, remaining from major commodity crops in the region, was tested in a laboratory wind tunnel using four levels of residue density. The impact of standing residue in controlling wind erosion was compared and analyzed in terms of residue density and their respective frontal area index. Our results show that residue at a density characteristic of the production environment (110 standing residue elements m −1 for winter wheat, 20 standing elements m −1 for canola, and 16 standing elements m −1 for chickpea) provided significant protection to the soil surface from wind erosion. Soil loss at this level of residue density was reduced by 73.3, 53.4, and 60.9% for respectively winter wheat, spring canola, and chickpea (frontal area indexes are 0.172, 0.104, and 0.026 respectively) compared with a surface without residue. The soil surface was found to be at significant risk from wind erosion when residue densities of the three crop types were < 50% of the typical production amounts. Although not consistently significant, soil loss decreased as wind direction shifted from parallel to perpendicular with the standing residue row. The APEX model adequately simulated winter wheat and spring canola residue protection but had low accuracy in representing chickpea residue effects relative to the wind tunnel experiments. In contrast, the RWEQ model appeared inadequate in simulating soil loss for the winter wheat and canola treatments but adequately represented chickpea residue effects. Differences in model accuracy for different crop types must be considered by producers and managers to determine whether model information used to select practices to control wind erosion are likely to result in under-or over-protection of soil resources.
... Several studies have demonstrated that vegetation has the ability to increase the entrapment of mobile sand and dust, and decrease soil loss by the wind as a result of reduction of soil erodibility and wind speed [9,10,11,12]. Lancaster and Baas [13] revealed that vegetation increases both the threshold wind shear velocity and the aerodynamic roughness length. According to Fryrear et al. [14] theory for the transport phenomenon over a bare land, the absolute maximum particle transport rate is reached after a certain distance under the condition of no limited field length. ...
This study deals with the vegetation change in Kuwait from 1974 compared to recent vegetation map and their capabilities on trapping mobile sand, dust and carbon dioxides (Co2). The average cost of one cubic meter removal of encroached sand around infrastructures in Kuwait is 1.32 USD. The capability of trapping sand is much higher for Haloxylon sp. than Stipagrostis sp. by 100%. The areas that were covered by Haloxylon in 1974 lose recently 4385 km2 for the benefits of Stipagrostis causing the formation of a new mobile sand corridor. The total estimated annual cost for the vegetation change is 35,429,379 USD obtained from the costs of sand encroachment, CO2 consumption loss and solar energy efficiency loss. Rehabilitation for areas that were occupied with large size canopy vegetation will surely cause a decrease in the aeolian activities and air temperature, lower the albedo and increase the precipitation and solar energy production.
... evidence, high agreement)(Touré et al. 2019;Amundson et al. 2015; Borrelli et al. 2017;Pierre et al. 2017).Amadou et al. (2011) note that even a minimal cover of crop residues (100 kg ha -1 ) can substantially decrease wind erosion. ...
... When vegetation is sparse, standing residue is significantly more effective than flat residue for wind erosion control (Hagen, 1996). Wind erosion studies have focused on the effects of sparse vegetation after harvesting (Wolfe and Nickling, 1993;Lancaster and Baas, 1998;He et al., 2017), land degradation and conversion (Li et al., 2005), and dry land conditions (Musick and Gillette, 1990;Toure et al., 2011). These are all conditions that make the soil surface vulnerable to wind erosion. ...
Maintaining vegetative cover on the soil surface is the most widely used method for control of soil loss by wind erosion. We numerically modeled airflow through artificial standing vegetation (i.e., simulated wheat plants) using computational fluid dynamics (CFD). A solver (simpleFoam within the OpenFOAM software architecture) was used to simulate airflow through various three-dimensional (3D) canopy structures in a wind tunnel, which were created using another open-source CAD geometry software (Salomé ver. 7.2). This study focused on two specific objectives: (1) model airflow through standing vegetation using CFD, and (2) compare the results of a previous wind tunnel study with various artificial vegetation configurations to the results of the CFD model. Wind speeds measured in the wind tunnel experiment differed slightly from the numerical simulation using CFD, especially near positions where simulated vegetation was present. Effective drag coefficients computed using wind profiles did not differ significantly (p <0.05) between the experimental and simulated results. Results of this study will provide information for research into other types of simulated stubble or sparse vegetation during wind erosion events. Keywords: 3-D canopy structure, OpenFOAM, Wind erosion, Wind tunnel studies.
Highlights Keywords: 3-D canopy structure, OpenFOAM, Wind erosion, Wind tunnel studies.
Measured airflow through a simulated canopy was successfully modeled using CFD software. Keywords: 3-D canopy structure, OpenFOAM, Wind erosion, Wind tunnel studies.
Effective drag coefficients did not differ between the experimental and simulated results. Keywords: 3-D canopy structure, OpenFOAM, Wind erosion, Wind tunnel studies.
Results of this study provide 3-D simulation data of wind flow through a plant canopy. Keywords: 3-D canopy structure, OpenFOAM, Wind erosion, Wind tunnel studies.
... Threshold velocity was determined with statistic method which has been successfully applied and verified by Kurosaki and Mikami (2007), Abdourhamane Touré et al. (2011) and Bergametti et al. (2016). The equation is as follows: ...
Soil and the atmospheric conditions are important factors that affect wind speed threshold of surface dust emissions. Based on the observed data of surface dust emissions in the Taklimakan Desert collected from March 2008 to February 2018, the effects of soil moisture, air humidity (vapor pressure), and air temperature on wind speed threshold were analyzed in this study. The results showed that the accumulated time of dust emissions over the decade was 3609.8 h, thereby accounting for 4.1% of the total observation time. In addition, the duration of dust emission in the four seasons were consistent with the pattern of summer > spring > autumn > winter. When the soil moisture is above 3.0–4.0%, the wind speed threshold for dust emission increases with increasing soil moisture. When the vapor pressure is above 10–15 kPa, the wind speed threshold for dust emission increases with increasing vapor pressure. There was a negative correlation between air temperature and the wind speed threshold for dust emission. When the air temperature was higher than 0.0 °C, the soil moisture decreased with increasing air temperature, and the wind speed threshold for dust emission was lower.
... Zhang [38] indicated that this ability is related to air pressure and influenced by the interaction between near-surface wind flow and roughness elements. Toure [39] pointed out that the potential for wind erosion decreases for aerodynamically-rougher surfaces. The results of the present study indicate that z 0 is correlated with the surface roughness properties of the underlying vegetation conditions (e.g., vegetation height), and decrease with increases in wind speed. ...
... The results play an important role in increasing the ability to control soil wind erosion and in realizing sustainable development of agriculture and grassland farming. Due to the increase in aerodynamic roughness that controls the threshold speed of aeolian erosion, even very low crop residue density of standing stalks (approximately 400 plants m −2 ), essentially in the form of millet stalks, can be a simple and sufficient method to reduce wind erosion (Amadou, Jean, Zibo, et al., 2011). ...
Crop residues, particularly standing stubble, can reduce soil erosion by wind in conservation tillage. To determine the optimum architecture of standing stubble for effective reduction of wind speeds near the soil surface, we quantitatively analyzed the interception effect of standing stubbles on soil erosion particles through pure wind experiments and sand‐bearing wind experiments. The relationship between stubble height, row number and interception rate was also revealed. The study was conducted in a wheat field under conservation tillage. Three representative heights of standing stubble were tested: 10, 20 and 30 cm. All had an average plant density of 400 plants m‐2, row spacing of 20 cm, and an average stubble cover of 52 %. A wind tunnel was placed over the stubble, and the central wind speeds were selected at 6, 9, 12, 15, and 18 m s‐1. During the experiments, the blowing time was set for 10min, and 1,404 g of local soil from the experimental area was fed into the wind tunnel. A sand collector was placed at the 16th, 19th, 23rd, 27th, and 31st stubble row away from the air inlet of the wind tunnel to measure the amount of soil eroded by wind at different stubble heights under conditions of blank and sand‐bearing winds. The concept of “interception rate” and its corresponding formula were proposed. The interception rates of 10‐cm and 20‐cm high stubbles were lower than those of 30‐cm stubble and decreased sharply, particularly at wind speeds > 12 m s‐1. Even at the wind speed of 18 m s‐1, the interception of wind erosion particles with 30‐cm high stubble at 16th, 19th, 23rd, 27th, and 31st rows were 20.85%, 41.55%, 60.46%, 80.59% and 85.96%, but only 5.75%, 24.76%, 39.32%, 62.39% and 76.04% with 10‐cm high stubble at the same positions. The interception effect was similar at the 27th and 31st rows. However, more than 31 rows would be required of the 10‐cm stubble and 20‐cm stubble to intercept the same amount of wind erosion particles as with 27 rows of the 30‐cm stubble. The soil particle interception rate increased with an increase in the number of rows of standing stubble and decreased with an increase in wind speed. As expected, with additional rows of stubble, the amount of soil intercepted increased. Therefore, for a farmland that uses 400 plants m‐2, leaves 30‐cm high stubble with 52 % cover, and has row spacing ≤ 20 cm, the effective amount of standing stubble should be more than 27 rows.
... Threshold velocity was determined with statistic method which has been successfully applied and verified by Park and In (2003), Kurosaki and Mikami (2007) and Toure et al. (2011). The equation is as follows: ...
A field experiment was conducted from 2 May 2010 to 1 May 2012 in the
Gurbantunggut Desert, the second largest desert in China, to investigate saltation activity
and its threshold velocity, and their relations with atmospheric and soil conditions. The
results showed that saltation activity occurred more frequently during 08:00–20:00 Local
Standard Time in spring and summer, with air temperatures between 20.0 and 29.0 �C,
water vapor pressures between 0.6 and 0.9 kPa, soil temperatures between 25.0 and
30.0 �C, and a soil moisture lower than 0.04 m3/m3. At 2 m height, the saltation threshold
velocity varied between 11.1 and 13.9 m/s, with a mean of 12.5 m/s. Threshold velocity
showed clear seasonal variations in the following sequence: spring (11.7 m/s) \autumn
(12.7 m/s)\summer (13.6 m/s). Affected by soil conditions, aeolian sand transport was
weak, with an average annual aeolian sand that transported across a section
(1.0 m 9 2.0 m) of less than 6.0 kg.
... With advances in research and application, agricultural by-products have been identified as a significant potential for bioenergy production. In addition, incorporation of straw into the soil plays a positive role in protecting the soil's ecological environment and combating soil erosion [5,6]. Some studies also found that continuing straw return will improve crop yield in the long term [7]. ...
Straw return has positive effect on improving soil ecological environment, and now there are serious ecological problems of arable land in China. Therefore, the concept of basic straw returning amount (BSRA) was put forward based on soil protection functions, and literature review and scenario analysis method were adapted to design BSRA. Based on this, a bottom up dynamic accounting model was built, which considering the changes of sown area, unit crop yield, planting structure and crop straw utilization. Then, the model was applied to assess the potentials of straw resources for bioenergy production. Usable straw resources in low, middle and high BSRA scenario are 265.70, 180.25 and 117.51 Tg respectively in 2030, mainly composed of paddy rice (Oryza sativa L.) stalk, maize (Zea mays L.) stalk and potato (Solanum tuberosum L.) stalk in low and middle scenario, and maize stalk, potato stalk and sugarcane (Saccharum officinarum) stalk in high scenario. Available crop residues mainly distribute in Sichuan, Henan, Heilongjiang and Shandong Province under the three scenarios. There will be interzone spatial transfer of usable straw resources, over time, mainly transferring to Northeast China, North China and Northwest China. The usable straw amount will have shape decrease in August and September as the increase of BSRA.
Wind erosion is a worldwide phenomenon for which several recent studies have shown that the intensity is expected to evolve because of climate and land use changes. Identifying the areas where wind erosion is the most active and its associated drivers may help to define efficient solutions to protect the environment from this hazard. The south of Tunisia is a region highly prone to wind erosion and presents a variety of landscapes and land uses in a relatively small area. Thus, from November 2012 to June 2016, six sites were instrumented to monitor wind erosion in the most extended land uses existing in southern Tunisia. The main results are that wind erosion: (i) is nil in the oasis, (ii) is weak (<120 kg m−1 yr−1) in the olive grove even if the surface is be ploughed up to eight times a year, (iii) is moderate (~1000 kg m−1 yr−1) in the barley field, for which the sowing date is a driving parameter of wind erosion seasonality, (iv) can occur in the Chott El Jerid except when it is flooded, (v) is the most intense in rangelands on sand (~2500 kg m−1 yr−1) and on flatbeds (>3000 kg m−1 yr−1).
In arid and semiarid areas with frequent dust storms, reasonable planting modes and crop residue measures could prevent dust storms that originate from local farmlands and address key factors for wind erosion control. The considered crop residue modes included all harvesting with no residue in a Cyperus esculentus L. (C.) monoculture setting (AH), retention of 1 ridge and harvesting of 3 ridges in a C. monoculture setting (RH), and all harvesting with retention of Setaria italica (L.) Beauv. var. germanica (Mill.) Schrad (S.) residue in an intercropping setting of 6S. ridges and 9C. ridges (ARS). The aerodynamic roughness z0 and friction velocity u* were measured with handheld anemometers, and sediment fluxes were measured by combining multichannel sand collectors under different crop residue modes. The results indicated that the total aeolian sediment flux increased with distance away from the final belt of crop residue under both the RH and ARS modes. S. residue reduced the total aeolian sediment flux by more than 50% compared to C. residue at a distance of 4.5 m away from the residue belt during the dust storm. The sand transported height under both the RH and ARS modes was lower than that under the AH mode, and the distance required for sand to be transported to the same flux behind the S. residue belt was more than that behind the C. residue belt. The frontal area index (FAI) is a parameter that can better reflect the wind erosion control ability of C. and S. residues. S. residue attained a higher wind weakening ability than C. residue according to the positive relationship between z0, u* and FAI. During the dust storm, the relationship between the total aeolian sediment flux and FAI changed from significant to insignificant as the sand transported height exceeded C. residue height. Using standing crops intercropping with C. can provide both income from C. and wind erosion control from standing crop residues. The results can provide useful guidance for local C. harvest and planting design.
Based on a large number of in‐situ measurements performed over a 9‐years period in two Sahelian stations, we investigate the drivers of the dust wet deposition in relation to the meteorological situations and the PM10 (Particulate Matter with diameter lower than 10 μm) surface concentrations. Precipitation associated with cold pools (CP) contribute to more than 90% of the precipitation amount associated with the collected wet deposition samples. The wet deposition events associated with these CP control by far the wet deposition, that is, 66% and 81%, depending on the station. The dust washout ratios (WR) corresponding to the most convective events under high level of dust concentrations were found to be in the range of 319–766 while WR of other kind of events are depending on the dilution effect. This range of value are in the lower range of WR previously estimated and used in dust modeling studies (200–2000).
The endemic argan woodlands cover large parts of South Morocco and create a characteristic landscape with areas of sparsely vegetated and bare soil surfaces between the single trees. This unique ecosystem has been under extensive agrosilvopastoral management for centuries and is now at risk of degradation caused by overgrazing and increasing scarcity and variability of rainfall.
To investigate susceptibility to wind erosion, we conducted an experimental-empirical study and quantified wind erodible material on five different associated surface types. The highest emission flux was measured on freshly ploughed surfaces (1875 g m-2 h-1), while older ploughed areas with a re-established crust produced a much lower emission flux (795 g m-2 h-1). Extensive tillage may have been a sustainable practice for generations, but increasing drought and uncertainty of rainfall now leads to an acute risk of severe soil erosion and dust production. The typical crusted surfaces characterised by residual rock fragment accumulation and wash processes produced the second highest emission flux (1354 g m-2 h-1). Collected material from tree-shaded areas (933 g m-2 h-1) was revealed as a considerable source of organic material possibly affecting substrate conditions positively on a larger regional scale. Lowest flux was measured on rock fragment covered surfaces (301 g m-2 h-1).
The data show that open argan woodland may be a considerable source for wind erosion and dust production, depending on surface characteristics strongly related to management. An adapted management must include the conservation of the argan trees to offer a promising approach to prevent severe wind erosion and dust production and mitigate possible impacts of land-use change and climate-change related shifts in wind and rainfall patterns.
It is common to distinguish two main types of erosion: natural erosion and accelerated erosion by human action. Erosion can also be classified into mechanical erosion and chemical erosion based on the processes. This chapter focuses on the accelerated and mechanical erosion of continental soils and the main principles of soil conservation. It briefly discusses three consequences of erosion, distinguishing between on‐site and off‐site effects, and historical consequences. On‐site effects mainly concern soil quality. Off‐site impacts concern impacts downstream of detachment areas, and therefore transport and deposition. Any erosion process involves three mechanisms: soil particle detachment, transport and deposition. Streambank erosion is also an important source of sediment. Wind erosion affects the climate, geochemical cycles, as well as air quality and human health. The main difficulty of modeling arises from the multiplicity of scales and processes that must be clearly defined each time. There are a variety of approaches: statistical, physically based, and hybrid.
An accurate threshold velocity ut for sand movement is essential for predicting wind erosion events and calculating the magnitude of horizontal sand flux. We compared various approaches for determining ut using data observed during 4–31 July 2009 in the hinterland of Taklimakan Desert. We used the Gaussian time fraction equivalence method developed by Stout with an interval of 1 day to obtain ut values of 3.03–5.62 m/s (the approach of observations through field experiments). The Kurosak (ut50%) method yielded values of 3.71–5.74 m/s (the approach of statistical calculations). The Marticorena and Shao models gave ut values of 4.87–4.90 and 5.82–6.78 m/s, respectively (the approach of model parameterizations). To test the accuracy of these threshold velocities above, we estimated the total horizontal sand flux and the duration of sand saltation. Estimates of the total horizontal sand flux were 1311.9, 1166.4, 1279.9, and 661.6 kg/m for the Stout, Kurosak, Marticorena, and Shao methods, respectively, while the observed value was 732.9 kg/m. The correlation coefficient between observed and estimated values based on the Stout, Kurosak, Marticorena, and Shao methods was 0.75, 0.79, 0.77, and 0.83, respectively. The estimated duration of sand saltation using the ut been as Stout, Kurosak, Marticorena, and Shao was 8211, 6575, 7567, and 3463 min, of which 6208, 5646, 5986, and 3346 min were correct reports, respectively, and the observed value was 7663 min. We discuss the advantages and disadvantages of these methods combining those results above, which can be used to advise researchers to improve the threshold velocity used in future studies.
Research to determine the off-site downwind impact of wind erosion of soil was conducted in the Columbia Plateau region of Washington State. As part of this project, mathematical relationships were determined for the combined effectiveness of flat plant residue and soil surface random roughness, then compared to similar equations derived from previous research. Data were obtained using a portable wind tunnel 1.0 m wide, 1.2 m high with a working section 7.3 m long. Eroded soil was collected using a vertical slot sampler 0.75 m high, 0.003 m wide and near isokinetic at three tunnel velocities. The mass of eroded soil was expressed as a ratio of that from an adjacent smooth, bare soil, then regressed against measured flat residue surface cover and random roughness. Predicted soil loss ratio values from the derived equation compared to the measured values with a slope of 0.96 and and r2 of 0.51. The derived equation was very similar to that of previous research from a region with quite different characteristics which suggests universality of the relationships.
In the frame of the African Monsoon Multidisciplinary Analysis-Special Observation Period 0 (AMMA SOP0) field campaign, a sophisticated instrumental setup specially designed for documenting the characteristics of aerosols in the surface boundary layer has been operated for 1 month (13 January to 13 February 2006) at the Banizoumbou supersite in Niger. The analysis of the long-term routine sunphotometer measurements shows that the intensive measurements were performed during a transition phase between a period when biomass burning aerosols are present in the atmospheric column and another when mineral dust is clearly predominant. On the basis of ground-level observations, three types of events can be defined. (1) Type B events are those corresponding to advection of Biomass burning plumes from southern Sahel. They represent only 3% of the measurement period. (2) Type D events are those during which mineral dust concentrations are larger than 200 mug m-3. They represent 35% of the measuring time, and coincide with advection of mineral dust from distant sources to the measurement site. The composition of the dust differs significantly from one episode to the other, depending on the trajectory of air mass that transported it. (3) Type L events are those that can be explained by occurrence of local erosion and that are of short duration. During them, the aerosol size distribution is dominated by a coarser mode (9 mum) than during the events of the D type dominated by a finer mode (4.5 mum). Elemental analysis also shows that mineral dust is relatively rich in Fe and poor in Ca, which is typical of the Sahelian aerosols.
Under rainfall, a crust forms at the surface of most soils. For soils rich enough in clay (clay content >5%), it is well known that a physical crust is responsible for a supply limitation of particles available for wind erosion. Sandy soils are very prone to crusting as well as to wind erosion. Indeed, structural ‘sieving’ crusts develop on sandy soils even after light rainfalls. This kind of crust shows a loose sand layer at the surface overlying a thin layer where fine particles are concentrated. The main objectives of this study were to determine whether such a crust formation limits the availability of (1) sand grains for entrainment into saltation and (2) fine particles for uptake into a vertical wind erosion flux (suspension in atmosphere of particles
The Sahelian belt is known to be a region where atmospheric levels of suspended mineral dust are among the highest observed on Earth. In the framework of the AMMA (African Monsoon Multidisciplinary Analysis) International Program, a transect of 3 ground based stations, the "Sahe- lian Dust Transect" (SDT), has been deployed in order to obtain quantitative information on the mineral dust content and its variability over the Sahel. The three stations, namely Banizoumbou (Niger), Cinzana (Mali) and M'Bour (Sene- gal) are aligned around 14 N along the east-westward main pathway of the Saharan and Sahelian dust towards the At- lantic Ocean. We discuss data collected between January 2006 and December 2008 to investigate the main charac- teristics of the mineral dust concentration over West Africa and their connection with the dominant meteorological situ- ations. The succession of the dry season during which the Sahel is under the influence of the dry Harmattan wind and the wet season induced by the entrance of the monsoon flow is clearly identified from the basic meteorological parameters (air temperature and moisture, wind direction). Atmospheric dust concentrations at the three stations exhibit a similar sea- sonal cycle, with a monthly maximum during the dry season and a minimum occurring during the rainy season, indicating that the general pattern of dust concentration is similar at re- gional scale. This seasonal cycle of the dust concentrations is not phased with the seasonal cycle of surface wind veloc- ity locally measured, suggesting that it is mainly controlled by Saharan dust transport. Local dust emissions induced by strong surface winds are responsible for the occurrence of ex- tremely high daily concentrations observed at the beginning of the rainy season. A decrease in the dust concentration is observed when moving from Niger to Senegal.
A household survey, combined with an extensive literature search of baseline studies, complemented on-station research on spatial soil and crop growth variability with a farmer's viewpoint. Farmers explained spatial variability in crop growth in terms of differences in soil types, soil fertility and degradation, as well as their cultivation and management practices. Farmers counteract spatial crop growth variability by within-field fallow; kraaling; spot applications of manure, crop residuals and household waste; by internationally moving their settlements; and by exploiting the micro-environmental differences around specific tree and shrub species.
Field studies conducted at Owens Lake, California, provide direct measurements of sand flux on sand sheets with zero to 20 per cent cover of salt grass. Results from 12 different sand transport events show that aerodynamic roughness length and threshold wind shear velocity increase with vegetation cover as measured by vertically projected cover and roughness density (lambda). This results in a negative exponential decrease in sediment flux with increasing vegetation cover such that sand transport is effectively eliminated when the vertically projected cover of salt grass is greater than 15 per cent. A general empirical model for the relation between sand flux and vegetation cover has been derived and can be used to predict the amount of vegetation required to stabilize sand dune areas. (C) 1998 John Wiley & Sons, Ltd.
Wind erosion is regarded as a major contributor to the desertification process in the Sahel, yet little quantitative information is available for that region on soil losses by wind erosion under different land management practices. A 3-yr, on-farm experiment was, therefore, set up to assess the effect of ridging and either banded or broadcast millet stover mulches (2000 kg ha-1) on soil loss in a millet-cowpea intercrop. For wind directions approximately perpendicular to the orientation of ridges and residue bands, sediment mass balances were calculated from the change in horizontal sediment mass fluxes measured across the experimental plots with Big Spring Number Eight sand traps. Mass balance calculations for 16 events over 3 yr indicated an average soil loss of 17.5, 15.4, and 18.0 Mg ha-1 on control plots, and deposition of 15.5, 15.3, and 7.4 Mg ha-1 on banded residue plots in 1995, 1996, and 1997, respectively. Broadcast and banded residue mulches were not significantly different (P = 0.05) in terms of their sediment trapping efficiency. During the same time period, ridges reduced soil losses by an average of 57% compared with the control plots, but their efficiency was reduced to less than 15% after 100 mm of cumulative rainfall as ridges collapsed. Linear regression analysis using the incoming sand fluxes as the independent variable was used to estimate potential soil losses for all events with sediment fluxes <25 kg m-1 irrespective of wind direction. The calculations indicated potential soil losses of up to 79 Mg ha-1 on control plots and deposition of 41 Mg ha-1 on broadcast residue plots in a single year. For wind erosion control, broadcast millet stover mulching constituted the most effective control technique because it effectively protected the soil against erosion and its trapping efficiency is expected to be independent of wind direction.
African Monsoon Multidisciplinary Analysis (AMMA) is an international project to improve our knowledge and understanding of the West African monsoon (WAM) and its variability with an emphasis on daily-to-interannual time scales. AMMA is motivated by an interest in fundamental scientific issues and by the societal need for improved prediction of the WAM and its impacts on West African nations. Recognizing the societal need to develop strategies that reduce the socioeconomic impacts of the variability of the WAM, AMMA will facilitate the multidisciplinary research required to provide improved predictions of the WAM and its impacts. This will be achieved and coordinated through the following five international working groups: i) West African monsoon and global climate, ii) water cycle, iii) surface-atmosphere feedbacks, iv) prediction of climate impacts, and v) high-impact weather prediction and predictability. AMMA promotes the international coordination of ongoing activities, basic research, and a multiyear field campaign over West Africa and the tropical Atlantic. AMMA is developing close partnerships between those involved in basic research of the WAM, operational forecasting, and decision making, and is establishing blended training and education activities for Africans.
In sahelian regions, termites are the most important component of soil macrofauna, as judged by population density and their impacts on soil structure. This paper is an attempt to quantify the effect of termite activity on infiltration in natural conditions and to provide an analysis of the various processes involved. A mulch was used to attract termites to crusted soil sites. Runoff measurements under natural rainfall during 4 years were used to estimate the effect of termites on infiltration, which was increased by a mean factor 2–3. The relationship between the intensity of termite activity and infiltration was studied at the scale of the rainfall event. We found that although infiltration increased with termite activity, at least 30 foraging holes per square meter were necessary for the effect to be significant. Additional experiments improved the understanding of this enhanced infiltration. Galleries and their openings were characterized by their sizes and their hydraulic properties. Tension infiltration measurements and simulated rainfalls with aqueous methylene blue showed that the termite effect significantly persisted through the degradation of the soil surface crust. In addition, it was shown that the influence of the large macropores made by termites is better described as a runoff interception process than by ponded infiltration. This conclusion has important consequences for modelling the effect of a population of large macropores on infiltration.
The vulnerability of dryland surfaces to wind erosion depends importantly on the absence or the presence and character of surface roughness elements, such as plants, clasts, and topographic irregularities that diminish wind speed near the surface. A model for the friction velocity ratio has been developed to account for wind sheltering by many different types of co-existing roughness elements. Such conditions typify a monitored area in the central Mojave Desert, California, that experiences frequent sand movement and dust emission. Two additional models are used to convert the friction velocity ratio to the surface roughness length (zo) for momentum. To calculate roughness lengths from these models, measurements were made at 11 sites within the monitored area to characterize the surface roughness element. Measurements included (1) the number of roughness species (e.g., plants, small-scale topography, clasts), and their associated heights and widths, (2) spacing among species, and (3) vegetation porosity (a measurement of the spatial distribution of woody elements of a plant). Documented or estimated values of drag coefficients for different species were included in the modeling. At these sites, wind-speed profiles were measured during periods of neutral atmospheric stability using three 9-m towers with three or four calibrated anemometers on each. Modeled roughness lengths show a close correspondence (correlation coefficient, 0.84–0.86) to the aerodynamically determined values at the field sites.
Au Sahel, l'érosion éolienne constitue une menace importante pour l'utilisation durable des ressources en terres. Elle présente une dynamique saisonnière marquée en relation avec les cycles climatiques intraannuels et l'évolution du couvert végétal. Au Niger, pays sahélien servant d'exemple pour cette revue de la littérature, l'érosion éolienne est, de plus, favorisée par la dominance de sols sableux à faible teneur en matière organique et par les pratiques culturales qui contribuent à maintenir un faible couvert du sol pendant la période la plus critique en fin de saison sèche et au début de la saison des pluies. L'érosion éolienne se traduit par des pertes en terres parfois considérables à l'échelle de la parcelle expérimentale et du champ. Du fait que l'essentiel des sédiments érodés se redéposent localement, le bilan à l'échelle du terroir villageois serait encore actuellement positif, bénéficiant d'apports extérieurs de poussières. Les agriculteurs sont conscients de leur impact sur l'environnement et prennent un ensemble de mesures qui contribuent, parfois indirectement, à réduire le risque d'érosion éolienne : paillage, maintien des pailles de mil dans les champs, préservation de la régénération naturelle de la végétation, maintien de la végétation en bordure de champ, défrichage des champs sans brûlis, etc. Cet article passe en revue les avantages et les limitations de ces différentes techniques ainsi qu'un certain nombre d'autres techniques évaluées dans le cadre de nombreux projets de recher-che et de développement au Niger. À l'avenir, il est vraisemblable que la lutte contre l'érosion éolienne ne pourra se concevoir qu'au travers de la mise en oeuvre d'un ensemble de mesures simples reposant sur le savoir-faire et les moyens locaux et apportant des bénéfices immédiats autres que le contrôle de l'érosion, par exemple en termes de fertilité des sols ou de sous-produits intéressants.
The Sahelian belt is known to be a region where the mineral dust content is among the highest observed on Earth. In the framework of the AMMA (African Monsoon Multidisciplinary Analysis) International Program, a transect of 3 ground based stations, the "Sahelian Dust Transect" (SDT), has been deployed in order to obtain quantitative information on the mineral dust content and its variability over the Sahel. The three stations, namely Banizoumbou (Niger), Cinzana (Mali) and M'Bour (Senegal) are aligned around 14° N along the east-west main pathway of the Saharan and Sahelian dust towards the Atlantic Ocean.
We discuss data collected between January 2006 and December 2008 to investigate the main characteristics of the mineral dust concentration over West Africa and their connection with the dominant meteorological situations. The succession of the dry season during which the Sahel is under the influence of the dry Harmattan wind and the wet season induced by the entrance of the monsoon flow is clearly identified from the basic meteorological parameters (air temperature and moisture, wind direction). Atmospheric dust concentrations at the three stations exhibit a similar seasonal cycle, with a monthly maximum during the dry season and a minimum occurring during the rainy season, indicating that the general pattern of dust concentration is similar at regional scale. This seasonal cycle of the dust concentrations is not phased with the seasonal cycle of surface wind velocity suggesting that it is mainly controlled by Saharan dust transport. A decrease in the dust concentration is observed when moving from Niger to Senegal. However, local dust emissions induced by strong surface winds are responsible for the occurrence of extremely high daily concentrations observed at the beginning of the rainy season.
In the West African Sahel, few direct estimates are currently available on the extent of soil losses by
wind erosion in traditionally managed farmers fields. Field scale sediment balances in western Niger were
derived from airborne sediment fluxes measured using BSNE sand traps. Results from a first study in 1997
indicated that sediment fluxes in a cultivated field increased linearly over distances = 80 m, irrespective of
wind power. Sediment deposition in an adjacent fallow was well described by an exponential decay
function with a near constant trapping efficiency coefficient of 0.1 m-1 for incoming sediment fluxes = 10
kg m-1. Mass balances up to -17.5 and + 10.5 Mg ha-1 were measured in a single storm in the field and
fallow, respectively. Starting in 1998, a second study was setup in a newly cleared, 8 ha farmer's field
equipped at 87 locations with BSNE sand traps. Whereas a net sediment balance of +5.4 Mg ha-1 was
measured in 1998, the experimental field was subject to a net soil loss of -5.0 Mg ha-1 in 1999. This was
attributed to changes in ground cover and differences in sediment influx from adjacent fields.
Mulching for wind erosion control in Sahelian farming systems is limited by low biomass production and use of crop residues for other purposes. The aim of this study was to test the effectiveness in soil protection created by two low amounts of crop residues. A field experiment was conducted in southwest Niger, on a Psammentic Paleustalf (sandy, siliceous, isohyperthermic) during the early rainy seasons of 1994 and 1995. Particle mass transport was quantified in two plots of 55 by 70 m. During the first storms of both seasons, the plots were without a mulch cover. Afterward, one plot was covered with fiat pearl millet [Pennisetum glaucum (L.) R. Br.] stalks. The application rates were 1500 and 1000 kg ha-1 during the first and second seasons, respectively. To quantify the mulch effect, mass transport rate differences between the two bare plots were quantified with a multiple linear regression model (R2 = 0.89), using wind speed (7.412.3 m s-1), wind direction, and storm duration (464-3835 s). Total mass transport rates were reduced from 365.2 to 132.9 g m-1 s-1 (63.6%) with 1500 kg ha-1, and from 325.1 to 188.0 g m-1 s-1 (42.2%) with 1000 kg ha-1 of crop residues. Soil protection tended to decrease with increasing wind speed. Linear regression indicated that the reduction in mass transport becomes zero at wind speeds of 11.1 and 16.0 m s-1 for the 1000 and 1560 kg ha-1 covers, respectively. The 1000 kg ha-1 cover even enhanced sediment transport by 6.5% during one storm with a wind speed of 11.3 m s-1. The 1500 kg ha-1 mulch cover reduced sediment transport from 49.7 to 80.2% during five storms with wind speeds varying from 8.3 to 10.6 m s-1, and is therefore recommended as the better application rate for wind erosion control in the Sahel.
Field observations of wind-blown particle transport are often characterized by a considerable spatial variation, which makes quantitative modeling of wind erosion difficult. This study examines how the horizontal distribution, or pattern, of mask transport can be determined from a limited number of point measurements. Twenty-one sediment catchers were installed in an experimental plot in the Sahelian zone of Niger, on a sandy, siliceous, isohyperthermic Psammentic Paleustalf. Mass transport values during four storms ranged from 24.0 to 213.6 kg m-1, 7.2 to 26.0 kg m-1, 9.6 to 68.9 kg m-1, and 68.9 to 282.7 kg m-1. Geostatistical theory was applied to produce stoma based maps by modeling the spatial correlation structure with the variogram. To estimate the variogram from 21 observations, the four storms were treated as independent temporal replicates. Two geostatistical mapping techniques were applied. Kriging (a spatial interpolation technique) produced maps of mass transport providing the best possible estimates of net soil losses from the plot, equal to 12.5, 2.0, 4.6, and 26.8 Mg ha-1, respectively. To overcome smoothing, possible realizations of actual mass transport were created by stochastic simulations with simulated annealing. The simulated maps reproduced the statistical properties of the observations and allowed a distinction between erosion and deposition areas within the experimental plot.
The effect of turbulent flow structures on saltation sand transport was studied during two convective storms in Niger, West Africa. Continuous, synchronous measurements of saltation fluxes and turbulent velocity fluctuations were made with a sampling frequency of 1 Hz. The shear stress production was determined from the vertical and streamwise velocity fluctuations. The greatest stress-bearing events were classified as turbulent structures, with sweep, ejection, inward interaction, and outward interaction described according to the quadrant technique. The classified turbulent structures accounted for 63·5 per cent of the average shear stress during the first storm, and 56·0 per cent during the second storm. The percentage of active time was only 20·6 per cent and 15·8 per cent, respectively. High saltation fluxes were associated with sweeps and outward interactions. These two structures contribute positively (sweeps) and negatively (outward interactions) to the shear stress, but have in common that the streamwise velocity component is higher than average. Therefore, the horizontal drag force seems primarily responsible for saltation sand transport, and not the shear stress. This was also reflected by the low correlation coefficients (r) between shear stress and saltation flux (0·12 and 0·14, respectively), while the correlation coefficients between the streamwise velocity component and saltation flux were much higher (0·65 and 0·57, respectively). © 1998 John Wiley & Sons, Ltd.
For the study of field wind erosion, detailed observations of wind-blown sediment transport in the field are needed. The objective of this study was to determine the best method to quantify the mass of wind-blown material moving past a fixed point during four storms. Twenty-one Modified Wilson and Cooke (MWAC) sediment catchers were installed in a pearl millet [Pennisetum glaucum (L.) R. Br.] field in the Sahelian zone of Niger, on a sandy, siliceous, isohyperthermic Psammentic Paleustalf. Each catcher trapped materials at seven heights between 0.05 and 1.00 m. The vertical profiles of measured horizontal mass fluxes were described by two different models, a three-parameter power function and a five-parameter combined model, which is a combination of an exponential function and a power function. For all four storms, both models described accurately the mass fluxes between 0.05 and 0.26 m, but fitted mass fluxes at 0.50, 0.75, and 1.00 m deviated from measured fluxes. Deviations were 21.1, 45.2, and 60.6% for the power function and 12.4, 18.5, and 30.0% for the combined model. Mass transport rates were calculated by integrating the mass flux profiles across height. The differences in calculated mass transport rates were small, but because of the better fit, the combined model was preferred. Correcting for the trapping efficiency of the MWAC catchers (0.49) and multiplying by the storm duration resulted in total mass transport values, which are equal to the mass of soil passing a strip of 1-m width perpendicular to the mean wind direction. The average mass transport values were 102.7, 15.5, 31.8, and 149.8 kg m-1, respectively, for the four storms.
Large-scale simulation of the soil-derived
dust emission in semi-arid regions needs to account for the influence of the
soil moisture on the wind erosion threshold. Soil water retention consists of
molecular adsorption on the soil grain surface and capillary forces between the
grain. Interparticle capillary forces (characterized by the moisture tension)
are the main factor responsible for the increase of the wind erosion threshold
observed when the soil moisture increases. When the soil moisture content is
close to but smaller than the maximum amount of adsorbed water, w'
(depending on the soil texture), these capillary forces are considered as not
strong enough to significantly increase the erosion threshold. An expression of
the moisture tension as a function of soil moisture and w' is derived
from retention curves. From this expression, a parametrization of the ratio of
the wet to dry erosion thresholds has been developed as a function of soil
moisture and soil texture. The coefficients of this parametrization have been
determined by using experimental data from the literature. An empirical
relationship between w' and soil clay content has been established. The
erosion threshold ratios simulated for different soil textures were found to be
in good agreement with the experimental data.Key words. Atmospheric composition and structure
(Aerosols and particles) · Hydrology (soil moisture)
In the Sahel, sandy soils are widespread and support not only most of the pearl millet production, the major staple crop in the region, but also grass production for livestock. Parent sediments of these soils have an aeolian origin and are hence prone to wind erosion. Still, their clay content, even though very low, allows physical crust formation during rainfall leading to runoff and water erosion. Squall lines, major rainfall events of the rainy season, are usually preceded by intense wind. Wind and water erosions are therefore closely associated in both time and space, but they are rarely studied simultaneously. Erosion measurements were performed for two years (2001, 2002) on a small catchment of grazing land (1,4 ha) at Katchari, Burkina Faso, a location typical of the Sahel area with under 500 mm annual rainfall. Wind erosion occurs at the onset of the rainy season, from May to 15th of July, when soil cover is the lowest and before the growth of vegetation. On this non-cultivated area, the same dynamic unfolds as that recorded in millet fields in other sahelian studies. Water erosion occurs throughout the rainy season, but certain intense events produce most of the total annual erosion. Wind causes the largest sediment fluxes leading to both erosion (up to 20 Mg/ha per year) and deposition (up to 30 Mg/ha per year) depending on the area in the catchment. Water erosion is one order of magnitude lower than wind erosion and is more intense where wind erosion is highest. Hence, the same area is eroded by both wind and water. Conversely, areas where aeolian deposition occurs are less affected by water erosion and correspond to fertile islands where vegetation grows. From this study, it comes out that there is on the whole no land degradation at the catchment scale, but an intense aeolian and water dynamic leading to substantial spatial variability typical of sahelian landscapes.
During the 2006 and 2007 special observing periods of the African Monsoon Multidisciplinary Analysis campaign an original experimental system has been implemented in Banizoumbou (Niger) for measuring the size-resolved dust emission flux in natural conditions and documenting the possible influence of wind speed on its size distribution. The instrumental set-up, associated methodology, and the quality tests applied to the data set are described before the results acquired during 2 events of the Monsoon type and 1 of the convective type are analyzed in detail. In good agreement with the theory of sandblasting, it is found in all cases that saltation must take place for a vertical emission flux to be detected. During a particular erosion event, the magnitude of the vertical flux is controlled by the surface roughness, which conditions the saltation threshold, and by the wind friction velocity. The dust flux released by the high energy convective event is also found to be much richer in very fine (*. This is interpreted as a possible result of the rather long duration (15') over which wind fluctuations must be averaged for computing u*, which could make it an inadequate parameter for representing the very short response-time physical processes that are at the origin of fine dust emission at the measurement sites.
A soil-derived dust emission scheme has been designed to provide an explicit representation of the desert dust sources for the atmospheric transport models dealing with the simulation of the desert dust cycle. Two major factors characterizing the erodible surface are considered: (1) the size distribution of the erodible loose particles of the soil which controls the erosion threshold and the emission strength and (2) the surface roughness which imposes the efficient wind friction velocity acting on the erodible surface. These two parameters are included in a formulation of the threshold wind friction velocity by adapting a size-dependent parameterization proposed by Iversen and White (1982) and by applying to the rough erodible surfaces a drag partition scheme derived from Arya (1975). This parameterization of the threshold friction velocity has been included in an horizontal flux equation proposed by White (1979). This allows to attribute a specific production rate to each soil size range for each type of surface. The dust flux F is then considered as a fraction of the total horizontal flux G, the value of the ratio F/G being imposed, at this time, by the soil clay content. In summary, the computed mass fluxes depend on the soil size distribution, the roughness lengths, and the wind friction velocity. The different steps of this scheme have been independently validated by comparison with relevant experimental data. Globally, the agreement is satisfying, so that the dust fluxes could be retrieved with less uncertainties than those observed in previous simulations of the desert dust cycle.
The effect of turbulent flow structures on saltation sand transport was studied during two convective storms in Niger, West Africa. Continuous, synchronous measurements of saltation fluxes and turbulent velocity fluctuations were made with a sampling frequency of 1Hz. The shear stress production was determined from the vertical and streamwise velocity fluctuations. The greatest stress-bearing events were classified as turbulent structures, with sweep, ejection, inward interaction, and outward interaction described according to the quadrant technique. The classified turbulent structures accounted for 63·5 per cent of the average shear stress during the first storm, and 56·0 per cent during the second storm. The percentage of active time was only 20·6 per cent and 15·8 per cent, respectively. High saltation fluxes were associated with sweeps and outward interactions. These two structures contribute positively (sweeps) and negatively (outward interactions) to the shear stress, but have in common that the streamwise velocity component is higher than average. Therefore, the horizontal drag force seems primarily responsible for saltation sand transport, and not the shear stress. This was also reflected by the low correlation coefficients (r) between shear stress and saltation flux (0·12 and 0·14, respectively), while the correlation coefficients between the streamwise velocity component and saltation flux were much higher (0·65 and 0·57, respectively).
