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The hydrology of inland valleys in the sub-humid zone of West Africa: Rainfall-runoff processes in the M'b?? experimental watershed
Abstract
Inland valleys with wet lowlands are an important water source for farming communities in the sub-humid zone of West Africa. An inland valley and surrounding contributing watershed area located in the sub-humid zone near M'bé in central Côte d'Ivoire was instrumented to study surface runoff and base flow mechanisms. Four flumes at different distances down the main stream and more than 100 piezometers were installed. Measurements were taken during two rainfall seasons in 1998 and 1999. Under initial wet conditions, a typical single-peak hydrograph was observed. Under low antecedent moisture conditions, however, runoff was characterized by a double-peaked hydrograph. The first peak, which occurred during the storm, was caused by rain falling on the saturated valley bottom. The second peak was delayed by minutes to hours from the first peak and consisted of rain flowing via the subsurface of the hydromorphic zone that surrounds the valley bottom. The duration of the delay was a function of the water table depth in the hydromorphic zone before the storm. The volume of the second peak constituted the largest portion of the stream flow. Copyright © 2003 John Wiley & Sons, Ltd.
... In the tropical-humid Sudanian climate, top soils are usually permeable, due to microfauna bioturbation, or soil work from farmers (Giertz and Diekkrüger, 2003;Richard, 2012;Robert, 2012). As a consequence, the infiltration-excess overland flow is very limited in the area (e.g., Giertz et al. (2006); Masiyandima et al. (2003). The top-soil layer, H1, is therefore permeable, with sand-like soil hydrodynamic properties. ...
... It is beyond the scope of this study to analyse the effect of this layer (H2) on the hydrological processes, because it would likely necessitate an entire study. We therefore chose to consider a rather sandy, permeable layer, to mimic a hardpan unit behavior (as part of the polycyclic soil profile described by Faure and Volkoff (1998)), and favor quick lateral subsurface transfer, which have been evidenced in the area (e.g Hector et al. (2015); Masiyandima et al. (2003); Séguis et al. (2011)). ...
... This allows the formation of perched water tables during the rainy season (Masiyandima et al., 2003). Such shallow drainage system is the main contributor to baseflow, which represent roughly about 80% of total streamflow for such catchment (Giertz et al., 2006;Hector et al., 2015;Kamagaté et al., 2007;Séguis et al., 2011). ...
Inland valleys are seasonally waterlogged headwater wetlands, widespread across West-Africa. Their role in the hydrological cycle in the humid, hard rock-dominated Sudanian area is not yet well understood. Thus, while in the region recurrent floods are a major issue, and hydropower has been recognized as an important development pathway, the scientific community lacks a precise knowledge of streamflow (Q) generation processes and how they could be affected by the presence of inland valleys. Furthermore, inland valleys carry an important agronomic potential, and with the strong demographic rates of the region, they are highly subjected to undergo land cover changes. We address both the questions of the hydrological functioning of inland valleys in the Sudanian area of West-Africa and the impact of land cover changes on these systems through deterministic sensitivity experiments using a physically-based critical zone model (ParFlow-CLM) applied on a synthetic catchment which comprises an inland valley. Model forcings are based on 20 years data from the AMMA-CATCH observation service and parameters are evaluated against multiple field data (Q, evapotranspiration – ET –, soil moisture, water table levels, and water storage). The hydrological model applied to the conceptual lithological/pedological model proposed in this study reproduces the main behaviors observed on a highly instrumented elementary catchment. We found that yearly water budgets were highly sensitive to the vegetation distribution: average yearly ET for a tree-covered catchment (944 mm) exceeds that of herbaceous-cover (791 mm). ET differences between the two covers vary between 12 and 24 % of the precipitation of the year for the wettest and driest year, respectively. As a consequence, the tree-covered catchment produces a yearly Q budget 28 % lower on average as compared to a herbaceous-covered catchment, ranging from 20 % for the wettest year to 47 % for a dry year. Trees also buffer interannual variability in ET by 26 %. On the other hand, pedological features (presence – or absence – of the low permeability layer commonly found below inland valley, upstream and lateral contributive areas) had limited impact on yearly water budgets but marked consequences on intraseasonal hydrological processes (sustained/unsustained baseflow in the dry season, catchment water storage redistribution ...). Therefore, subsurface features of inland valleys have potentially significant impacts on downstream water-dependent ecosystems and water uses as hydropower generation, and should focus our attention.
... Most research in tropical dry forests has focused on quantifying the catchment scale water balance [Lugo et al., 1978;de Ara ujo and Gonz alez Piedra, 2009;Montenegro and Ragab, 2010], while other work has focused on identifying the primary forms of runoff generation [Sandstr€ om, 1996;Masiyandima et al., 2003;Mugabe et al., 2007]. In tropical dry forests, runoff is strongly controlled by the hydraulic properties of the surface and shallow subsurface soils. ...
... In tropical dry forests, runoff is strongly controlled by the hydraulic properties of the surface and shallow subsurface soils. It has been shown that runoff is dominated by infiltration-excess (Hortonian) overland flow (HOF), saturation excess overland flow (SOF) [Sandstr€ om, 1996;McCartney et al., 1998;Mugabe et al., 2007], and limited subsurface stormflow [Masiyandima et al., 2003]. These studies provide a foundation of knowledge concerning the mechanisms by which runoff is generated. ...
... The 191 mm of cumulative rainfall prior to streamflow activation recorded at our site falls within the 64-545 mm range observed at other tropical dry forest catchments [Ewel and Whitmore, 1973;Masiyandima et al., 2003;Mugabe et al., 2007]. Although the cumulative rainfall has been used to assess the timing of streamflow activation in tropical dry forests, it is not the most accurate method, as the interannual variation in the frequency, depth, intensity, and duration of storm events has been shown to directly affect the rate at which storage deficits are satisfied and consequently the amount of rainfall needed to activate streamflow [Mugabe et al., 2007]. ...
In forested catchments the exceedance of rainfall and antecedent water storage thresholds is often required for runoff generation, yet to our knowledge these threshold relationships remain undescribed in tropical dry forest catchments. We therefore identified the controls of streamflow activation and the timing and magnitude of runoff in a tropical dry forest catchment near the Pacific coast of central Mexico. During a 52 day transition phase from the dry to wet season, soil water movement was dominated by vertical flow which continued until a threshold soil moisture content of 26% was reached at 100 cm below the surface. This satisfied a 162 mm storage deficit and activated streamflow, likely through lateral subsurface flow pathways. High antecedent soil water conditions were maintained during the wet phase but had a weak influence on stormflow. We identified a threshold value of 289 mm of summed rainfall and antecedent soil water needed to generate >4 mm of stormflow per event. Above this threshold, stormflow response and magnitude was almost entirely governed by rainfall event characteristics and not antecedent soil moisture conditions. Our results show that over the course of the wet season in tropical dry forests the dominant controls on runoff generation changed from antecedent soil water and storage to the depth of rainfall.
... As the runoff generation mechanism in a catchment is mainly governed by the rainfall intensity and the soil characteristics of the basin [45,46], incorrect approximation of rainfall (amount and intensity) and average approximation of soil parameters could lead to erroneous simulations of the peak discharges during an extreme event. Although temporal variation of rainfall intensity could also affect the resultant simulated runoff, this is difficult to investigate because detailed rainfall data (e.g., hourly accumulated rainfall) are not available in the Gin River Basin. ...
... Although temporal variation of rainfall intensity could also affect the resultant simulated runoff, this is difficult to investigate because detailed rainfall data (e.g., hourly accumulated rainfall) are not available in the Gin River Basin. In addition, the cumulative effect of recording errors in the observed data, or errors in estimations of overall rainfall and evaporation, could cause discrepancies between simulated and observed peak discharges [9,45,46]. ...
As flooding is inevitable and becoming increasingly frequent, efficient flood management strategies should be developed to manage floods, especially in developing countries. Rainfall-Runoff-Inundation (RRI) model, which is based on a diffusive wave model, was applied to Gin River Basin, Sri Lanka using daily rainfall data. The RRI model was calibrated and validated for three past flood events (2003, 2016, and 2017) based on observed discharge data and inundation maps developed from ground survey data and satellite images. The Nash–Sutcliffe efficiency (NSE) values for river discharge obtained at the downstream gauging station were greater than 0.7 during both the calibration and validation experiments. Simulated inundation data showed good agreement with the limited observational records. The Critical Success Index (CSI) value for inundated extent in large flood event (May 2017) within downstream was greater than 0.3. Incorporation of embankment information significantly improved the accuracy of the simulation of inundation extent during large flood events (May 2017). The CSI value without embankment information for large flood event (May 2017) within downstream decreased to around 0.1. On the other hand, the embankment information was less useful for smaller flood events caused by less extreme rainfall. Inclusion of embankment information for large flood events enhanced the model performance, thus ensuring the availability of accurate inundation information for efficient flood risk planning and management in the basin.
... Quick-flow peaks are due to saturationexcess overland flows originating from the saturated area located at the bottom of the small valley near the Strengbach outlet (Ladouche et al., 2001). This type of hydrological functioning has been observed, for example for two forested catchments in Canada and Luxembourg (McDonnell and Taylor, 1987;Westhoff et al., 2011), several arctic catchments (McNamara et al., 1998), a sub-humid catchment in Africa (Masiyandima et al., 2003), and an alpine catchment (Zillgens et al., 2007). However, in some cases, a delayed response to precipitation can occur. ...
... For instance, the largest delayed-flow event of the studied period discharged 150000 m 3 of water between December 22, 2012, and January 19, 2013, while only 1700 m 3 of water was discharged during the largest quickflow event (May 29, 2016, between 1:50 p.m. and 6:10 p.m.). A major contribution of the delayed-flow peaks to runoff has already been observed for other catchments in other geological and/or hydrological contexts (Pearce and O'Louglin, 1984;McDonnell and Taylor, 1987;Masiyandima et al., 2003;Zillgens et al., 2007). ...
... In the past, many studies conducted in West Africa have focused on their agro-ecological characterization (Andriesse et al., 1994;Andriesse and Fresco, 1991), the assessment of their agropotential and the potential constraints on crop production (Djagba et al., 2013;Giertz et al., 2012;Ogban and Babalola, 2003;Totin et al., 2013), and the response of crop performance to agronomic management (Schmitter et al., 2015;;Touré et al., 2009). However, few studies address the hydrology of these wetlands by describing the major processes involved within different physiographic units, assessing rainfall-runoff processes in their surrounding drainage areas and analyzing the frequency of floods in the valley bottoms (Kyei-Baffour et al., 2013;Masiyandima et al., 2003;etc.). Moreover, the lack of studies dealing with water quality is noticeable in inland valley streams. ...
... In summary, surface and subsurface flows are the dominant hydrological processes in the inland valley of Kounga where the land use is predominantly agriculture. They represent an essential portion of the streamflow and may play an important part in agricultural water management, especially in lowland rice production (Masiyandima et al., 2003). However, as a result of agricultural intensification, runoff generation and flooding risks may increase in Kounga due to its currently high level of cultivation and shallow groundwater table in the lowland areas. ...
Achieving sustainable agricultural intensification in inland valleys while limiting the impacts on water quantity and water quality requires a better understanding of the valleys’ hydrological behavior with respect to their contributing watersheds. This study aims at assessing the dynamics of hydrological processes and nitrate loads within inland valleys that are experiencing different land uses. To achieve this goal, an HRU-based interface (ArcSWAT2012) and a grid-based setup (SWATgrid) of the Soil Water Assessment Tool (SWAT) model were applied to three headwater inland valley watersheds located in the commune of Djougou in central Benin that are characterized by different proportions of cultivated area. Satisfactory model performance was obtained from the calibration and validation of daily discharges with the values of R² and NSE mostly higher than 0.5, but not for nitrate loads. The annual water balance reveals that more than 60% of precipitation water is lost to evapotranspiration at all sites, amounting to 868 mm in Kounga, 741 mm in Tossahou, and 645 mm in Kpandouga. Percolation (302 mm) is important in the Kpandouga watershed which is dominated by natural vegetation at 99.7%, whereas surface runoff (105 mm) and lateral flow (92 mm) are the highest in the Kounga watershed having the highest proportion of agricultural land use (14%). In all the studied watersheds, nitrate loads are very low (not exceeding 4000 KgN per year) due to the low fertilizer application rates, and the water quality is not threatened if a standard threshold of 10 mg/l NO3-N is applied. The results achieved in this study show that SWAT can successfully be used in spatial planning for sustainable agricultural development with limited environmental impact on water resources in inland valley landscapes.
... Direct runoff is the sum of saturation excess flow (Dunn flow) and runoff caused by infiltration capacity excess (Horton flow). The importance of direct runoff in West Africa has been presented in detail by several authors (Windmeijer & Andriesse, 1993;van de Giesen et al., 2000;Masiyandima et al., 2003). It seems that at the level of the Volta Basin, most direct runoff is generated as Dunn flow from the saturated flood plainss that have relatively fixed areas. ...
... Coefficients of variation indicate high variability for baseflow, and low variability for direct runoff. These results confirm the low contribution from Hortonian overland flow and the relatively large contribution from wetland areas (Masiyandima et al., 2003). As the wetland areas vary very little in size, their contribution to runoff shows low variability, which is reproduced well by the modelled direct runoff. ...
Rainfall in West Africa is often typified as being subject to large inter-annual variability. For the Volta Basin, this variability is actually not very large. The coefficient of variation for rainfall is only 0.08 (1931-1995) with an average of 400 km3 year-1. Yet, a much higher coefficient of variation of 0.38 (1931-1995) is found for runoff with an average of 43 km3 year-1. The basin shows a nonlinear response that amplifies small changes in rainfall into large changes in runoff. A simple runoff model for the Volta Basin was formulated with a Nash-Sutcliffe efficiency of 72%. This model implies that once rainfall exceeds a threshold of 342 km3 year-1, more than 50% of the exceedence runs off. This threshold behaviour makes the water resources of the Volta Basin highly sensitive to both climatic variability and anthropogenic impacts. First, the impact of climate variability is quantified. Subsequently, development of the storage capacity of the Basin is evaluated on the basis of the water balance over the period 1966-1995 with an adapted Thornthwaite-Mather model. Results show that there is no change in hydrological behaviour of the basin that could be attributed to human impact. This lack of a clear anthropogenic signal can partially be explained by two contrary developments, increase in surface water storage in reservoirs and decrease in soil moisture storage due to soil and vegetation decline.
... In the Sudanian region, a substantial drop in streamflow was observed, about twice as large as the precipitation deficit, e.g., a 40% streamflow deficit versus a 15-20% precipitation deficit in the upper Ou em e catchment in Benin [Lebel and Vischel, 2005;Descroix et al., 2009]. In this area, infiltration rates are higher than in the Sahel [Valentin et al., 2004], and streamflow is governed by deep and shallow subsurface flow with a minor contribution from Hortonian flow and saturation overland flow [Masiyandima et al., 2003;Giertz et al., 2006 understood, which prevents the community from anticipating system responses to expected climate changes and increasing human pressure on land cover. ...
... While there is no consensus on the relative permeability between fallow and cultivated areas [Giertz et al., 2005;Richard, 2014], both found in the Nalohou catchment, the high average permeability of top soils largely prevents Hortonian runoff. This is a common feature in Sudanian West Africa [e.g., Masiyandima et al., 2003]. ...
In basement catchments of sub-humid West Africa, baseflow is the main component of annual streamflow. However, the important heterogeneity of lithology hinders the understanding of baseflow generation processes. Since these processes are linked with water storage changes (WSCs) across the catchment, we propose the use of hybrid gravity data in addition to neutron probe-derived water content and water levels to monitor spatiotemporal WSC of a typical crystalline basement headwater catchment (16 ha) in Benin. These behaviors are shown to provide insights into hydrological processes in terms of water redistribution toward the catchment outlet. Hybrid gravimetry produces gravity change observations from time-lapse microgravity surveys coupled with gravity changes monitored at a base station using a superconducting gravimeter and/or an absolute gravimeter. A dense microgravity campaign (70 surveys of 14 stations) covering three contrasted years was set up with a rigorous protocol, leading to low uncertainties (< 2.5 µGal) on station gravity determinations (with respect to the network reference station). Empirical orthogonal function analyses of both gravity changes and WSCs from neutron probe data show similar spatial patterns in the seasonal signal. Areas where storage and water table show a capping behavior (when data reach a plateau during the wet season), suggesting threshold-governed fast subsurface redistribution, are identified. This observed storage dynamics, together with geological structures investigated by electrical resistivity tomography and drill log analysis make it possible to derive a conceptual model for the catchment hydrology. This article is protected by copyright. All rights reserved.
... The riparian forest as simulated here contributes to 37 % of the annual hillslope transpiration, and reaches 57 % in the dry season, whereas it only covers 5 % of the hillslope area. 1993; Masiyandima et al., 2003;Giertz and Diekkrüger, 2003). More recently a series of hydrodynamic, geochemical and subsurface geophysical investigations (Kamagaté et al., 2007;Séguis et al., 2011b) have shown the contribution of Published by Copernicus Publications on behalf of the European Geosciences Union. ...
... Knowing that riparian forests are a key element of the landscape in northern Benin (Natta et al., 2002), this paper addresses the interplay between riparian forests and the underlying groundwater and its impacts on the hydrological processes, which control the local and mesoscale water cycle. Numerous studies have demonstrated that riparian forests use a part of the groundwater flow that, ultimately, feeds the streamflow, thus riparian forests decrease the water yield of the basin (e.g. ...
Forests are thought to play an important role in the regional dynamics of the West African monsoon, through their capacity to extract water from a permanent and deep groundwater table to the atmosphere even during the dry season. It should be the case for riparian forests too, as these streambank forests are key landscape elements in Sudanian West Africa. The interplay of riparian forest and groundwater in the local hydrodynamics was investigated, by quantifying their contribution to the water balance. Field observations from a comprehensively instrumented hillslope in northern Benin were used. Particular attention was paid to measurements of actual evapotranspiration, soil water and deep groundwater levels. A vertical 2-D hydrological modelling approach using the Hydrus software was used as a testing tool to understand the interactions between the riparian area and the groundwater. The model was calibrated and evaluated using a multi-criteria approach (reference simulation). A virtual experiment, including three other simulations, was designed (no forest, no groundwater, neither forest nor groundwater).
The model correctly simulated the hydrodynamics of the hillslope regarding vadose zone dynamics, deep groundwater fluctuation and actual evapotranspiration dynamics. The virtual experiment showed that the riparian forest transpiration depleted the deep groundwater table level and disconnected it from the river, which is consistent with the observations. The riparian forest and the deep groundwater table actually form an interacting transpiration system: the high transpiration rate in the riparian area was shown to be due to the existence of the water table, supplied by downslope lateral water flows within the hillslope soil layer. The simulated riparian transpiration rate was practically steady all year long, around 7.6 mm d−1. This rate lies within high-end values of similar study results. The riparian forest as simulated here contributes to 37% of the annual hillslope transpiration, and reaches 57% in the dry season, whereas it only covers 5% of the hillslope area.
... In the Sudanian region, a substantial drop in streamflow was observed, about twice as large as the precipitation deficit, e.g., a 40% streamflow deficit versus a 15-20% precipitation deficit in the upper Ou em e catchment in Benin [Lebel and Vischel, 2005;Descroix et al., 2009]. In this area, infiltration rates are higher than in the Sahel [Valentin et al., 2004], and streamflow is governed by deep and shallow subsurface flow with a minor contribution from Hortonian flow and saturation overland flow [Masiyandima et al., 2003;Giertz et al., 2006 understood, which prevents the community from anticipating system responses to expected climate changes and increasing human pressure on land cover. ...
... While there is no consensus on the relative permeability between fallow and cultivated areas [Giertz et al., 2005;Richard, 2014], both found in the Nalohou catchment, the high average permeability of top soils largely prevents Hortonian runoff. This is a common feature in Sudanian West Africa [e.g., Masiyandima et al., 2003]. ...
Advances in groundwater storage monitoring are crucial for water
resource management and hydrological processes understanding. The
evaluation of water storage changes (WSC) often involve point
measurements (observation wells, moisture probes, etc.), which may be
inappropriate in heterogeneous media. Over the past few years, there has
been an increasing interest in the use of gravimetry for hydrological
studies. In the framework of the GHYRAF (Gravity and Hydrology in
Africa) project, 3 yr of repeated absolute gravity measurements using a
FG5-type gravimeter have been undertaken at Nalohou, a Sudanian site in
northern Benin. Hydrological data are collected within the long-term
observing system AMMA-Catch. Once corrected for solid earth tides, ocean
loading, air pressure effects, polar motion contribution and non-local
hydrology, seasonal gravity variations reach up to 11 μGal,
equivalent to a WSC of 260-mm thick infinite layer of water. Absolute
temporal gravity data are compared to WSC deduced from neutron probe and
water-table variations through a direct modelling approach. First, we
use neutronic measurements available for the whole vertical profile
where WSC occur (the vadose zone and a shallow unconfined aquifer). The
RMSD between observed and modelled gravity variations is 1.61 μGal,
which falls within the error bars of the absolute gravity data. Second,
to acknowledge for the spatial variability of aquifer properties, we use
a 2-D model for specific yield (Sy) derived from resistivity mapping and
Magnetic Resonance Soundings (MRS). The latter provides a water content
(θMRS) known to be higher than the specific yield.
Hence, we scaled the 2-D model of θMRS with a single
factor (α). WSC are calculated from water-table monitoring in the
aquifer layer and neutronic measurements in the vadose layer. The value
of α is obtained with a Monte-Carlo sampling approach, minimizing
the RMSD between modelled and observed gravity variations. This leads to
α = Sy/θMRS = 0.63 ± 0.15, close to what
is found in the literature on the basis of pumping tests experiments,
with a RMSD value of 0.94 μGal. This hydrogeophysical experiment is a
first step towards the use of time-lapse gravity data as an integrative
tool to monitor interannual WSC even in complicated subsurface
distribution.
... In the Sudanian region, a substantial drop in streamflow was observed, about twice as large as the precipitation deficit, e.g., a 40% streamflow deficit versus a 15-20% precipitation deficit in the upper Ou em e catchment in Benin [Lebel and Vischel, 2005;Descroix et al., 2009]. In this area, infiltration rates are higher than in the Sahel [Valentin et al., 2004], and streamflow is governed by deep and shallow subsurface flow with a minor contribution from Hortonian flow and saturation overland flow [Masiyandima et al., 2003;Giertz et al., 2006 understood, which prevents the community from anticipating system responses to expected climate changes and increasing human pressure on land cover. ...
... While there is no consensus on the relative permeability between fallow and cultivated areas [Giertz et al., 2005;Richard, 2014], both found in the Nalohou catchment, the high average permeability of top soils largely prevents Hortonian runoff. This is a common feature in Sudanian West Africa [e.g., Masiyandima et al., 2003]. ...
Advances in water storage monitoring are crucial to characterize the
spatial variability of hydrological processes. Classical water storage
investigation methods often involve point measurements (piezometers,
neutron probes, humidity sensors...), which may be irrelevant in
heterogeneous mediums. Over the past few years, there has been an
increasing interest in the use of gravimeters for hydrological studies.
Water mass redistribution leads to variations in the Earth's gravity
field which can be measured by gravimetry. In the framework of the
GHYRAF (Gravimetry and Hydrology in Africa) project, 3 years of repeated
absolute gravity measurements have been undertaken at Nalohou, a
Sudanian site in northern Benin. Hydrological monitoring is carried out
within the long-term observing system AMMA-Catch (an observatory of RBV,
the French critical zone exploration network). Seasonal gravity
variations in link with the hydrological cycle can reach 11 μgal at
this site, equivalent to a 26cm thick infinite layer of water. The
vadose zone and a shallow unconfined aquifer in weathered metamorphic
rocks are responsible for most of the water storage. For the first time
in the climatic context of the West African monsoon, gravity data are
compared to the time evolution of the water storages deduced from
neutron probes and water-table variations. The approach is two-fold:
first, total storage variations are estimated from neutron probe-derived
moisture through the whole vertical profile monitored at the gravimetric
site and uniformly extended according to the topography. Results show a
very good fit with gravity data, enlightening the fact that absolute
gravimeters are sensitive to total water storage variations from the
soil surface to the aquifer. The second approach introduces a spatial
variability: it was undertaken to check a structural model for specific
yield of the aquifer, based on magnetic resonance soundings (MRS) and
spatialized with resistivity data (TDEM). We distinguish the soil
moisture variations estimated by neutron probes in the vadose zone and
the groundwater storage variations calculated from water-table
observations and specific yield estimations. This experiment allows
discussing each layer (i.e. soil and water table fluctuation zone)
relative contribution to gravity signal in term of amplitude and
relevant lateral extension. Again, a conclusive fit shows the interest
of gravimetric measurements to strengthen a structural model. This
hydro-geophysical experiment shows that time-lapse gravity data can
effectively be used as an integrative tool to monitor the long-term
water storage variations in tropical West Africa. In a close future,
gravimetric measurements could become a calibration variable in
hydrological modeling as streamflow discharge do today.
... The deep West African soils can store a large amount of water that can be utilized by grasses, crops and trees. Only when the deep soil profile is close to saturation does water percolate below the root zone to feed groundwater and thereby river flow 12 . See Masiyandima et al. 12 for a further description of the most relevant hydrological processes. ...
... Only when the deep soil profile is close to saturation does water percolate below the root zone to feed groundwater and thereby river flow 12 . See Masiyandima et al. 12 for a further description of the most relevant hydrological processes. If the rainfall is spread out evenly over the season, little or no recharge may occur. ...
Impacts of climate change vary from region to region. The 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) mentions that drier areas will be affected by more droughts while the rainfall regime, in general, will become 'rougher'. In West Africa, specifically the area below the Sahel, the climate change signal may be more subtle. Anecdotal evidence from farmers suggests that the onset of rainy season has been shifting forward in time over the past two generations. Recently, detailed atmospheric mod-elling over the region shows that in the near future too, the onset of rainy season will shift to later periods in the year, roughly from April towards May. The end of rainy season as well as the total amount of rainfall will remain more or less fixed. This implies that adapta-tion strategies should be twofold. The first part of a comprehensive adaptation strategy would be a con-tinuation of the efforts to produce faster growing rain-fed crop cultivars, mainly corn and sorghum. The second part would consist of increased water storage during the wet season for use during dry season.
... HOF can account for up 75% of the event water (Sandström, 1996) and is typically generated over bare rock, crusted and low conductivity soils (Bonell and Williams, 1986;McCartney et al., 1998;Mugabe et al., 2007). Although less common, SOF also occurs in tropical dry forests ( Figure 4) and is generated under wet antecedent conditions where the soil profile becomes saturated either above shallow impeding layers along the hillslope (Mugabe et al., 2007;Kabeya et al., 2008) or in deeper near-stream soil profiles (McCartney et al., 1998;Masiyandima et al., 2003). Masiyandima et al. (2003) solely identify SSF as a primary runoff mechanism, contributing more to total runoff than overland flow. ...
... Although less common, SOF also occurs in tropical dry forests ( Figure 4) and is generated under wet antecedent conditions where the soil profile becomes saturated either above shallow impeding layers along the hillslope (Mugabe et al., 2007;Kabeya et al., 2008) or in deeper near-stream soil profiles (McCartney et al., 1998;Masiyandima et al., 2003). Masiyandima et al. (2003) solely identify SSF as a primary runoff mechanism, contributing more to total runoff than overland flow. These authors suggest that SSF was highly affected by antecedent soil moisture conditions, where delivery to the stream was faster and of a greater magnitude under wet than dry conditions. ...
Abstract
The hydrology of tropical dry forests have been poorly characterised when compared to their humid temperate and wet tropical counterparts. Despite accounting for more than 42% of all tropical forests and roughly 19% of the Earth's total forest, tropical dry forest represent less than 1% of the forest hydrology literature. The need for substantial hydrological research in tropical dry forests is extremely important, given that many tropical dry forest regions are currently water stressed due to high population densities and rapid land use change. Furthermore, future climate change scenarios are expected to have significant implications for the hydrological functioning of these catchments and will likely enhance pressures on already limited water resources. This paper provides an overview on the state of hydrological knowledge, particularly runoff generation, of tropical dry forests. We further highlight the research gaps and identify research priorities for tropical dry forests, and issue a call for increased hydrological research efforts in these forests.
... The list of parameters to be included in the model is not exhaustive. This method is made of statistics with several variants depending on the belonging of the basin to a climatic division, a geographical position, a division for a country or a group [26]; [27]. ...
The purpose of this study is to determine the hydrological characteristics and the morphometric parameters of the Sokouraba watershed (BV) where the climatic hazards make the water resource increasingly insufficient. This watershed has never been the subject of morphometric and hydrological characterization in the past. The present study made it possible thus, to better understand the functioning of the hydrographic networks as well as the quantity of water likely to be used for the construction of a dam in this watershed. According to the hydrologic analysis results, from 1960 to 2019, the annual precipitation values range from 347,3 to 1596,6 mm with an average of 1099,28 mm. The maximum daily rainfall values for the same period vary between 42 and 287 mm with an average of 82.85 mm. The values of wet decennial and wet quinquennial rainfall are respectively 1380 and 1280 mm. The dry five-year and dry ten-year rainfall values are 917 mm and 821 mm respectively. According to the watershed characterization results, the calculated Sokouraba watershed surface is 77,34 Km² and its perimeter is 44,3 Km. The values of the length and the width of the equivalent rectangle of the basin are respectively 17,81 Km and 4,34 Km. The longitudinal slope and the overall slope index values are 3.24 m/km and 3.20 m/Km respectively. The value of the corrected overall slope index of the watershed is 7.05 m/Km while the Gravelius compactness index is 1.42. This value of KG higher than 1 indicates that the watershed of sokouraba is of elongated shape. The specific gradient value of the watershed is 61.99. This value is included between 50 and 100 m, thus according to the classification of the reliefs the latter, the relief of the catchment area of Sokouraba can be qualified as moderate. The values of the length of the main watercourse and the total length of the watercourses calculated are respectively 15,41Km and 63,25 Km. The calculated value of the drainage density of the catchment area is 0.21 Km-1. The value of the attenuation coefficient and the ten-year rainfall calculated in this study are 0,78 and 100,062 mm respectively. The value of the decennial runoff coefficient thus calculated is 27,83%.
... After some preliminary tests, the K s range has been narrowed around low K s values where the model has shown to be highly sensitive. ▪ Experiment 5: The lateral redistribution of water in or above the near subsurface layer has been shown to be highly dependent on the extent of clayey areas or inland valleys (Giertz et al., 2006;Hector et al., 2015Hector et al., , 2018Masiyandima et al., 2003). According to the abovementioned literature review, we study the model sensitivity to the extent of a low-permeability layer in the subsurface, hypothesizing that its extension is a function of local topography. ...
Abstract Hard rock aquifers (HRAs) in West Africa (WA) are located within a thick regolith layer. The representation of thick tropical regolith in integrated hydrological models lacks consensus on aquifer geometries and parameter ranges. Our main objective was to determine the knowledge requirements on saturated hydraulic conductivity (Ks) to model the critical zone (CZ) of HRAs in WA. A parametric sensitivity analysis with a focus on the representation of the Ks heterogeneity of the regolith was conducted with a critical zone model (Parflow‐CLM [Community Land Model]) of the Upper Ouémé catchment in Benin (14,000 km2) at a 1‐ × 1‐km2 resolution. The impact of parameter changes in the near subsurface (0.3‐to‐5‐m depth) and in the deeper regolith aquifer (24‐ and 48‐m maximum depth) was assessed in five modeling experiments. Streamflow was largely dependent on Ks and on clay distribution in the near subsurface and less on the properties of the deeper subsurface. Groundwater table depths and amplitudes were controlled by vegetation and topography as observed on instrumented hillslopes and for Ks within the literature range. Experiments with higher Ks suggested a Ks threshold where dynamics become less determined by one‐dimensional vertical and more determined by lateral processes. Such heterogeneity impacts from smaller scales need to be accounted for when hydrological models are upscaled to larger domains (1‐ × 1‐km2 resolution or coarser). Our findings highlight the need for a new conceptual approach to represent clay distribution in order to develop catchment‐scale CZ models of HRAs in WA that capture the observed processes.
... However, for more than a decade, farmers inexorably and on annual basis try with little success to grow rice on the BIV. Alongside the abundant literature in West Africa on inland valleys, that have differently characterized inland valleys including causes of success and failure in their use (Andriesse and Fresco 1991;Hector et al. 2018;Windmeijer and Andriesse 1993;Andriesse et al. 1994;Masiyandima et al. 2003;Danvi et al. 2016); the current study proposes, as a complementary approach, a hydrological diagnostic of the BIV infrastructures. Therefore, the overall objective of the study is to contribute to a sustainable and effective agricultural development of inland valleys in the West African region. ...
Several developed inland valleys for rice production were abandoned due to poor design or implementation. The Bankandi inland valley (BIV) is a contour bunds system developed in 2006 by a development project, currently experiencing a systematic waterlogging. This study assessed: (1) the waterlogging vs. changing hydro-climatic conditions relationship; (2) the hydrological design and implementation of water control infrastructures; and (3) how digital elevation models (DEMs) data could be used for inland valleys development. To investigate the waterlogging vs. changing hydro-climatic conditions (precipitation and discharge), the conceptual HBV model was applied; coupled with break and trend detections tests. To evaluate the accuracy of the location of drainage flume and contour bunds, a topographic survey using a D-GPS was performed. To explore free DEMs as support tool in the development of inland valley, the Shuttle Radar Topography Mission 1 and the Advanced Spaceborne Thermal Emission and Reflection Radiometer 1, were used. The results show that: (1) the waterlogging was not related to changing environmental conditions; (2) major flaws including bunds not implemented on contour lines contribute to the waterlogging; (3) free DEMs were not accurate enough for valley development. The overall diagnostic of BIV entails conducting basic hydrological investigations prior to implementation.
... In essence, farming climate change adaptation refers to the more efficient use of nutrients and the reduction of greenhouse gas emmissions (Hillel and Rosenzweig, 2011). Based on this, measures in the West African Sudan savannah could include minimum tillage, intercropping, relay cropping, increase in the use of synthetic fertilizers, mulching, rotation with legumes, supplemental irrigation, improvement of water storage, utilization of surface and shallow groundwater, etc. (Bationo and Vlek 1998;Schlesinger 1999;Steenhuis et al. 2003;FAO, 2008;Naab et al., 2012). The application of these technical measures has produced uneven results. ...
Farming is the main economic activity in the West African savannah.
It is, however, characterized by high variability. The situation is fueled by
poor soil fertility, uncertain water availability, limited infrastructure and
persistent institutional weakness. The effects of climate change, which are particularly acute in the region, have exacerbated the situation. As an initiative aiming to enhance the scientific knowledge in the region, the West African Science Service on Climate Change and Adapted Land Use (WASCAL) was established.
This chapter summarizes the examples of the WASCAL research on
farmer climate change adaptation carried out during the last six years. It
offers combined disciplinary and interdisciplinary insights on how
farmers respond to climate change and its impacts, revealing the
feasibility of the assessed measures and the pre-conditions that these should fulfill to perform successfully. Two catchments/political units across the Sudan savannah belt were studied, i.e., the Dassari
arrondissement in north-western Benin and the Dano department in
south-western Burkina Faso. The ecological similarities and institutional
and political differences were contrasted and discussed.
After crosschecking the farmer perceptions on climate change with
their preferred farming adaptation measures, some of them were analyzed in depth. These indicate no causality between climate records, experienced hazards and household harm. Declines in yields and also pest outbreaks, although accurately estimated, do not relate to climate change only. Similarly, farming coping measures are widespread and not exclusively climate related, and appear to prioritize food security, inflow of cash and household overall capitalization in this order. The assessed coping measures strengthen rural households’ resilience, albeit differently. In contrast, largely approved measures such as switching to resistant species and varieties or exploitation of the effects of residual fertilization did not withstand field validation.
In this context, key for feasible farming climate change coping
measures is the provision of short-term economic benefits matching the
local socio-ecological conditions and harmonizing with ongoing
traditional practices. Furthermore, key actors for operationalization are the agricultural extension institutions.
... However, for more than a decade, farmers inexorably and on annual basis try with little success to grow rice on the BIV. Alongside the abundant literature in West Africa on inland valleys, that have differently characterized inland valleys including causes of success and failure in their use [1,[12][13][14][15]; the current study proposes, as a complementary approach, a hydrological diagnostic of the BIV infrastructures. Therefore, the overall objective of the study is to contribute to a sustainable and effective agricultural development of inland valleys in the West African region. ...
... To cope with water scarcity, the use of supplemental irrigation, improvement of storage capacities, utilization of surface and shallow groundwater and strengthening of community-based management are recommended [28][29][30]. ...
Climate change is a great threat to the already climate-unstableWest Africa. Current and potential impacts are especially hard on farming in the Sudan savannah, thus adaptation is widely advised and encouraged, and already occurring. In the study sites Dassari, Benin, and Dano, Burkina Faso, farmers' climate change perceptions and practiced coping measures were qualitatively and quantitatively recorded. Analyses included statistical testing to detach anecdotal responses from factual decisions. Results reveal that responses regarding climate change perception and adaptation are frequently subjective, conjectural and inconsistent. Farmers' acknowledge that adaptations to climate change impacts are diverse, but site specific. Measures do not causally respond to the type of hazard, nor to its impacts, but instead tend to address wide-ranging demands, such as household food security, income generation and capitalization. Hence, causally linking hazards, impacts and responses can be misleading, and measures can thus be ineffective. After our findings, key qualities of effective coping measures are short-term economic returns, compatibility with local ecological, social and institutional settings and agreeing with the customary farming traditions. With respect to operability, the national agricultural extension services are still the most relevant instances. Considering these aspects can support local farming adaptation and also increase the general resilience of the households.
... In this study, we did not consider the depth of the groundwater table in individual farmer's fields as supplement to rainfall in the boundary analysis of rainfed systems. It has been reported that the depth is highly variable across and within the fields (Masiyandima et al., 2003;Worou et al., 2013), which can affect yield within small distances and across years (Wade et al., 1999). Consequently, observed high yields, particularly those in rainfed lowlands of drier sites, may have been related to shallow groundwater tables (Boling et al., 2007) with reduced water percolation rates (Fukai et al., 1995). ...
Rice (Oryza spp.) is the major staple food for most countries in West Africa, but local production does not meet demand. Rice is grown mainly by smallholder farmers, and yields are generally low with high temporal and spatial variability. Low yields have been attributed to unfavorable climate conditions, poor soil quality, and sub-optimum agricultural practices. The objectives of this study were to assess variation in yields of three major rice production systems (irrigated lowland, rainfed lowland, and upland) across three climatic zones (semi-arid, sub-humid, and humid), and identify factors affecting that variation. We analyzed data on yield, climate, soil, and agricultural practices for 1305 farmers’ fields at 22 sites in 11 West African countries between 2012 and 2014. A boundary function approach was used to determine attainable yields. Random forest algorithm was used to identify factors responsible for yield variation. Average rice yield was 4.1, 2.0, and 1.5 t ha⁻¹ in irrigated lowland, rainfed lowland, and rainfed upland systems, respectively, with maximum attainable yields of 8.3, 6.5, and 4.0 t ha⁻¹. Yield difference between attainable and average yield tended to be higher in irrigated and rainfed lowland systems. In those two systems, yields were highest in the semi-arid zone, while no difference in yields among climatic zones was apparent for upland rice. High rice yields were associated with high solar radiation, high maximum temperature, intermediate air humidity, multiple split nitrogen (N) fertilizer applications, high frequency of weeding operations, the use of certified seeds, and well-leveled fields in the irrigated lowland system. Minimum temperature, solar radiation, rainfall, construction of field bunds, varietal choice, and the frequency of weeding operations were determinants of rice yield variation in the rainfed lowland system. Varietal choice, bird control, and frequency of weeding operations affected rice yields in the upland system. Improving access to inputs, improving input use efficiencies, and site-specific management strategies are recommended as priority interventions to boost rice yields at regional scale independent of production system and climatic zone.
... Padilla et al., 2015). In certain contexts, runoff might be characterised by double peak hydrographs only under low antecedent moisture conditions, when the groundwater levels are low and the subsurface flow peak occurs after the overland flow peak (Masiyandima et al., 2003). In the end, all these explanations are linked to subsurface storage and how it maps temporally and spatially throughout the catchment. ...
Double peak hydrographs are widespread phenomena but poorly understood mechanistically. In many cases, saturation-excess overland flow in the near-stream areas is assumed to control the initial peak, while the delayed peak is explained by subsurface flow in the soil or sediment cover or groundwater flow on fractured bedrock. Here we explore the mechanisms that control the generation of double peak hydrographs in a forested headwater catchment. We made use of the extensive high-resolution hydrometric time series collected in the catchment to estimate catchment storage and causal linkages. We found that double peak hydrographs occurred only after a certain amount of catchment storage was exceeded. The amount of this storage threshold was consistent over a 3-year period. The non-linear relationship between storage and discharge led to hysteretic relationships between both variables, and these hysteretic relationships were different for the different hydrograph types (single or double peak hydrographs). Discharge peaked before catchment storage during single peak hydrographs suggesting that single peaks were mainly generated by water quickly reaching the stream during precipitation pulses. It was catchment storage that peaked first during double peak hydrographs and consequently generated the delayed peak in the hydrograph. Our results also showed that double peak hydrographs were controlled in different proportions by contrasting landscape units (defined along a hillslope sequence). Hillslopes were connected to the stream at low discharge values, whereas the plateau dominated discharge generation when storage reached a certain threshold value.
... Hence, soil fertility can be improved through the addition of key nutrients and through soil conservation practices (Bationo and Vlek 1998;Schlesinger 1999;FAO 2008;Naab et al. 2012). The impact of water scarcity can be reduced through supplemental irrigation, better storage capacities, utilization of surface water and shallow groundwater, and strengthening of community-based management (Steenhuis et al. 2003;Sandwidi 2007;Mdemu et al. 2010). ...
The effects of climate change are widely threatening West African farming. The case of the farmers in the West African Sudan savannah is particularly severe due to the specific ecological vulnerability and the political and socioeconomic instability in the region.
Since 2012, the West African Science Service on Climate Change and Adapted Land Use (WASCAL) has been leading a regional effort to enhance the understanding, data availability and building capabilities to strengthen the ability of agricultural systems in coping with the effects of climate change. One of the activities focuses on determining the drivers of farmers’ adaptation.
The case studies in this research in Benin and Burkina Faso identified major climate hazards, farmers’ perceptions and adaptation measures. Research methods included ethnography, quantitative and mixed analyses.
Results show mixed mainstream and specific perceptions of climate change and its effects. The mainly perceived impacts relate to yield decline, pest increase and water scarcity. Adaptation is widespread, but practices are diverse and often neither correlated with the type of hazards nor with impacts.
This indicates that it is necessary to acknowledge the role of adaptation to problems not only caused by climate change, and to consider adaptation as an enhancing factor of social-ecological resilience. Furthermore, the focus on technological changes in the local context should be directed towards enhancing adaptive and transforming institutions. Ongoing studies on agroforestry, subsidies provision and livestock demonstrate such strategies.
... Hence, soil fertility can be improved through the addition of key nutrients and through soil conservation practices (Bationo and Vlek 1998;Schlesinger 1999;FAO 2008;Naab et al. 2012). The impact of water scarcity can be reduced through supplemental irrigation, better storage capacities, utilization of surface water and shallow groundwater, and strengthening of community-based management (Steenhuis et al. 2003;Sandwidi 2007;Mdemu et al. 2010). ...
For implementation of climate change adaptation in communities on the islands of Java and Lombok, Indonesia, a new extension approach was designed and carefully further developed while establishing it. It is called “Science Field Shops (SFSs)”. In these “Shops”, farmers, scientists/scholars and (where possible, so ideally) local extension officers meet to discuss and solve vulnerabilities and actual local problems expressed by farmers. In such context, agricultural extension is defined best as: “bringing new knowledge to farmers” and this is coordinated at these SFSs.
We use SFSs to temporarily bridge the gap in availability and training of extension intermediaries by using farmer facilitators (FFs). Farmers are confused by consequences of climate change and want answers on questions that are related to local climate problems but also many other issues in growing their crops, among which rice is most important. Giving and discussing answers and predictions demand real dialogues in an agrometeorological learning approach to response farming.
Farmers start to believe in their attempts to understand and reduce yield differences with the past and between them, by actively learning about consequences of climate change and how we can jointly fight them in such and otherwise changing environments. Anthropology and climatology are combined.
... Hence, soil fertility can be improved through the addition of key nutrients and through soil conservation practices (Bationo and Vlek 1998;Schlesinger 1999;FAO 2008;Naab et al. 2012). The impact of water scarcity can be reduced through supplemental irrigation, better storage capacities, utilization of surface water and shallow groundwater, and strengthening of community-based management (Steenhuis et al. 2003;Sandwidi 2007;Mdemu et al. 2010). ...
Achieving food security in its totality continues to be a challenge not only for the developing nations, but also for the developed world. The difference lies in the magnitude of the problem in terms of its severity and proportion of the population affected. In developed nations the problem is alleviated by providing targeted food security interventions, including food aid in the form of direct food relief, food stamps, or indirectly through subsidized food production. Similar approaches are employed in developing countries but with less success. The discrepancy in the results may be due to insufficient resource base, shorter duration of intervention, or different systems most of which are inherently heterogeneous among other factors.
... The first peak of double-peak hydrographs is often attributed to saturation-excess or infiltration-excess overland flow (Burt and Butcher, 1985), or the immediate reaction of the riparian zone (e.g. Masiyandima et al., 2003), and is usually not the main focus of investigations. The reasons described for the second peak are more diverse. ...
Preferential flow is omnipresent in natural systems. It links multiple scales from single pores to entire hillslopes and potentially influences the discharge dynamics of a catchment. However, there is still a lack of appropriate monitoring techniques and thus, process understanding. In this study, a promising combination of 2D time-lapse ground-penetrating radar (GPR) and soil moisture monitoring was used to observe preferential flow processes in highly structured soils during a hillslope-scale irrigation experiment. The 2D time-lapse GPR data were interpreted using structural similarity attributes, highlighting changes between individual time-lapse measurements. These changes are related to soil moisture variations in the subsurface. In combination with direct measurements of soil moisture, the spatial and temporal characteristics of the resulting patterns can give evidence about subsurface flow processes. The response dynamics at the hillslope were compared to the runoff response behavior of the headwater catchment. The experiment revealed a fast establishment of hillslope-scale connectivity despite unsaturated conditions, with high response velocities of up to 10−3 m s−1 or faster, and a high portion of mobile water. These processes substantially impact the overall catchment response behavior. While the presented approach is a good way to observe the temporal dynamics and general patterns, the spatial characteristics of small-scale preferential flow path could not be fully resolved.
... Much of the Odaw catchment area is built up and many of the streams are channelized. Rainfall in Accra occurs in the form of intensive storm events, which cause local flooding (BGR-GSD, 2006, Masiyandima et al., 2003, Hayward and Oguntoyinbo, 1987). Several factors contribute to the flooding problem. ...
Abstract:
The impacts of global climate change on communities and economies the world over has been a topical issue in recent years. One of the notable and most devastating impacts of climate change over the years has been flooding. Every year, reports from all over the world give an indication of the devastation caused by floods through loss of lives and property in both rural and urban areas. The problem in the urban centres is usually exacerbated by unsustainable development practices evident through the sprawling of slums, deforestation and building in waterways which are all mainly due to poor land use planning. The study examined the combined impact of climate change and unsustainable development on the frequency and severity of flood events in Ghana in the face of rapid population growth. The study reviewed related studies in other flood-prone areas of the world looking at attempts that have been made and how successful and otherwise these attempts have been and recommendations made accordingly.
Keywords: Unsustainable development, climate change, floods
... Hayward and Oguntoyinbo (1987) alluded to this variability in rainfall and observed that much rainfall in West Africa is derived from storms or showers lasting three to six hours, when possibly 100 drops in excess of 2 mm diameter may fall per cm 2 , assuming the intensity of 25 mm hr -1 , which will produce runoff and soil loss in open farmland. Similarly, Manley and Askew (1993) and Masiyandima et al. (2003) corroborated the intense nature of the rainstorms, with the former realizing that the rainfall intensities can be high in the tropics inducing erosion and flooding whilst the latter reported that rainfall events can last less than 30 minutes in the humid and dry zone of West Africa. This variability in rainfall intensities according to Jackson (1978) can exist between nearby locations and this he illustrated in a tropical catchment in Tanzania (East Africa) where for time periods of a month or longer, rainfall totals differed by between 16%-33% at nearby locations (from < 1 km-20 km apart), even when the rainfall stations are in an area of uniform relief and their long term averages are similar. ...
... In the sub-humid zones of West Africa, infiltration rates are generally higher than in the Sahel due to the higher organic matter content of the top soils (Valentin et al., 2004). At the hillslope scale, previous studies showed that most infiltration excess water infiltrates before reaching an expanse of open water (van de Giesen et al., 2000;Masiyandima et al., 2003;Giertz et al., 2006). Consequently, Hortonian runoff should contribute little to total streamflow. ...
We review the main results of land-surface studies obtained in the three sites of the long-term observing system AMMA-CATCH. Runoff in the Sahel enhances the variability of energy partitioning between non-infiltrative areas where sensible heat is dominant and infiltrative areas where soil water availability increases the latent flux. In terms of water resources, an increase in runoff over the past 50 years, already reported for the exoreic Sahel, was revealed in the endoreic Sahel. In the Sudanian domain, the subsurface origin of streamflow could explain its decrease over the same period. Copyright © 2011 Royal Meteorological Society
... This suggests that interflow is the dominant process generating streamflow, which is supported by the fact that all hydrographs show a 15-30 minute delay between rainfall and runoff peaks and continuation of the flow signal after the rain has stopped both before and after fire – clear signs of interflow and baseflow. Another reason why overland flow is likely negligible is that all hydrographs lack the distinct double peak associated with the occurrence of interflow both and overland flow [Masiyandima et al., 2003]. This is despite the widespread occurrence of soil water repellency throughout the year [Stoof et al., 2011a]. ...
... Some options to effectively meeting these challenges are (i) making better use of inland valleys, (ii) utilizing shallow groundwater resources, (iii) introducing supplemental irrigation, (iv) raising storage capacity with the help of small reservoirs, and (v) combining these strategies. Due to accumulation of surface runoff and interflow from surrounding areas, inland valleys provide an environment less susceptible to rainfall variability (Steenhuis et al., 2003). Use of shallow groundwater and supplemental irrigation (Mdemu et al., 2010;Laube et al., 2012) are key-interventions to overcome or at least to reduce the impact of drought periods on agricultural yield. ...
Farming is the main livelihood activity in semi-arid rural West Africa, involving the largest portion of
the population, contributing significantly to the regional economy and intimately intertwined with
current environmental problems. Key vulnerabilities of the Sudan Savanna include its ecological
fragility, institutional weakness, high levels of poverty and food insecurity, and political and economic
instability, now aggravated by climate change. The characterization of current farming and cropping
systems in the Sudan Savanna is the key for understanding and proposing meaningful adaptation
strategies at the field, farm, local and national levels. This review begins by examining the
agroecological (biophysical) profile, detailing climatic, edaphological and hydrological qualities. Next,
the main socioeconomic features: demography, culture, and organizational and economic institutions
are summarized, followed by a characterization of the main farming and cropping systems and
associated management. The paper concludes by offering an outlook on targeted activities,
interventions and strategies for cropping and farming systems to cope and adapt to climate change
and variability, as well as soil fertility challenges within the current socio-ecological context.
... The agricultural land-use was representative for the region with homogenous vegetation cover for each site (upland rice, maize, and sorghum). Strictly speaking, the direct observations are limited to plots and do not include complete slopes although watershed scale studies point towards similar results (Masiyandima et al., 2003). We focused on plot studies because this allowed us to vary, at each site, only one variable, namely slope length, while keeping all other variables the same. ...
Bimodal runoff behavior, characterized by two distinct peaks in flow response, often leads to significant stormflow and associated flooding. Understanding and characterizing this phenomenon is crucial for effective flood forecasting. However, this runoff behavior has been understudied and poorly understood in semi-humid regions. In this study, we investigated the response characteristics and occurrence conditions of bimodal hydrograph based on the hydrometric and isotope data spanning 10 years in a semi-humid forested watershed in North China. The main findings include: 1) the onset of the bimodal hydrograph exhibits a threshold behavior, with delayed streamflow peaks occurring when the sum of event rainfall (P) and antecedent soil moisture index prior to the rainfall (ASI) exceeds 200 mm; 2) isotopic hydrograph separation reveals that delayed stormflow process is primarily driven by pre-event water, with increasing contributions of pre-event water during catchment wetting-up; 3) the dynamic variation in groundwater level precedes that of streamflow, establishing a hysteretic relationship wherein groundwater level peaks before streamflow during delayed stormflow. These findings, supported by onsite observations, emphasize the dominance of shallow groundwater flow in the generation of delayed stormflow.
Inland valleys are seasonally waterlogged headwater wetlands, widespread
across western Africa. Their role in the hydrological cycle in the humid,
hard-rock-dominated Sudanian savanna is not yet well understood. Thus, while
in the region recurrent floods are a major issue, and hydropower has been
recognized as an important development pathway, the scientific community
lacks precise knowledge of streamflow (Q) generation processes and how they
could be affected by the presence of inland valleys. Furthermore, inland
valleys carry an important agronomic potential, and with the strong
demographic rates of the region, they are highly subject to undergoing land
cover changes. We address both the questions of the hydrological functioning
of inland valleys in the Sudanian savanna of western Africa and the impact of
land cover changes on these systems through deterministic sensitivity
experiments using a physically based critical zone model (ParFlow-CLM)
applied to a virtual generic catchment which comprises an inland valley.
Model forcings are based on 20 years of data from the AMMA-CATCH observation
service and parameters are evaluated against multiple field data (Q,
evapotranspiration – ET –, soil moisture, water table levels, and water
storage) acquired on a pilot elementary catchment. The hydrological model
applied to the conceptual lithological/pedological model proposed in this
study reproduces the main behaviours observed, which allowed those virtual
experiments to be conducted. We found that yearly water budgets were highly
sensitive to the vegetation distribution: average yearly ET for a
tree-covered catchment (944 mm) exceeds that of herbaceous cover (791 mm).
ET differences between the two covers vary between 12 % and 24 % of
the precipitation of the year for the wettest and driest years, respectively.
Consequently, the tree-covered catchment produces a yearly Q amount of
28 % lower on average as compared to a herbaceous-covered catchment,
ranging from 20 % for the wettest year to 47 % for a dry year. Trees
also buffer interannual variability in ET by 26 % (with respect to
herbaceous). On the other hand, pedological features (presence – or absence
– of the low-permeability layer commonly found below inland valleys,
upstream and lateral contributive areas) had limited impact on yearly water
budgets but marked consequences for intraseasonal hydrological processes
(sustained/non-sustained baseflow in the dry season, catchment water storage
redistribution). Therefore, subsurface features and vegetation cover of
inland valleys have potentially significant impacts on downstream
water-dependent ecosystems and water uses as hydropower generation, and
should focus our attention.
To examine claims that reforesting degraded Imperata grassland in Leyte (Philippines) made streamflow perennial again, we studied the hydrological behaviour of a 23‐year‐old mixed‐species ‘reforest' between June 2013 and May 2014. Typhoon Haiyan, one of the strongest events ever, caused major damage to the site in November 2013. Average daily apparent water use (ET) was 5.0 mm d‐1 pre‐typhoon and 3.2 mm d‐1 after disturbance. Corresponding ratios of period‐total quickflow Qq to precipitation were 16% and 44%. Quickflow volume and peak discharge increased rapidly once a threshold value of ~250 mm for soil water storage in the top 60 cm was exceeded. Before disturbance, quickflow consisted predominantly of lateral subsurface flow due to high soil hydraulic conductivities down to 60 cm. After disturbance, shallow groundwater rose regularly to within 10 cm of the surface on foot‐slopes and saturation overland flow was observed during several large storms. Comparing estimated annual ET and Qq for undisturbed conditions for the reforest and a nearby degraded Imperata grassland micro‐catchment suggested that the extra infiltration following reforestation (~240 mm yr‐1) exceeded the extra ET by the reforest (100–185 mm yr‐1), implying a net positive trade‐off (55–140 mm yr‐1) and tentatively confirming local claims of improved dry‐season flow.
Dans un contexte de changement climatique dont les projections régionales sont incertaines, de forte variabilité inter-annuelle du cycle hydrologique, de forte croissance démographique et de changement d'occupation des sols, les questions relatives au cycle hydrologique et à la ressource en eau actuels et à venir sont cruciales. Dans un tel contexte, ce travail de thèse approfondit la connaissance du fonctionnement hydrologique du bassin versant de l'Ouémé supérieur (situé en climat soudanien au Bénin), en considérant l'ensemble des termes et des processus du cycle hydrologique.Dans un premier temps, le fonctionnement hydrologique de l'Ouémé supérieur est analysé à l'échelle d'un versant grâce au modèle Hydrus 2D. Cette analyse de processus s'appuie sur un ensemble complet de mesures (précipitations, évapotranspiration, humidité du sol, niveau piézométrique, débit en rivière) obtenues dans le cadre de l'observatoire hydrométéorologique AMMA-CATCH. Les simulations montrent que la forêt ripisylve vidange la nappe profonde et la déconnecte ainsi du réseau hydrographique. L'apport d'eau de la nappe profonde permet une transpiration de la forêt ripisylve toute l'année, y compris en saison sèche. Les écoulements en rivière ne sont pas produits uniquement par exfiltration d'écoulements latéraux de subsurface non saturés, les bas-fonds jouent probablement un rôle.À méso-échelle, nous cherchons à quantifier l'impact de la variabilité spatiale de la conductivité hydraulique à saturation sur le bilan hydrologique et l'évapotranspiration. Le modèle numérique utilisé est nTopAMMA, formalisme dérivé de TopMODEL. À partir de mesures de terrain, nous avons mis en évidence que la variabilité spatiale de la conductivité hydraulique à saturation est corrélée à l'occupation du sol du bassin d'étude. La prise en compte de cette variabilité spatiale dans le modèle nTopAMMA montre que l'état hydrique et l'évapotranspiration simulés localement par le modèle dépendent aux trois-quarts de la topographie et pour un quart de la conductivité hydraulique à saturation.Adoptant une approche ascendante, nous confrontons la représentation élaborée à l'échelle du versant à la modélisation hydrologique à méso-échelle. Nous analysons les processus et flux verticaux du modèle nTopAMMA. L'amélioration de la modélisation du cycle hydrologique de l'Ouémé supérieur par le modèle nTopAMMA nécessite (i) la prise en compte des hétérogénéités du bassin versant, (ii) la modification du formalisme de l'évapotranspiration, (iii) la diversification des sources de prélèvements évapotranspiratoires et (iv) l'intégration de la nappe d'altérites.
Soils of rainfed rice fields have unique characteristics. Inland valleys are extensively found in West African savannas, and their hydromorphic valley bottoms are intensively used for rainfed rice-culture during rainy seasons when become wetlands. Data of rainfall and soil moisture are perennially collected in a study site located in Northern Region of Ghana. The soil of the site is classified as Dystric Planosols. The soil moisture retention curve at a depth of the soil is identified from observed volumetric water content and matric head. Since the soil is variably saturated, it is important to clarify abruptly changing hydraulic properties of the soil near saturation. A bimodal model including eight empirical parameters turns out to be suitable for this purpose. The vertically one-dimensional Richards equation is considered to reproduce infiltration process of rainwater into the soil. The saturated water content, one of the empirical parameters, and saturated hydraulic conductivity are assumed to be functions of the depth of soil. Then, the finite element method is applied for solving the Richards equation to simulate the process. The simulation results are in good accordance with actually observed data. The physically-based model serves as a tool for comprehending formulation process of the wetlands at early stages of rainy seasons.
Gravity irrigation is very frequent in West Africa while the amount of water used is rarely follow up or analyzed. Such a work was carried out in the case of the 16 ha Tropical Savannah irrigated rice valley bottom scheme of Tiefora in Burkina Faso (West Africa). Using the flow equation of the concrete weir at the headwork, daily water use volumes were calculated as time series covering three irrigation seasons (two dry and one wet). The moving average trend analysis reveals that during both rainy season (1200 mm of rainfall) and dry season (no rainfall), the main canal gate is almost never closed, keeping a minimum discharge of 200 m3/day for 4 ha (50 mm/day vs. a local evapotranspiration of 7 mm/day). That stresses the necessity of a more rigorous water management. Furthermore, the autocorrelation analysis using ARIMA model showed that the irrigation cycle that ensures equity in water distribution among farm plots is 20 days instead of five. The knowledge of this fact can defuse potential conflicts about equity among farmers: the lack of water in day 4 may be compensated later. during the 20-day cycle. It appeared that a simple water level measuring device - installed at the headwork of the main irrigation canal - can produce a time series to which autoregressive moving average model can be applied to yield, at low cost, a thorough assessment of water management in this surface irrigation system.
Forests are thought to play an important role in the regional dynamics of the West African monsoon, through their capacity to extract water from permanent aquifers located deep in the soil and pump it into the atmosphere even during the dry season. This is especially true for riparian forests located at the bottom of the hillslopes. This coupling between the riparian forests and the permanent aquifers is investigated, looking for quantifying its contribution to the catchment water balance. To this end, use is made of the observations available from a comprehensively instrumented hillslope through the framework of the AMMA-CATCH (African Monsoon Multidisciplinary Analysis – Coupling the Tropical Atmosphere and the Hydrological Cycle) observing system. Attention is paid to measurements of actual evapotranspiration, soil moisture and deep groundwater level. A vertical 2-D approach is followed using the physically-based Hydrus 2-D model in order to simulate the hillslope hydrodynamics, the model being calibrated and evaluated through a multi-criteria approach. The model correctly simulates the hydrodynamics of the hillslope as far as soil moisture dynamics, deep groundwater fluctuation and actual evapotranspiration dynamics are concerned. In particular, the model is able to reproduce the observed hydraulic disconnection between the deep permanent groundwater table and the river. A virtual experiment shows that the riparian forest depletes the deep groundwater table level through transpiration occurring throughout the year so that the permanent aquifer and the river are not connected. Moreover the riparian forest and the deep groundwater table form a coupled transpiration system: the riparian forest transpiration is due to the water redistribution at the hillslope scale feeding the deep groundwater through lateral saturated flow. The annual cycle of the transpiration origin is also quantified. The riparian forest which covers only 5% of the hillslope generates 37% of the annual transpiration, this proportion reaching 57% during the dry season. In a region of intense anthropogenic pressure, forest clearing and its replacement by cropping could impact significantly the water balance at catchment scale with a possible feedback on the regional monsoon dynamics.
Empirical studies of residential flood vulnerability have generally concentrated on either the geophysical characteristics or the socio-economic characteristics of a given region, rather than a combination of the two. In addition, studies using subjective assessments of flood vulnerability are not very common. However, due to the fact that people’s perceptions influence their risk behaviour, and therefore their vulnerability, understanding perceptions about a phenomenon is very significant for the design of effective communication as well as mitigation, coping, and adaptation strategies. This study uses a digital map (to calculate mean elevation, slope, proximity to lagoon, sea, and drain length by area) and the EDULINK Round II Household Survey (for socio-demographic characteristics of households) for the analysis. Perception of flood vulnerability is derived from responses from heads of households to the question, “Do you perceive your household to be vulnerable to floods?” The responses are either in the affirmative or negative. Results show that even when a subjective assessment of vulnerability is undertaken, it is geophysical characteristics that have significant associations with perceptions of flood vulnerability.
The Volta Basin covers 400,000 km2 of the West-African savanna zone. Ghana lies downstream and contains 42% of the basin. Most of the upstream part of the basin lies in Burkina Faso (43% of total), and the remaining 15% lies in Mali, Côte d'Ivoire, Togo, and Benin. Average rainfall is 1000 mm per year of which around 9% or 36 km3 becomes available as runoff in the Volta River. Small variations in rainfall cause relatively large variations in runoff. The Volta Basin is undergoing rapid changes in land use and water resource development, mainly driven by the high population growth of 3% per year. However, different countries pursue economic development in different ways. At independence in 1957, Ghana's leaders saw industrialization as essential to development and electric power from the Volta Dam as central to that industrialization. In 1964, the Volta Dam was built and Ghana's economic growth in the mining, industrial, and service sectors has depended on the dam's hydropower ever since. In contrast, land-locked Burkina Faso has less industrial potential and seeks to develop through its agriculture, both for subsistence and export crops. Given the extremely unreliable rainfall, irrigation development is seen as the only way to increase agricultural production. In general, irrigation in Burkina Faso takes the form of many small scale, village-based schemes of which the downstream impact is difficult to gauge. A minor drought in 1997 and 1998 caused the level of Lake Volta to drop, resulting in widespread power outages. In the ensuing public discussion, hydraulic development in Burkina Faso was seen as one of the potential causes of the lack of water. No firm data were available to substantiate this claim. In fact, over-withdrawals in previous years combined with climate variability were more likely culprits. A recently initiated multi-disciplinary research project will be presented that seeks to provide a scientific basis on which future discussions between the two countries concerning shared water resources can take place.
Understanding a discharge hydrograph is one of the leading interests in catchment hydrology. Recent research has provided credible information on the importance of bedrock groundwater on discharge hydrographs from headwater catchments. However, intensive monitoring of bedrock groundwater is rare in mountains with steep topography. Hence, how bedrock groundwater controls discharge from a steep headwater catchment is in dispute. In this study, we conducted long-term hydrological observations using densely located bedrock wells in a headwater catchment underlain by granitic bedrock. The catchment has steep topography affected by diastrophic activities. Results showed a fairly regionalized distribution of bedrock aquifers within a scale of tens of meters, consisting of upper, middle, and lower aquifers, instead of a gradual and continuous decline in water level from ridge to valley bottom. This was presumably attributable to the unique bedrock structure; fault lines developed in the watershed worked to form divides between the bedrock aquifers. Spatial expanse of each aquifer and the interaction among aquifers were key factors to explain gentle and considerable variations in the base flow discharge and triple-peak discharge responses of the observed hydrograph. A simple model was developed to simulate the discharge hydrograph, which computed each of the contributions from the soil mantle groundwater, from the lower aquifer, and from the middle aquifer to the discharge. The modeling results generally succeeded in reproducing the observed hydrograph. Thus, this study demonstrated that understanding regionalized bedrock aquifer distribution is pivotal for explaining discharge hydrograph from headwater catchments that have been affected by diastrophic activities.
Impacts of climate change vary from region to region. The 4th Assessment Report of the IPCC mentions that drier areas will be affected by more droughts and that the rainfall regime, in general, will become "rougher". In West Africa, specifically the area below the Sahel, the climate change signal may be more subtle. Anecdotal evidence from farmers suggest that the onset of the rainy season has been shifting forward in time over the past two generations. Recently, detailed atmospheric modeling over the region (Jung and Kunstmann, 2008) shows that also in the near future, the onset of the rainy season will shift to later in the year, roughly from April towards M ay. The end of the rainy season as well as the total amount of rainfall will remain more or less fixed. This implies that adaptation strategies should be twofold. The first part of a comprehensive adaptation strategy would be a continuation of the efforts to produce faster growing rainfed crop cultivars, mainly corn and sorghum. The second part would consist of increased water storage during the wet season for use in the dry season. River runoff in West Africa is very sensitive to the rainfall distribution. When the same amount of rain falls within a shorter period, as is suggested by climate projections, runoff will show an important increase. Also the recharge of groundwater will improve under these circumstances. Storage of surface runoff in small reservoirs would be an important part of climate change adaptation. Extensive use of (shallow) groundwater in the dry season could be a second, highly complimentary adaptation strategy. The development of large dams would probably be less successful given the flatness of the landscape and the move towards decentralized development in most West African countries. Shortening of the rainy season will reduce rainfed agriculture, which is the dominant mode of food production in the region. Use of surface and groundwater in the dry season may partially offset this negative effect. Success of any of these adaptation strategies will to a large extent depend on institutional and socio-economic developments within the region.
Shallow groundwater affects terrestrial ecosystems by sustaining river base-flow and root-zone soil water in the absence of
rain, but little is known about the global patterns of water table depth and where it provides vital support for land ecosystems.
We present global observations of water table depth compiled from government archives and literature, and fill in data gaps
and infer patterns and processes using a groundwater model forced by modern climate, terrain, and sea level. Patterns in water
table depth explain patterns in wetlands at the global scale and vegetation gradients at regional and local scales. Overall,
shallow groundwater influences 22 to 32% of global land area, including ~15% as groundwater-fed surface water features and
7 to 17% with the water table or its capillary fringe within plant rooting depths.
In an area of low intensity rainfall and permeable soils, three hillside
plots were instrumented for a study of runoff-producing mechanisms.
Runoff from the plots was measured at the ground surface, the base of
the root zone, and in the zone of perennial groundwater seepage. Data on
soil moisture, water-table elevation, and piezometric head were also
collected during natural and artificial storms. The data showed that, as
the infiltration capacity of the soil exceeded the rainfall intensities
that occurred and that were applied, overland flow generated by the
mechanism described by Horton did not occur. Although soils and
topography were those generally thought to be conducive to subsurface
stormflow, the runoff produced by this mechanism was too small, too
late, and too insensitive to fluctuations of rainfall intensity to add
significantly to stormflow in the channel at the base of the hillside.
When the water table rose to the surface of the ground, however,
overland flow was generated on small areas of the hillside. Only when
this overland flow occurred were significant amounts of stormflow
contributed to the channel by the hillside. The return periods of storms
that would produce such overland flow were found to be very large.
As part of the construction of a simulation model to test an acid precipitation neutralization mechanism, the stream hydrograph was separated into its base flow and event water components using stable environmental isotopes of water, naturally occurring conservative tracer. Three snowmelt events and one storm event during the winter and spring of 1984 were studied at an instrumented watershed in the Hubbard Brook Experimental Forest, New Hampshire. Conditions for use of the isotopic tracer were not always met, however. During the latter part of the snowmelt and the storm the isotopic content of the groundwater and event water were not distinguishable. Furthermore, the isotopic content of the meltwater varied considerably over time, thereby reducing the precision of the hydrograph separation. Frequent sampling of the meltwater is mandatory to assess this variability. Because the concentration of major cations and anions was measured as well, chemical tracers could be compared to the isotopic tracer, when the isotopic hydrograph separation was reliable, to test whether the chemical tracer was conservative. Dissolved silica was found to act as a conservative tracer for this watershed.
The geographic sources and hydrologic flow paths of stormflow in small catchments are not well understood because of limitations in sampling methods and insufficient resolution of potential end members. To address these limitations, an extensive hydrologic dataset was collected at a 10 ha catchment at Panola Mountain Research Watershed near Atlanta, GA, to quantify the contribution of three geographic sources of stormflow. Samples of stream water, runoff from an outcrop, and hillslope subsurface stormflow were collected during two rainstorms in the winter of 1996, and an end-member mixing analysis model that included five solutes was developed. Runoff from the outcrop, which occupies about one-third of the catchment area, contributed 50–55% of the peak streamflow during the 2 February rainstorm, and 80–85% of the peak streamflow during the 6–7 March rainstorm; it also contributed about 50% to total streamflow during the dry winter conditions that preceded the 6–7 March storm. Riparian groundwater runoff was the largest component of stream runoff (80–100%) early during rising streamflow and throughout stream recession, and contributed about 50% to total stream runoff during the 2 February storm, which was preceded by wet winter conditions. Hillslope runoff contributed 25–30% to peak stream runoff and 15–18% to total stream runoff during both storms. The temporal response of the three runoff components showed general agreement with hydrologic measurements from the catchment during each storm. Estimates of recharge from the outcrop to the riparian aquifer that were independent of model calculations indicated that storage in the riparian aquifer could account for the volume of rain that fell on the outcrop but did not contribute to stream runoff. The results of this study generally indicate that improvements in the ability of mixing models to describe the hydrologic response accurately in forested catchments may depend on better identification, and detailed spatial and temporal characterization of the mobile waters from the principal hydrologic source areas that contribute to stream runoff. Copyright © 2001 John Wiley & Sons, Ltd.
Effective control of nonpoint source pollution from contaminants transported by runoff requires information about the source areas of surface runoff. Variable source hydrology is widely recognized by hydrologists, yet few methods exist for identifying the saturated areas that generate most runoff in humid regions. The Soil Moisture Routing model is a daily water balance model that simulates the hydrology for watersheds with shallow sloping soils. The model combines elevation, soil, and land use data within the geographic information system GRASS, and predicts the spatial distribution of soil moisture, evapotranspiration, saturation-excess overland flow (i.e., surface runoff), and interflow throughout a watershed. The model was applied to a 170 hectare watershed in the Catskills region of New York State and observed stream flow hydrographs and soil moisture measurements were compared to model predictions. Stream flow prediction during non-winter periods generally agreed with measured flow resulting in an average r(2) of 0.73, a standard error of 0.01 m(3)/s, and an average Nash-Sutcliffe efficiency R-2 of 0.62. Soil moisture predictions showed trends similar to observations with errors on the order of the standard error of measurements. The model results were most accurate for non-winter conditions. The model is currently used for making management decisions for reducing non-point source pollution from manure spread fields in the Catskill watersheds which supply New York City's drinking water. Copyright (C) 1999 John Wiley & Sons, Ltd.
For many years, hydrologists and engineers alike have attempted to describe and model the processes that generate streamflow from rainfall. Modern understanding of runoff processes expanded rapidly in the twentieth century. Horton (1933) proposed that the soil surface partitions rainfall into overland flow and ground water. Simply stated, when rainfall rate exceeds the infiltration capacity of the soil, the excess water flows over the surface to become the primary source of storm flow. Base flow, that which maintains streams between rains, issues from ground water aquifers sloping gently to the channels. During the 1930s and 1940s, Horton and his coworkers firmly established the idea that infiltration was dominant in the runoff process. The central criticism of the Hortonian concept is that too much emphasis was placed on rain that fails to infiltrate the soil, and not enough on that which does infiltrate.
EÄective control of nonpoint source pollution from contaminants transported by runoÄ requires information about the source areas of surface runoÄ. Variable source hydrology is widely recognized by hydrologists, yet few methods exist for identifying the saturated areas that generate most runoÄ in humid regions. The Soil Moisture Routing model is a daily water balance model that simulates the hydrology for watersheds with shallow sloping soils. The model combines elevation, soil, and land use data within the geographic information system GRASS, and predicts the spatial distribution of soil moisture, evapotranspiration, saturation-excess overland flow (i.e., surface runoÄ), and interflow throughout a watershed. The model was applied to a 170 hectare watershed in the Catskills region of New York State and observed streamflow hydrographs and soil moisture measurements were compared to model predictions. Stream flow prediction during non-winter periods generally agreed with measured flow resulting in an average r2 of 0.73, a standard error of 0.01 m3/s, and an average Nash-SutcliÄe eÅciency R2 of 0.62. Soil moisture predictions showed trends similar to observations with errors on the order of thestandarderrorofmeasurements. The modelresultsweremost accuratefor non-winterconditions. Themodel iscurrentlyusedformakingmanagementdecisionsforreducing non-pointsourcepollutionfrommanurespread fields in the Catskill watersheds which supply New York City's drinking water. Copyright #1999 John Wiley & Sons, Ltd.
The temperature of a stream under thick snow cover in northern Hokkaido Island, Japan, was found to be 3° to 4°C during the snowmelt flood. Because this was considerably higher than the temperature of snowmelt (0°C), and the stream was perfectly insulated from heat sources from above by snow cover, it was reasoned that soil heat was furnishing most of the heat in runoff.An attempt was made to “separate” the daily snowmelt hydrograph into surface and subsurface components, using the conservation low of mass and heat. Surface runoff begins when the daily snowmelt hydrograph rises, but accounts for only 15–20% of streamflow even at daily peak runoff. As the hydrograph recedes, all of the flow is subsurface in origin, according to heat-mixing analysis. The results are contrary to the assumptions underlying conventional hydrograph analysis, but are consistent with the results by environmental isotope analysis and with the variable source-area concept of runoff in headwaters.
Five mathematical models for predicting subsurface flow were compared to discharge measurements made by Hewlett and Hibbert (1963) on a uniform sloping soil trough at the Coweeta Hydrologic Laboratory. The models included one- and two-dimensional finite element models based on the Richards equation, a kinematic wave model, and two simple storage-discharge models based on the kinematic wave and Boussinesq assumptions. The simple models simulated the subsurface response and water table positions as well as the more complex models based on the Richards equation and were much more economical to use from the point of view of computational costs. Such models have features that would allow them to be incorporated into more complex watershed models, thus placing hydrologic prediction on a more physically correct and less empirical footing.
Hortonian runoff was measured from plots with lengths of 1·25 and 12 m, and at watershed level for rainstorms during the 1996 rainy season in cental Côte d'Ivoire, Africa. A clear reduction in runoff coefficients was found with increasing slope lengths, giving order of magnitude differences between runoff measurements at point level (1 m2: 30–50% of total rain) and watershed level (130 ha: 4% of total rain). Runoff reduction from 1·25 and 12 m slopes was reproduced for each major runoff-producing rainstorm at two different sets of plots, but the reduction was erratic for rainfall events which produced little runoff. In addition, runoff reduction varied wildly from one rainstorm to the next. In the analysis, we show that the spatial variability of runoff parameters causes the erratic behaviour during rainstorms with little runoff. During the more important, larger runoff-producing events, which give 78% of total runoff, the temporal dynamics of the rainfall–runoff process determine the reduction of runoff coefficients from longer slopes. A simple infiltration/runoff model was used to simulate the field results, thereby confirming the importance of rainfall dynamics as an explanatory factor for measured reduction of runoff coefficients. Copyright © 2000 John Wiley & Sons, Ltd.
Simultaneous monitoring of conservative and non-conservative tracers in streamflow offers a valuable means of obtaining information on the age and flow paths of water reaching the basin outlet. Previous studies of stormflow generation in a small forested basin on the Canadian Shield used isotopic (IHS) and geochemical hydrograph separations (GHS) to infer that some event water during snowmelt reaches the stream via subsurface pathways, and that surface water runoff is generated by direct precipitation on to saturated areas (DPSA) in the stream valley. These hypotheses were tested for rainfall inputs using simultaneous IHS (18O) and GHS (dissolved silica) of basin stormflow, supplemented by hydrochemical and hydrometric data from throughflow troughs installed on basin slopes. Comparison of pre-event and subsurface water hydrographs did not provide conclusive evidence for subsurface movement of event water to the stream, owing to the appreciable uncertainty associated with the hydrograph separations. However, IHSs of runoff at the soil–bedrock interface on basin slopes indicated that event water comprised 25–50% of total runoff from areas with deep soil cover, and that these contributions supplied event water flux from the basin in excess of that attributable to DPSA. The surface water component of stormflow estimated from the GHS was also largely the result of DPSA. GHS assumes that dissolved silica is rapidly and uniformly taken up by water infiltrating the soil and that water moving via surface pathways retains the low dissolved silica level of rainfall; however, neither assumption was supported by the hillslope results. Instead, results suggest that the observed depression of silica levels in basin stormflow previously attributed to dilution by DPSA was partly a function of transport of dilute event water to the channel via preferential pathways. Implications of these results for the general use of simultaneous IHS and GHS to infer hydrological processes are discussed. © 1997 by John Wiley & Sons, Ltd.
This paper addresses the application of a data-based mechanistic (DBM) modelling approach using transfer function models (TFMs) with non-linear rainfall filtering to predict runoff generation from a semi-arid catchment (795 km2) in Tanzania. With DBM modelling, time series of rainfall and streamflow were allowed to suggest an appropriate model structure compatible with the data available. The model structures were evaluated by looking at how well the model fitted the data, and how well the parameters of the model were estimated. The results indicated that a parallel model structure is appropriate with a proportion of the runoff being routed through a fast flow pathway and the remainder through a slow flow pathway. Finally, the study employed a Generalized Likelihood Uncertainty Estimation (GLUE) methodology to evaluate the parameter sensitivity and predictive uncertainty based on the feasible parameter ranges chosen from the initial analysis of recession curves and calibration of the TFM. Results showed that parameters that control the slow flow pathway are relatively more sensitive than those that control the fast flow pathway of the hydrograph. Within the GLUE framework, it was found that multiple acceptable parameter sets give a range of predictions. This was found to be an advantage, since it allows the possibility of assessing the uncertainty in predictions as conditioned on the calibration data and then using that uncertainty as part of the decision-making process arising from any rainfall-runoff modelling project. Copyright © 2001 John Wiley & Sons, Ltd.
Previous studies of the downslope movement of soil moisture have been largely confined to two-dimensional slope profiles—the influence of contour curvature (three-dimensional) on moisture movement has received little attention. An automatic tensiometer system has been used to monitor soil moisture status continuously in a single hollow and adjacent spurs. Maximum saturated hillslope flow is shown to coincide with the stream discharge ‘throughflow peak’. Relative to the hollow, little saturation is evident at the base of the spurs. This subcatchment model is applied to a major portion of the catchment, showing the general importance of topography in the control of stream discharge.
The influence of bedrock subsurface flow on storm runoff generation was investigated in Japan in two regions in Japan underlain by three bedrock types. One region, with approximately 2500 m of relief, is located in the Japan Alps, central Japan (In a region), where six small forest-covered watersheds underlain by granite and Mesozoic shale were studied. Two of these watersheds were instrumented to monitor stream discharge and the other four are springs occurring at the bedrock exposure. The other study area is in northern Kyoto prefecture, western Japan (Oe region). Here, spring distribution and seasonal flow were monitored in two steep basins underlain by serpentinite rocks. Distinctly different runoff responses were observed: the granite watershed exhibited a large and rapid runoff peak that coincided with the rainfall peak (type 1); and the shale and serpentinite basins exhibited small initial runoff peaks followed by a maximum peak discharge five to ten times greater with a convex recession limb (type 2). Runoff response from bedrock springs had characteristics similar to type 1 hydrographs; however, discharge increased only when the antecedent precipitation index was large and the runoff peak was delayed between 10 h and 1 week after rain events. The specific discharge of the springs decreased with increase in altitude in the Oe region, especially in dry periods. This decline in discharge with elevation suggests that a deep subsurface flow system through bedrock fissures contributes to the storm water flow in serpentinate basins. When comparing runoff response and spring discharge, slow or double peak runoff response may be a good indicator of the influence of bedrock subsurface outflow on storm runoff generation in steep mountainous regions. Copyright © 2001 John Wiley & Sons, Ltd.
No reliable method exists to establish the relative proportions of surface-runoff and groundwater discharge from analysis of hydrographs1–3). However, it is possible to measure the amounts of tritium in the rainwater, the groundwater, and the outflow from a particular drainage basin. From such data, the part of the total discharge due to direct runoff can be established.After a short description of the method and an analysis of the accuracy to be expected, three applications have been given. At Pont des Blaves, the greater part of the flood waters passing on 11/7/1968 appeared to consist of displaced groundwater. To a lesser extent this proved also to be the case with the discharge at Charmoisy (2/8/1968), situated within the same drainage basin. On the other hand, the discharge at Verthier (23 to 29/12/1968) was largely due to precipitation.Earlier radiometric methods have used 90Sr and 137Cs as indicators4–6). Tritium, however, is to be preferred; unlike the other two elements, it is not adsorbed by the soil.It is to be expected that the proposed method will give more insight into the water balance and especially into the hydrological cycle in drainage basins.RésuméDans l'analyse de l'hydrogramme d'une crue, il n'existe pas de méthode expérimentale permettant d'évaluer les parts respectives des eaux de ruissellement et des eaux souterraines dans le débit des rivières1–3).Nous fondant sur la mesure des teneurs en tritium dans les précipitations, la rivière et les eaux souterraines d'un même bassin versant, nous avons pu évaluer, à chaque instant, la participation de la pluie directement ruisselée au débit de la rivière.
Snowmelt hydrographs for the 8 ha Martinelli Basin on Niwot Ridge, Colorado Front Range, show a well marked diurnal periodicity for much of the summer melt season. The discharges have been separated into ground-surface and subsurface components on the basis of the major ion concentrations and specific conductance of the streamwater. The results suggest a high (up to 50%) contribution by subsurface routing through the superficial mantle of the basin, even during the highest flows of the melt season. Both surface and subsurface components retain the diurnal pattern of the hydrograph, with the latter lagged by only about one hour. This suggests that even subsurface routing gives a quick hydrologic response from this basin.
This paper describes the development of a deterministic-empirical model that can simulate total streamflow and the components of the stream hydrograph in near-stream wetland regions. The model treats the watershed in terms of three zones with differing storage characteristics, and accommodates the dynamic flow system that occurs owing to the influence of the capillary fringe. It accounts for the expansion and contraction of near-stream saturated regions, the large and rapid rise in the water table and the development and decay of the water-table mound, which result from the decreasing storage capacity toward the stream. The overland flow and the groundwater flow components of the stream are calculated accounting for the increased hydraulic gradient toward the stream and the development and expansion of the seepage face. Using relatively modest input, the model incorporates the effect of the capillary fringe mechanism for the purpose of improving the runoff prediction capability in shallow water-table regions.
Analysis of the storm event rainfall and runoff data and other physical data from the undisturbed Geebung Creek catchment shows that its hydrologic behaviour is well described by the variable source area concept. Runoff is believed to occur mainly from saturated source areas that are fed by rapid subsurface flow from upslope. O'Loughlin's theoretical steady-state saturation zone model has been applied to the catchment to identify the size and location of zones of saturation in the landscape under a range of wetness conditions and field observations confirm the general predictions made by the model. Linkage of the results from this model with a simple rainfall—runoff model provides a means of modelling the hydrologic response of the catchment to precipitation in terms of the dynamic expansion and contraction of the saturated source areas. The results of the rainfall—runoff simulation on the Geebung Creek catchment are very good, indicating that this methodology may have considerable potential for evaluating the impacts of land use change on the hydrology of certain types of catchments, at the process level.
An analysis of the factors influencing infiltration and runoff was carried out using data obtained under simulated rainfall conditions on 87 plots in arid and semi-arid areas of West Africa, arranged in a hierarchical sequence. One of the key factors to emerge from this typological classification is the type of surface crust. Together with faunal activity, vegetation cover and surface roughness, this parameter permits the inclusion of a ‘unit surface’ factor in the classification system. Each unit surface is characterized in terms of genetic, morphological and hydrological properties.At a higher level, the combination of these unit surfaces allows a ‘soil surface features unit’ to be defined, based on an original mapping method. The samples, once defined in this system, were found to correspond to homogeneous criteria both in terms of evolution dynamics and hydrological behaviour. Furthermore, such cartography could be successfully extrapolated through remote sensing image analysis, making possible a classification of the runoff capability of small watersheds based on objective and standardized criteria.
Over the last thirty years, French hydrologists have observed flow characteristics at the outlets of small tropical watersheds, characteristics which vary between arid zones, humid zones and equatorial rainforest zones. The main physical phenomena related to the various types of runoff generation have been observed and analysed. The generation of runoff and subsurface runoff following tropical rainstorms depends on soil surface formations, texture and structure as well as vegetation cover, geological substrata, weathered material and climate.Analysis is also made of experimental rainfall-runoff relationships, the shape of flood hydrographs and the conditions necessary for the occurrence of steady base flow. Scale effects between experimental plot and watershed have been studied to identify any effect this may have upon runoff generation and flow. Tables detail watershed characteristics and relationships between these and rainfall and runoff.
For some years the author has used the term “infiltration” to describe the process involved where water soaks into or is absorbed by the soil. Absorption, imbibition, and percolation are often used in much the same sense. It seems better to confine the use of “percolation” to the free downward flow by gravity of water in the zone of aeration—a process for which a distinctive term is needed. “Absorption” includes the entrance of air as well as water, both liquid and vapor, into the soil (see Patten and Gallagher, Absorption of vapors and gases by soils, U.S. Dep. Agric., Bul. 51, Bur. Soils, Wash., 1908; also Charles H. Lee, On absorption and transpiration, Trans. Amer. Geophys. Union, 1932, pp. 288–298). “Infiltration” is limited to water in the liquid form and is more accurately descriptive of the physical processes by which rain enters the soil. “Water‐penetration” is also sometimes used as if synonymous with infiltration. Its use should be restricted to the depth below soil‐surface reached by the given surface infiltration.
The paper describes the natural framework of the Booro-Borotou (1.36 km2) basin: geomorphology, geology and soil, vegetation and surface features, precipitation and evapotranspiration. Several experiments (rainfall simulation, surface runoff traps), together with the observation of internal storage and floods, show the complexity of runoff processes. Discussion confirms that, in this elementary watershed, water emerging as runoff travels by multiple routes.
Paradise postponed-challenges facing the reconciliation of tracer, isotopic, hydrometric evidence for the study of catchment-scale runoff processes
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Buttle JM. 2001. Paradise postponed-challenges facing the reconciliation of tracer, isotopic, hydrometric evidence for the study of
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Modeling subsurface storm flow on steeply sloping forested watersheds Scale effects of Hortonian overland flow and rainfall–runoff dynamics in a West African catena landscape
- Pg Sloan
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Detailed soil survey and land evaluation of the WARDA experimental area, Bouak´ e, Cˆ ote d’Ivoire
- Hydrol
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Hydrograph analysis for areas with mainly groundwater runoff. In Drainage Principles and Applications. II. Theories of Field Drainage and Watershed Runoff . ILRI Publication 16(II) ILRI: Wageningen
- De Zeeuw
De Zeeuw JW. 1983. Hydrograph analysis for areas with mainly groundwater runoff. In Drainage Principles and Applications. II. Theories of Field Drainage and Watershed Runoff. ILRI Publication 16(II). ILRI: Wageningen; 323– 357.
Etude hydrologique de petits bassins en
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Dubreuil PL. 1960. Etude hydrologique de petits bassins en Côte d'Ivoire. Rapport général. ORSTOM Service Hydrologique.
Estimation of discharges of ten years floods for catchments with a surface less than 200 sq. km in West Africa. ORSTOM: Paris. Sloan PG, Moore ID. 1984. Modeling subsurface storm flow on steeply sloping forested watersheds
- Rodier Ja Auvray
Rodier JA, Auvray C. 1965. Estimation of discharges of ten years floods for catchments with a surface less than 200 sq. km in West Africa. ORSTOM: Paris. Sloan PG, Moore ID. 1984. Modeling subsurface storm flow on steeply sloping forested watersheds. Water Resources Research 20: 1815– 1822.
Detailed soil survey and land evaluation of the WARDA experimental area, Bouaké, Cô d'Ivoire. Report 22. The Winand Staring Centre: Wageningen, The Netherlands. Hibbert AR, Troendle CA. 1987. Streamflow by variable source area
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- Ema Smaling
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Copyright 2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 1213– 1225 (2003) Hakkeling RTA, Smaling EMA, Diatta S. 1989. Detailed soil survey and land evaluation of the WARDA experimental area, Bouaké, Cô d'Ivoire. Report 22. The Winand Staring Centre: Wageningen, The Netherlands. Hibbert AR, Troendle CA. 1987. Streamflow by variable source area. In Forest Hydrology and Ecology at Coweeta, Swank WT, Crossley DA Jr (eds). Springer-Verlag: New York; 111–128.
Hydrograph analysis for areas with mainly groundwater runoff
- J W De Zeeuw
De Zeeuw JW. 1983. Hydrograph analysis for areas with mainly groundwater runoff. In Drainage Principles and Applications. II. Theories
of Field Drainage and Watershed Runoff. ILRI Publication 16(II). ILRI: Wageningen; 323-357.
Estimation of discharges of ten years floods for catchments with a surface less than 200 sq. km in West Africa
- Rodierja Auvrayc
Etude hydrologique de petits bassins en Côte d'Ivoire. Rapport général
- Dubreuilpl