Article

Water erosion risk assessment in South Africa: A proposed methodological framework

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Abstract

With the increase in human impacts on the environment, especially in terms of agricultural intensification and climate change, erosion processes need to be assessed and continually monitored. In many countries, but particularly in developing countries such as South Africa, standardized methodological frameworks that deliver comparable results across large areas as a baseline for regional scale monitoring are absent. Due to limitations of scale at which techniques can be applied and erosion processes assessed, this study describes a multi-process and multi-scale approach for soil erosion risk assessment under South African conditions. The framework includes assessment of sheet-rill erosion at a national scale based on the principles and components defined in the Universal Soil Loss Equation; gully erosion in a large catchment located in the Eastern Cape Province by integrating 11 important factors into a GIS; and sediment migration for a research catchment near Wartburg in KwaZulu-Natal by means of the Soil and Water Assessment Tool. Three hierarchical levels are presented in the framework, illustrating the most feasible erosion assessment techniques and input datasets that are required for application at a regional scale with proper incorporation of the most important erosion contributing factors. The methodological framework is not interpreted as a single assessment technique but rather as an approach that guides the selection of appropriate techniques and datasets according to scale dependency and modelled complexity of the erosion processes.

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... Stability was later confirmed by Olivier et al. (2018Olivier et al. ( , 2019), who found active gully processes, but no evident increase in extent and a negligible 0.003 t ha À1 yr À1 soil loss rate. The rate is significantly lower than the national soil loss prediction (Le Roux & Sumner, 2013) and the natural baseline (Reinwarth et al., 2019) described in SA ( Figure 12a). Compared to similar global geomorphic sites, the soil loss in the Swartland is appreciably lower, for example, soil loss measured in a discontinuous gully (0.1 t ha À1 yr À1 in New South Wales, Australia by Crouch, 1990) and where soil conservation methods were successfully implemented (5-2 t ha À1 yr À1 in Wisconsin, USA by Trimble, 1999 (Figure 12a). ...
... Introducing more variables restricted the studies to shorter temporal scales but allowed more complex efforts to understand gully occurrence(Bernini et al., 2021;Le Roux & Sumner, 2012;Mararakanye & Le Roux, 2012;Mararakanye & Nethengwe, 2012;Olivier et al., 2016;Seutloali et al., 2016).Topographic variables are frequently combined with imagery in a GIS environment(Grellier et al., 2012;Kakembo et al., 2009;Mararakanye & Sumner, 2017;Olivier, 2013;Olivier et al., 2018;Seutloali et al., 2016). Although other physiographic factors were used less often initially, more recently, they have been used more prominently, for example, geological(Bernini et al., 2021;Le Roux & Sumner, 2012;Mararakanye & Sumner, 2017), soil(Mararakanye & Sumner, 2017;Seutloali et al., 2016), land-use/ cover(Bernini et al., 2021;Du Plessis et al., 2020;Le Roux & Sumner, 2013), climate ...
... with two soil loss thresholds(McPhee & Smithen, 1984;Reinwarth et al., 2019), a modelled soil loss rate on a national scale(Le Roux & Sumner, 2013), and global gully data that shares geomorphic similarity to the research site of interest (Figure 12). The two thresholds consist of sustainable soil loss tolerance estimated between 5 and 10 t ha À1 yr À1(McPhee & Smithen, 1984) and a natural baseline calculated as 0.01 to 0.64 t ha À1 yr À1(Reinwarth et al., 2019). ...
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Gully erosion is an environmental problem recognised as one of the worst land degradation processes worldwide. Insight into regional gully perturbations is required to combat the serious on‐ and off‐site impacts of gullying on a catchment management scale. In response, we intersect different perspectives on gully erosion‐specific views in South Africa (SA), a country that exhibits various physiographic properties and spans 1.22 million km2. While the debate surrounding gully origin continues, there is consensus that anthropogenic activities are a major contemporary driver. The anthropogenic impact caused gullying to transcend climatic, geomorphic, and land‐use boundaries, although it becomes more prominent in central to eastern SA. Soil erodibility plays a crucial role in what extent gully erosion severity is attained from human impact, contributing to the east‐west imbalance of erosion in SA. Soil erosion rates from gullying and badlands are limited but suggest that it ranges between 30 t ha‐1 y‐1 and 123 t ha‐1 y‐1 in the more prominent areas. These soil loss rates are comparable to global rates where gullying is concerning; moreover, they are up to four orders of magnitude higher than the estimated baseline erosion rate. On a national scale, the complexity of gullying is evident by different temporal timings of (re)activation or stabilising and different evolution rates. Continued efforts are required to understand the intricate interplay of human activities, climate, and preconditions determining soil erodibility. In SA, more medium‐ to long‐term studies are required to understand better how changing control factors affect gully evolution. More research is needed to implement and appraise mitigation measures, especially using indigenous knowledge. Establishing (semi)‐automated mapping procedures would aid in gully monitoring and assessing the effectiveness of implemented mitigation measures. More urgently, the expected changes in climate and land‐use necessitate further research on how environmental change affects short‐term gully erosion dynamics.
... Indices such as the Transformed Soil Adjusted Vegetation Index (TSAVI), Modified Soil Adjusted Index (MSAVI) and Soil Adjusted Vegetation Index (SAVI) were established as so as to accurately map soil erosion features especially in regions with low vegetation (Huete, 1988;Nadal-Romero et al., 2012;Le roux Jacobus & Sumner, 2013;Phinzi et al., 2021;Taruvinga, 2009). For example, Taruvinga (2009) and Phinzi and Ngetar (2017) used Landsat data derived vegetation indices to compare the accuracy of NDVI and SAVI in mapping soil erosion distribution in South Africa. ...
... We expected that, like taxonomy-based indicators that are well established, tolerant and sensitive signature traits of the EPTs would be identified for long-term monitoring of river and streams draining semi-urban catchments. The Tsitsa River catchment in the Eastern Cape of South Africa is located in a semi-urban catchment and is among the few rivers in the country in their near-natural condition (Le Roux & Sumner, 2013). However, increasing pollution from nearby rural communities, informal settlements, domestic (e.g. ...
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... We expected that, like taxonomy-based indicators that are well established, tolerant and sensitive signature traits of the EPTs would be identified for long-term monitoring of river and streams draining semi-urban catchments. The Tsitsa River catchment in the Eastern Cape of South Africa is located in a semi-urban catchment and is among the few rivers in the country in their near-natural condition (Le Roux & Sumner, 2013). However, increasing pollution from nearby rural communities, informal settlements, domestic (e.g. ...
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The taxonomy-based response pattern of macroinvertebrates to pollution gradient is well established, with tolerant taxa increasing in impacted conditions, while sensitive taxa increase with decreasing deterioration, typical of rural pollution. This study identified rural indicator and sensitive traits of Ephemeroptera, Plecoptera and Trichoptera (EPT) taxa by examining their trait distribution pattern in relation to rural pollution. Physicochemical parameters and EPT were sampled seasonally from August 2016 to April 2017. Eight sites were selected and categorised into three site groups. Site group 1 served as the least impacted site group and Site group 2 as the moderately influenced, whereas Site group 3 was the most impacted. Seven traits were selected and categorised into 27 trait modalities. The response of EPT traits to physicochemical parameters was analysed using the simultaneous analysis of the informationcontained in three tables: R (environmental charac-teristics of samples), L (taxa distribution across sam-ples) and Q (species traits) (RLQ) and confirmed with fourth-corner analysis. Three trait attributes, large (10–20 mm), swimming, shredding, streamlined body shape and large body size (≥ 10–20), were consid-ered tolerant signature traits of semi-urban pollution. These trait attributes were associated with the influ-enced Site group 3 and indicated a significant positive affinity with at least one physicochemical indicator of increasing semi-urban pollution (NH4-N, NO3-N, NO2-N, PO4-P, EC, turbidity, temperature and pH). Conversely, small body size (< 10 mm), operculate gills, spherical body shape and a preference for sedi-ments were correlated with the least influenced Site group 1 and were considered sensitive traits of semi-urban disturbance. Overall, this study provided criti-cal insights into EPT responses to disturbance, reveal-ing that semi-urban activities influenced EPT traits differently in the Tsitsa River.
... Hence, vegetation cover mapping was conducted in the Amazon forest using vegetation indices in order to establish the extent of deforestation (Sano et al., 2010). Erosion hazard assessment was performed in Australia using the Revised Universal Soil Loss Equation in order to determine the extent of soil erosion hazard (Le Roux & Sumner, 2013). ...
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This study assessed the vegetation cover changes and soil erosion hazard in the Muzvezve sub-catchment located in Sanyati catchment, Zimbabwe. A descriptive research design was used in this study. Data was collected using Geographical Information System (GIS) and remote sensing, key informant interviews, and direct field observations. ArcGIS 10.5 was used to analyze the data collected using GIS and remote sensing. Content analysis was used to analyze data collected from direct field observations and semi-structured interviews. The results indicated that the major causes of deforestation and soil erosion in the sub-catchment were anthropogenic activities such as urban and agricultural expansion, resettlement programs, veld fires, brick molding, and mining. Vegetation cover changes fluctuated from 2008 to 2019, experiencing gains and losses in vegetation cover over the years. The erosion hazard fluctuated between the different erosion hazard classes from 2008 to 2019. These changes were attributed to land use and land cover changes, and conservation practices in the sub-catchment. Some of the strategies that were undertakento deal with deforestation and soil erosion were not effectively implemented. The study recommends more community engagement programs such as awareness campaigns in order to educate local people on sustainable forest and soil resource management.
... These authors also assert that severe soil erosion deteriorate soil fertility, reduce soil water holding capacity, adversely affect biodiversity, increased eutrophication and lead to increased land degradation. Given the increase of human impact on environment, more especially in the form of agricultural intensification and varying changes in climate, there is a need to continually assess and monitor soil erosion (Le Roux and Sumner, 2013). Communities whose livelihood depends on rain-fed agriculture are at risks of soil erosion, which fails sustain crop production due to loss in its fertility. ...
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... The modelled results of case study III concur that the cabbage plot in the upper reaches of the research catchment near Wartburg is a significant sediment source, but is counterbalanced by sinks, including the river channel and farm dams downstream. Insight is also provided into the applicability of SWAT in connectivity studies, explicitly describing how model assumptions affect outputs in the context of connectivity between sources and sinks (Le Roux 2012;Le Roux et al. 2013). ...
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Desertification and land degradation present formidable challenges to global sustainable development. This study employs advanced remote sensing (RS) methodologies to ascertain the extent of these phenomena within a specific locale. Integration of satellite imagery, geographic information systems, and machine learning facilitates a nuanced evaluation of key indicators—namely, plant cover, soil moisture, and land surface temperature. Focused on a defined geographical area, the research illuminates the multifaceted impacts of population expansion, climate variability, and unsustainable land management practices. The outcomes contribute a comprehensive understanding of the causative factors driving land degradation, thereby informing the formulation of judicious and sustainable land management strategies. The study underscores the efficacy of RS technologies in addressing pressing environmental concerns related to global land deterioration and desertification, providing actionable insights for policymakers, land managers, and conservationists. Furthermore, it lays the groundwork for subsequent scholarly investigations in this domain.
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Agricultural land-use is a leading cause of water quality deterioration, biodiversity loss and impairment of stream functionality. Understanding the mechanisms by which agricultural land-use impair stream ecosystems is important for their effective management, especially in Africa. In this study, a combination of analytical tools, including macroinvertebrate taxonomic- and trait-based community analysis, functional indices, functional feeding groups and stable isotopes were used to investigate the effects of an increasing gradient of agricultural disturbance on the community composition, functional diversity, and food web of aquatic macroinvertebrates in the Kat River. Eight sites grouped into four site categories that represent a decreasing gradient of agricultural pollution (LUC 1< LUC 2 < LUC 3 and LUC 4) were selected. Macroinvertebrates and physiochemical variables and aquatic and terrestrial basal food sources were sampled from the eight sites over four sampling occasions; dry (winter and spring) and wet (summer and autumn) periods using the SASS 5 protocols. The taxonomy-based analysis showed different responses of macroinvertebrates to agricultural disturbance, with taxa such as Lymnaea spp., L. columella, Appasus spp. Biomphalaria spp., Trithemis spp. and Oligochaeta identified as potentially tolerant indicators of agricultural pollution. These taxa were positively correlated with the highly disturbed LUC 1 sites, and increasing levels of NH4-N, NO2-N, temperature and TDS. Conversely, Caenis spp., Afroptilum spp., Pseudocloeon piscis, Pseudocloeon spp., Baeti harrisoni, and Potamonautes spp. were sensitive to agricultural pollution, indicating strong negative associations with LUC 1 sites and NH4-N, NO2-N, salinity, temperature and TDS. 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Erosion and sediment transport are natural catchment processes that play an essential role in ecosystem functioning by providing habitat for aquatic organisms and contributing to the health of wetlands. However, excessive erosion and sedimentation, mostly driven by anthropogenic activity, lead to ecosystem degradation, loss of agricultural land, water quality problems, reduced reservoir storage capacity and damage to physical infrastructure. It is reported that up to 25% of dams in South Africa have lost approximately 30% of their initial storage capacity to sedimentation. Therefore, excessive sedimentation transcends from an ecological problem to a health, livelihood and water security issue. Erosion and sedimentation occur at variable temporal and spatial scales; therefore, monitoring of these processes can be difficult and expensive. Regardless of all these prohibiting factors, information on erosion and sediment remains an urgent requirement for the sustainable management of catchments. Models have evolved as tools to replicate and simulate complex natural processes to understand and manage these systems. Several models have been developed globally to simulate erosion and sediment transport. However, these models are not always applicable in Africa because 1) the conditions under which they were developed are not as relevant for African catchments 2) they have high data requirements and cannot be applied with ease in our data-scarce African catchments 3) they are sometimes complicated, and there are little training available or potential users simply have no time to dedicate towards learning these models. To respond to the problems of erosion, sedimentation, water quality and unavailability of applicable models, the current research further develops, applies and evaluates an erosion and sediment transport model, the Water Quality and Sediment Model (WQSED), for integration within the existing water resources framework in South Africa and application for practical catchment management. The WQSED was developed to simulate daily suspended sediment loads that are vital for water quality and quantity assessments. The WQSED was developed based on the Modified Universal Soil Loss Equation (MUSLE), and the Pitman model is a primary hydrological model providing forcing data, although flow data from independent sources may be used to drive the WQSED model. The MUSLE was developed in the United States of America, and this research attempts to improve the applicability of the MUSLE by identifying key issues that may impede its performance. 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The additional evaluation of the WQSED simulated sediment yield rates against observed reservoir sediment rates showed a broad range of differences between the simulated and observed sediment yield rates. Differences between WQSED simulated sediment and observed reservoir sediment ranges from a low of 30% to a high of > 40 times. The large differences were partly attributed to WQSED being limited to simulating suspended sediment from sheet and rill processes, whereas reservoir sediment is generated from more sources that include bedload, channel and gully processes. Nevertheless, the model simulations replicated some of the regional sediment yield patterns and are assumed to represent sheet and rill contributions to reservoir sediment in selected catchments. The outcome of this study is an improved WQSED model that has successfully undergone preliminary testing and evaluation. Therefore, the model is sufficiently complete to be used by independent researchers and water resources managers to simulate erosion and sediment transport. However, the model is best applicable to areas where some observed data or regional information are available to calibrate the storage components and constrain model outputs. The report on potential MUSLE scale dependencies is relevant globally to all studies applying the MUSLE model and, therefore, can improve MUSLE application in future studies. The WQSED model offers a relatively simple, effective and applicable tool that is set to provide information to enhance catchment, land and water resources management in catchments of Africa.
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Runoff and erosion processes are often non-linear and scale dependent, which complicate runoff and erosion modelling at the catchment scale. One of the reasons for scale dependency is the influence of sinks, i.e. areas of infiltration and sedimentation, which lower hydrological connectivity and decrease the area-specific runoff and sediment yield. The objective of our study was to model runoff and erosion for a semi-arid catchment using a multi-scale approach based on hydrological connectivity. We simulated runoff and sediment dynamics at the catchment scale with the LAPSUS model and included plot and hillslope scale features that influenced hydrological connectivity. The semi-arid Carcavo catchment in Southeast Spain was selected as the study area, where vegetation patches and agricultural terraces are the relevant sinks at the plot and hillslope scales, respectively. We elaborated the infiltration module to integrate these runoff sinks, by adapting the parameters runoff threshold and runoff coefficient, which were derived from a rainfall simulation database. The results showed that the spatial distribution of vegetation patches and agricultural terraces largely determined hydrological connectivity at the catchment scale. Runoff and sediment yield for the scenario without agricultural terraces were, respectively, a factor four and nine higher compared to the current situation. Distributed hydrological and erosion models should therefore take account of relevant sinks at finer scales in order to correctly simulate runoff and erosion-sedimentation patterns.
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To date erosion scientists have failed to address — or have addressed inadequately — some of the ‘big questions’ of our discipline. For example, where is erosion occurring? Why is it happening, and who is to blame? How serious is it? Who does it affect? What should be the response? Can we prevent it? What are the costs of erosion? Our inability or reluctance to answer such questions damages our credibility and is based on weaknesses in commonly-used approaches and the spatial and temporal scales at which much research is carried out. We have difficulty in the recognition, description and quantification of erosion, and limited information on the magnitude and frequency of events that cause erosion. In particular there has been a neglect of extreme events which are known to contribute substantially to total erosion. The inadequacy and frequent misuse of existing data leaves us open to the charge of exaggeration of the erosion problem (a la Lomborg).
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Catchment sediment budgeting is an attempt to identify the sources, sinks and pathways of eroded material within catchments. However, the identification of these quantities is not straightforward, and the conceptual underpinnings of sediment budgets make unwarranted and untested assumptions about process stability. Many sediment budgets leave one or more quantities unmeasured and obtain estimates of them by subtraction (assuming budget closure). Consequently, errors in sediment budgets are often hidden and are not quantified. There has been an emphasis on suspended sediment, which, for management purposes, may not be useful. Sediment budgeting can act as a framework for a research agenda on catchment processes. What has been lacking from this agenda has been an adequate consideration of the time taken for sediment to travel via the various pathways to the catchment outlet. The storage term in such budgets has been used as a poor substitute for a thorough understanding of sediment velocity through catchments.
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Progress in regard to soil erosion, soil conservation, land suitability evaluation and land use planning in South Africa during the 25 year period 1978–2003 is reviewed, with special emphasis on the role of soil scientists. An attempt has been made not only to give a list of the work done, but to present an interpretive review that could hopefully serve as a reference source for future soil scientists.
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Gully erosion is an important form of soil erosion that contributes greatly to soil degradation and loss in South Africa (SA). It is a process whereby soil is removed through the concentration of surface or subsurface water which results into the formation of incised channels. However, little is known regarding the spatial extent of gullies in SA, especially at a national scale. Previous soil erosion assessment studies indicated the difficulties of modelling or automatically extracting gullies using remote sensing at regional scales due to the temporal and spatial complexity at which the phenomenon occurs. This study created a gully location map for SA by means of visual interpretation and vectorisation from Système Pour l'Observation de la Terre (SPOT) 5 imagery at a scale of 1:10,000 within a geographic information system. Results illustrate the extent of gully erosion in the Eastern Cape (161,517 ha), Northern Cape (160,885 ha), KwaZulu-Natal (92,543 ha), Free State (64,674 ha), Limpopo (58,669 ha), Western Cape (25,403 ha), Mpumalanga (17,420 ha) and North West (10,782 ha) provinces. Additional zonal calculations indicate that gullies are more prominent on land that is suitable for cultivation (4.3%) than on less suitable to unsuitable land (1.5%), subsequently undermining sustainable management of soil resources and food security. This is probably attributed to inappropriate agricultural activities such as clearing of vegetation and overstocking in certain agricultural areas. Despite some problems experienced during the interpretation and mapping phases (such as difficult distinction between gullies and dry river beds/channels, landslides and other erosion forms), results show an overall accuracy of 90% when compared to observations (n = 1019) in the field. Future studies should focus on quantifying different contributing factors in order to ease selection of appropriate management strategies.
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The invader shrub Pteronia incana has colonised extensive areas in communal villages, commercial farms and game reserves in the Eastern Cape Province, South Africa. The spatial and temporal trends in patchiness dynamics for P. incana and their implications for landscape connectivity and functionality were analysed at the hillslope scale in one of the affected catchments. High‐resolution imagery for 2001, 2004 and 2009 was used to analyse the temporal changes in patchiness loss. Spatial metrics, namely landscape shape index (LSI), percentage of landscape (PLand) and largest patch index (LPI) were used to depict patchiness trends. Vegetation patterning was examined in the field along transects in order to determine the landscape organisation index (LOI) and characterise selected components of the landscape function analysis. Sediment sinks in the form of run‐on zones were also surveyed. Temporal trends analysed between 2001 and 2009 showed an increase in P. incana patchiness loss, as confirmed by a 75 per cent increase in the LSI, a decrease in PLand and LPI from 42·64 to 38·02 per cent and 25·98 to 20·52 per cent, respectively. The average inter‐patch length increased, and LOI decreased in a downslope direction. A progressive increase in rill density per 200 m and the development of gullies on the lower slope elements were also observed. Such increased connectivity is an indication of a trajectory towards hillslope dysfunctionality. The presence of run‐on zones within microtopographic concavities on the hillslope units provides isolated elements of functionality in the hillslope systems. These zones are recommended as starting points in restoration efforts. Copyright © 2012 John Wiley & Sons, Ltd.
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Gully erosion is a degradation process affecting soils in many parts of the World. Despite the complexity of a series of collective factors across different spatial scales, previous research has not yet explicitly quantified factor dominance between different sized gullies. This factorial analysis quantifies the differences in factor dominance between continuous gullies (cgs) and discontinuous gullies (dgs). First, gullies (totaling 5273 ha) visible from SPOT 5 imagery were mapped for a catchment (nearly 5000 km2) located in the Eastern Cape Province of South Africa. Eleven important factors were integrated into a geographical information system including topographical variables, parent material‐soil associations and land use–cover interactions. These were utilized in a zonal approach in order to determine the extent factors differ between cgs and dgs. Factors leading to the development of cgs are gentle footslopes in zones of saturation along drainage paths with a large contributing area, erodible duplex soils derived from mudstones and poor vegetation cover due to overgrazing. Compared to cgs conditions, more dgs occur on rolling slopes where the surface becomes less frequently saturated with a smaller contributing area, soils are more stable and shallow. Factorial analysis further illustrates that differences in factor dominance between the two groups of gullies is most apparent for soil factors. A combination of overgrazing and susceptible mudstones proves to be key factors that consistently determine the development of cgs and dgs. Copyright © 2011 John Wiley & Sons, Ltd.
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Studies of land degradation in South Africa have seldom addressed the issue for the whole country. As part of the first step in developing a National Action Programme to combat desertification, a national review of the soil and veld degradation problem was conducted in 1997 and 1998. The results are based on the perceptions of agricultural extension workers and resource conservation technicians from the Department of Agriculture. They indicate that it is primarily in the communal areas along the eastern and northern escarpment and in some commercial districts along the Orange River that problems of soil degradation are greatest. Veld degradation is also higher in communal areas than commercial areas, although many commercial areas are susceptible to bush encroachment and alien plant invasions. A separate multiple regression analysis indicates that both biophysical and socio-economic factors are associated with high levels of soil and veld degradation. Magisterial districts which are most degraded are characterised by steep slopes and high mean annual temperatures, and a rural population in which many people are dependent on only a few wage earners. Although the interaction is poorly understood it appears that when there are high levels of poverty in susceptible environments, land degradation is greatest.
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Loss of topsoil is one of the principal soil degradation problems confronting agriculture throughout South Africa and receives special attention by policy-makers. For effective prevention and remediation, the spatial extent of the problem has to be established and monitored. Recent developments in the application of remote sensing and GIS to the study of soil erosion offer considerable potential in this regard. This paper outlines key technologies available for monitoring, and highlights the problems to be solved at a regional scale. The status of the technologies used in South Africa are reviewed and the more recent studies related to soil erosion are presented in a comparative context. Spatial, temporal and measurement variabilities are major constraints in erosion assessment. Previous erosion studies conducted in South Africa at the regional scale have disregarded important erosion factors and have overvalued less important ones. Different processes and interactions are likely to emerge as dominant when crossing scale boundaries. Such considerations highlight the need to establish a methodological framework to guide and standardize future regional soil loss monitoring efforts.
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Scientific planning for soil and water conservation requires knowledge of the relations between those factors that cause loss of soil and water and those that help to reduce such losses. The soil loss prediction procedure presented in this handbook provides specific guidelines which are needed for selecting the control practices best suited to the particular needs of each site. The procedure is founded on an empirical soil loss equation that is believed to be applicable wherever numerical values of it factors are available. KEYWORDS: TROPAG textbar Miscellaneous subjects textbar Climatology textbar Land Conservation and Management textbar USA (Mainland).
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The Guadalentin basin, located in the SE of Spain, has a semiarid climate and presents typical characteristics of Mediterranean landscapes vulnerable to land degradation processes and desertification risks. In such an environment, when the vegetation cover is low, the signal received by satellites is dominated by the spectral properties of soils. Changes in these properties can be interpreted in terms of varying soil surface conditions. These optical changes underline the major modifications affecting soil surface under land degradation processes. The present research uses remote sensing techniques to characterise land degradation based on two approaches: spectral mixture analysis and a set of indices describing the spectrum shape. It also presents an integrated approach for evaluating ecosystem vulnerability to land degradation, through the combined analysis of spectrally-derived land units and geomorphometric units. Specific objectives consist of evaluating the potential of extending the indices describing the spectrum shape to the short-wave infrared region, and of identifying landscape units according to their sensitivity to land degradation. Our results demonstrate that the spatial distribution of regional patterns of land degradation can be reliably mapped by using both indices describing the spectrum shape and spectral unmixing. The latter holds great potential for operational mapping of soil conditions and erosion features from optical images. Moreover, landscape-unit analysis shows that DEM (Digital Elevation Model) variables combined with spectral information are very useful for land degradation assessment. This approach allowed us to segment the landscape into different units according to their lithology and vegetation characteristics, as well as their susceptibility to water erosion.
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The efficiency of crop production is defined in thermodynamic terms as the ratio of energy output (carbohydrate) to energy input (solar radiation). Temperature and water supply are the main climatic constraints on efficiency. Over most of Britain, the radiation and thermal climates are uniform and rainfall is the main discriminant of yield between regions. Total production of dry matter by barley, potatoes, sugar beet, and apples is strongly correlated with intercepted radiation and these crops form carbohydrate at about 1.4 g per MJ solar energy, equivalent to 2.4% efficiency. Crop growth in Britain may therefore be analysed in terms of (a) the amount of light intercepted during the growing season and (b) the efficiency with which intercepted light is used. The amount intercepted depends on the seasonal distribution of leaf area which, in turn, depends on temperature and soil water supply. These variables are discussed in terms of the rate and duration of development phases. A factorial analysis of efficiency shows that the major arable crops in Britain intercept only about 40% of annual solar radiation and their efficiency for supplying energy through economic yield is only about 0.3%. Some of the factors responsible for this figure are well understood and some are immutable. More work is needed to identify the factors responsible for the large differences between average commercial and record yields.
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A Core Research Project within the Global Change and Terrestrial Ecosystems (GCTE) programme is the Soil Erosion Network. The objective of the Network is to identify the most suitable experimental and monitoring databases, and the most robust models for global change studies. Meetings at Oxford (1995) and Utrecht (1997) attempted to evaluate erosion models at both field and watershed scale using common data sets. While the results of the performance of the different models varied, useful conclusions of the model strengths and shortfalls were defined.
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A desertification monitoring system is developed that uses four indicators derived using continental-scale remotely sensed data: vegetation cover, rain use efficiency (RUE), surface run-off and soil erosion. These indicators were calculated on a dekadal time step for 1996. Vegetation cover was estimated using the Normalized Difference Vegetation Index (NDVI). The estimation of RUE also employed NDVI and, in addition, rainfall derived from Meteosat cold cloud duration data. Surface run-off was modelled using the Soil Conservation Service (SCS) model parametrized using the rainfall estimates, vegetation cover, land cover, and digital soil maps. Soil erosion, one of the most indicative parameters of the desertification process, was estimated using a model parametrized by overland flow, vegetation cover, the digital soil maps and a digital elevation model (DEM). The four indicators were then combined to highlight the areas with the greatest degradation susceptibility. The system has potential for near-real time monitoring and application of the methodology to the remote sensing data archives would allow both spatial and temporal trends in degradation to be determined.
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Our understanding about the regional variation of Sediment Yield (SY) in Europe and its scale dependency currently relies on a limited number of data for mainly larger river systems. SY is the integrated result of all erosion and sediment transporting processes operating in a catchment and is therefore of high value for environmental studies and monitoring purposes. Most global assessments of SY consider catchment area (A), climate and topography as the main explanatory variables. However, it is still unclear if these factors also control regional variations of SY within Europe. This paper aims at bridging this gap. Therefore, we i) present a large database of SY-values which was constructed through an extensive literature review; ii) describe the spatial patterns of SY across Europe; and iii) explore its relation with A, climate, and topography.
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Soil erosion in the Mfolozi catchment is already a serious problem and likely to be exacerbated by landuse changes incumbent on the Land Reform Programme. As peasant farmers particularly perceive gully erosion as a problem, this study sought to identify those parts of the catchment that are unsuitable for allocation to them because they are either already severely gully eroded or are susceptible to this form of erosion. Two principle data sources were available for use. Firstly, an unpublished geomorphological map showing the location of eleven length classes of gullies. And secondly, maps and associated memoirs delimiting and detailing the topographic, substrate and rainfall characteristics of the catchment's landtypes. Data on the veld type, bioclimatic and landuse characteristics of these landtypes was extracted from various additional sources. The density of each gully class within each of the possible 16 landtypes within each of the catchment's 43 subcatchments was recorded. Statistical analysis of the resultant gully and landtype data sets focused on the significance of differences between and groupings within components of each set, and on functional relationships between the two sets. In addition to identifying susceptible and very susceptible landtypes, the study identified ten subcatchments which are already severely gully eroded as well as fifteen subcatchments that contain substantial portions of susceptible landtypes.
Article
Water resources management is an important issue in southern African countries. With respect to water quality problems, the understanding of the dynamics of integrated soil erosion processes in river basins is of crucial importance. This study is on the delineation of response units (RUs) in the Mkomazi River catchment (KwaZulu/Natal; South Africa). It was carried out within the framework of an interdisciplinary EU-funded project aimed at developing an integrated water resources management system for water resources analyses and prognostic scenario planning in semi-arid catchments of southern Africa. Within this more general framework, particular attention was focused on the identification of sediment source areas. For this purpose, the RUs concept was applied to delineate erosion RUs (ERUs). The ERUs were subsequently used to identify heterogeneously distributed areas within a river catchment characterized by specific hydrological and erosion dynamics and features. Therefore, they can be applied as modelling entities in physically based hydrological and erosion models. Spatially distributed input data from the catchment were derived by remote sensing techniques and geographical information systems analyses. Taking into account the high amount of sediments produced by gully erosion, not considered in USLE-type models, special attention was focused on gully erosion using a dynamic gully erosion model. The example from southern Africa shows that the methods applied are able to identify areas affected by gully erosion. Furthermore, it was possible to estimate the entire amount of soil loss, including gully erosion. Copyright © 2003 John Wiley & Sons, Ltd.
Article
This paper predicts the geographic distribution and size of gullies across central Lebanon using a geographic information system (GIS) and terrain analysis. Eleven primary (elevation; upslope contributing area; aspect; slope; plan, profile and tangential curvature; flow direction; flow width; flow path length; rate of change of specific catchment area along the direction of flow) and three secondary (steady-state; quasi-dynamic topographic wetness; sediment transport capacity) topographic variables were generated and used along with digital data collected from other sources (soil, geology) to statistically explain gully erosion field measurements. Three tree-based regression models were developed using (1) all variables, (2) primary topographic variables only and (3) different pairs of variables. The best regression tree model combined the steady-state topographic wetness and sediment transport capacity indices and explained 80% of the variability in field gully measurements. This model proved to be simple, quick, realistic and practical, and it can be applied to other areas of the Mediterranean region with similar environmental conditions, thereby providing a tool to help with the implementation of plans for soil conservation and sustainable management. Copyright © 2007 John Wiley & Sons, Ltd.
Article
The goal of this paper is to test a new semi-lumped sediment delivery concept to consider deposition. With this method, the distance between sediment source and river channel is taken into account. It is based on the assumption that sediment delivery rates (SDR-values) are related to the length of the flow path of the sediment from source area to channel. For each subwatershed an average weighted distance to the river channel can be calculated whereby cells with little or no soil erosion have a high weight. The procedure was implemented in SWAT-G (Soil and Water Assessment Tool) and calibrated and validated for two watersheds in Belgium and Germany. The results point out that the proposed method is a significant improvement of the sediment routine compared to existing lumped equations such as the MUSLE. Copyright
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A conceptual, continuous time model called SWAT (Soil and Water Assessment Tool) was developed to assist water resource managers in assessing the impact of management on water supplies and nonpoint source pollution in watersheds and large river basins. The model is currently being utilized in several large area projects by EPA, NOAA, NRCS and others to estimate the off-site impacts of climate and management on water use, non-point source loadings, and pesticide contamination. Model development, operation, limitations, and assumptions are discussed and components of the model are described. In Part II, a GIS input/output interface is presented along with model validation on three basins within the Upper Trinity basin in Texas.
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Regional environmental models often require detailed data on topography, land cover, soil, and climate. Remote sensing derived data form an increasingly important source of information for these models. Yet, it is often not easy to decide what the most feasible source of information is and how different input data affect model outcomes. This paper compares the quality and performance of remote sensing derived data for regional soil erosion and sediment yield modelling with the WATEM-SEDEM model in south-east Spain. An ASTER-derived digital elevation model (DEM) was compared with the DEM obtained from the Shuttle Radar Topography Mission (SRTM), and land cover information from the CORINE database (CLC2000) was compared with classified ASTER satellite images. The SRTM DEM provided more accurate estimates of slope gradient and upslope drainage area than the ASTER DEM. The classified ASTER images provided a high accuracy (90%) land cover map, and due to its higher resolution, it showed a more fragmented landscape than the CORINE land cover data. Notwithstanding the differences in quality and level of detail, CORINE and ASTER land cover data in combination with the SRTM DEM or ASTER DEM allowed accurate predictions of sediment yield at the catchment scale. Although the absolute values of erosion and sediment deposition were different, the qualitative spatial pattern of the major sources and sinks of sediments was comparable, irrespective of the DEM and land cover data used. However, due to its lower accuracy, the quantitative spatial pattern of predictions with the ASTER DEM will be worse than with the SRTM DEM. Therefore, the SRTM DEM in combination with ASTER-derived land cover data presumably provide most accurate spatially distributed estimates of soil erosion and sediment yield. Nevertheless, model calibration is required for each data set and resolution and validation of the spatial pattern of predictions is urgently needed. Copyright © 2009 John Wiley & Sons, Ltd.
Article
Basin sediment yield is the product of all sediment producing processes and sediment transport within a basin. Consequently, the prediction of basin sediment yield should take into consideration all different erosion and sediment transport processes. However, traditional physics-based, conceptual, and empirical or regression models have not been able to describe all these processes due to insufficient systems knowledge and unfeasible data requirements. Therefore, the applicability of these models at the basin scale is troublesome. This paper first illustrates the relation between basin area, dominant erosion processes, and sediment yield by a combination of measured sediment yield at different spatial scales in Mediterranean environments. This clearly reveals that soil erosion rates measured at one scale are not representative for sediment yield at another scale level. Second, the most important semi-quantitative models developed for erosion and sediment yield assessments at the basin scale are reviewed. Most of these models use environmental factors to characterise a drainage basin in terms of sensitivity to erosion and sediment transport. Six of the nine models discussed (PSIAC, FSM, VSD, Gavrilovic, CSSM, WSM) include sheet-, rill-, gully, bank erosion, landslides, and connectivity, at least partly, in the assessment of basin sediment yield. The low data requirements and the fact that practically all significant erosion processes are considered makes them especially suited for estimating off-site effects of soil erosion. The other three models (EHU, CORINE, FKSM) focus mainly on sheet and rill erosion and provide quantitative descriptions of the sensitivity to erosion at basin or even regional scales. These models thus focus mainly on on-site problems of soil erosion. Most of the semi-quantitative models might benefit from a more quantitative description of factors used to characterise the basin. Though an equilibrium should be found between the extra effort and increase in model performance, the increased availability of spatially distributed topographic data as well as high-resolution satellite imagery will probably make this feasible in the near future.
Article
Accurate predictions of sediment yield from distributed models of runoff and sediment yield depends in part of how well matched the model structure is to input data spatial representation. This study investigated how model structure and input data representation affect sediment predictions made using the Soil and Water Assessment Tool (SWAT). The study focused on the integration of two specific components of SWAT: the Modified Universal Soil Loss Equation (MUSLE) and the hydrologic response unit (HRU). MUSLE, a watershed erosion model, was applied to different levels of watershed partitioning and alternative HRU schemes for a watershed and its two subwatersheds over a 4-year period of measured stream flow and sediment yield. The results show that across different levels of watershed partitioning, HRUs do not conserve sediment. Instead, HRUs introduce almost half of the variability in sediment generation, which has previously been attributed solely to input data aggregation. This occurs for two reasons. First, MUSLE defines a nonlinear relationship between sediment generation and HRU area, but the sediment load is scaled linearly from the HRU level to the subwatershed level. Second, HRUs aggregate land areas without regard for the surface connectivity assumptions, which are implicit in MUSLE. These conflicts caused by the integration of HRU and MUSLE makes it difficult to determine the effect of different land use on soil erosion. This study indicates that greater attention should be made to structuring the data inputs to match the underlying assumptions of sub-models within SWAT.
Article
Understanding the sediment delivery process at the drainage basin scale remains a challenge in erosion and sedimentation research. In the absence of reliable spatially distributed process-based models for the prediction of sediment transport at the drainage basin scale, area-specific sediment yield (SSY, t km(-2) y(-1)) is often assumed to decrease with increasing drainage basin area (A). As the measurement of A is relatively simple, this assumption is frequently used for prediction of SSY in ungauged basins. However, over the last two decades several studies reported a positive or non-linear relation between A and SSY Various authors have suggested diverse explanations for these opposing trends. This paper provides an overview of the different observed trends and summarizes the explanations for each trend. Furthermore, three typical trends are identified to conceptualize the main driving forces of the relation between A and SSY First of all, it is emphasized that erosion and sediment deposition processes are scale dependent, and going from small (<m(2)) to larger areas (km(2)) more erosion processes become active leading to a rise in SSY with increasing A. However, for larger areas (>km(2)) erosion rates generally decrease and deposition in sediment sinks increases due to decreasing slope gradients, and so SSY decreases with increasing A. Next, land-cover conditions and human impact determine if hillslope erosion is dominant over channel erosion or vice verso. In the first case, SSY is expected to decrease with increasing A, while in the latter case SSY will show a continuous positive relation with A. Only for very large areas (A > similar to 10(4) km(2)) a decrease in SSY is observed when drainage density decreases or channel banks are stabilized. Finally, spatial patterns in lithology, land cover, climate or topography can cause SSY to increase or decrease at any basin area and can therefore result in non-linear relations with A. Altogether, with increasing A often first a rise and then a decrease in SSY is observed. The decrease can be absent or can be postponed within a region due to local factors of which lithology, land cover, climate and topography are the most important ones. The large regional, local and even temporal variability in the trend between A and SSY implies that prediction of SSY based on A alone is troublesome and preferably spatially distributed information on land use, climate, lithology, topography and dominant erosion processes is required.
Article
This commentary brings together, as a virtual Special Issue, a number of recent papers in Earth Surface Processes and Landforms that are all related to issues of scale in soil erosion. Empirical concepts that were developed in the 1940s are now in need of re-thinking, and papers are increasingly exploring, through modelling and measurement, appropriate ways to recognize the mechanisms that connect processes across time and space scales. Issues include a more nuanced approach to selective transportation, responses to variability in surface and sub-surface conditions and the need to analyse measurements in ways that can be transferred between sites and storms. Copyright © 2010 John Wiley & Sons, Ltd.
Article
A spatially distributed soil erosion and sediment delivery model (WATEM/SEDEM) was applied to the Scheldt River Basin (19,000[no-break space]km2) using SRTM elevation data with a 3'' resolution, and CORINE Land Cover data which are available at a resolution of 100[no-break space]m. Transport capacity coefficients in the model were first calibrated using observed sediment yield data and WATEM/SEDEM predictions made with a higher resolution DEM derived from contour maps. When optimal transport capacity values are used, the calibrated model with SRTM data has an overall model efficiency of 0.79 for area-specific sediment yield and 0.95 for total sediment yield. R-square values between observed and predicted sediment yields are > 0.8. Optimal calibration values are much smaller than those obtained from a higher resolution model, illustrating the need for recalibrating distributed models when input data with different accuracies or resolution are used. Application of the calibrated model to the Scheldt River Basin estimated the total sediment supply from hillslopes to the river channels in the basin at 1.9 x 106[no-break space]t year- 1. Model results indicate a large spatial variability in hillslope sediment delivery, with the major sediment sources situated in the upper parts of the river basin. It is shown that the decrease in area-specific sediment yield with increasing catchment area can already be explained by the increasing importance of lower slope gradients in the lower parts of the river basin, without taking into account of floodplain sediment storage.
Article
Physically based, distributed hydrologic models are increasingly used in assessments of water resources, best management practices, and climate and land use changes. Model performance evaluation in ungauged basins is an important research topic. In this study, we propose a framework for developing Soil and Water Assessment Tool (SWAT) input data, including hydrography, terrain, land use, soil, tile, weather, and management practices, for the Upper Mississippi River basin (UMRB). We also present a performance evaluation of SWAT hydrologic budget and crop yield simulations in the UMRB without calibration. The uncalibrated SWAT model ably predicts annual streamflow at 11 USGS gauges and crop yield at a four-digit hydrologic unit code (HUC) scale. For monthly streamflow simulation, the performance of SWAT is marginally poor compared with that of annual flow, which may be due to incomplete information about reservoirs and dams within the UMRB. Further validation shows that SWAT can predict base flow contribution ratio reasonably well. Compared with three calibrated SWAT models developed in previous studies of the entire UMRB, the uncalibrated SWAT model presented here can provide similar results. Overall, the SWAT model can provide satisfactory predictions on hydrologic budget and crop yield in the UMRB without calibration. The results emphasize the importance and prospects of using accurate spatial input data for the physically based SWAT model. This study also examines biofuel-biomass production by simulating all agricultural lands with switchgrass, producing satisfactory results in estimating biomass availability for biofuel production.
Article
Water erosion creates negative impacts on agricultural production, infrastructure, and water quality across the world. Regional-scale water erosion assessment is important, but limited by data availability and quality. Satellite remote sensing can contribute through providing spatial data to such assessments. During the past 30 years many studies have been published that did this to a greater or lesser extent. The objective of this paper is to review methodologies applied for water erosion assessment using satellite remote sensing. First, studies on erosion detection are treated. This comprises the detection of erosion features and eroded areas, as well as the assessment of off-site impacts such as sediment deposition and water quality of inland lakes. Second, the assessment of erosion controlling factors is evaluated. Four types of factors are discussed: topography, soil properties, vegetation cover, and management practices. Then, erosion mapping techniques are described that integrate products derived front satellite remote sensing with additional data sources. These techniques include erosion models and qualitative methods. Finally, validation methods used to assess the accuracy of maps produced with satellite data are discussed. It is concluded that a general lack of validation data is a main concern. Validation is of utmost importance to achieve regional operational monitoring systems, and close collaboration between the remote sensing community and field-based erosion scientists is therefore required. (C) 2005 Elsevier B.V. All rights reserved.