Evaluating an integrated approach to catchment management to reduce soil loss and sediment pollution through modelling

Abstract. Soil erosion and sediment delivery cause many environmental problems posing a substantial financial burden upon society. Policy makers therefore look for a strategy to minimize their impact. The spatial nature of soil erosion and sediment delivery, as well as the variety of possible soil conservation and sediment control measures, requires an integrated approach to catchment management. To evaluate such management, a spatially distributed soil erosion and sediment delivery model is necessary. Such a model (WaTEM/SEDEM) was applied to three agricultural catchments in Flanders (Belgium). The model was first used to identify where the measures to control soil loss should be taken. Secondly, a scenario analysis was used to select the most effective set of techniques. The findings showed that soil conservation measures taken in fields are not only effective in reducing on-site soil loss, but also in drastically reducing sediment yield. Off-site sediment control measures appear to be much less effective in reducing sediment yield than previously thought. The results also suggest that data from field experiments cannot be extrapolated to a catchment scale.     

Modelling of snowmelt erosion and sediment yield in a small low-mountain catchment in Germany

Temporal variability and spatial heterogeneity of surface runoff generation triggers the dynamics of source areas of sediment and sediment-associated nutrient transport. Reliable modelling of hydrological special situations i.e. snowmelt is of high importance for the quality of erosion and sediment yield modelling. Data from the research catchment Schäfertal demonstrate the individuality of snowmelt events in terms of runoff coefficient and delivery ratio. This 1.44 km² low mountain catchment is characterised by a high portion of arable land with a winter grain/winter rape crop rotation. The integrated winter erosion and nutrient load model (IWAN) considers these dynamic aspects by coupling a hydrological model with a sediment load model. Cell size of this raster-based approach is 10 × 10 m². Additionally, snowmelt rill erosion is simulated with a newly developed physically based model that is firstly applied on a catchment scale. A sensitivity analysis of this model system component demonstrates the plausibility of the model approach and the overall robustness of the model system IWAN. The results of the long-term hydrological modelling from 1991 to 2003 are reliable and form the basis for the simulation of six snowmelt events which were observed in the Schäfertal catchment. The estimated total runoff volumes for these events match the observations well. The modelled overland runoff coefficients vary from 0.001 to 0.72. The mean values of cell erosion, which were modelled with one set of parameters for all six events range from 0.0006 to 0.96 t ha^− 1. The total modelled erosion for the events with unfrozen soil and low amount of surface runoff is of a factor 50 below those with partly frozen soil. In addition to these distinctions, the major differences are caused by flow accumulation in shallow depressions in variable parts of the catchment. However, the validation of these results on the single event scale is restricted due to limited spatial data. Total simulated sediment yield at the catchment outlet was as high as 13.84 t which underestimates the observed values, with the exception of one event. Oversimplification of the modelled channel processes may be a reason. The temporal variability and spatial heterogeneity of the surface roughness parameter, which was identified to be sensitive, also causes uncertainty in the parameter estimation. Despite these findings, the model system IWAN was applied successfully on the catchment scale and the simulated results are reliable.     

Modelling of snowmelt erosion and sediment yield in a small low-mountain catchment in Germany

Temporal variability and spatial heterogeneity of surface runoff generation triggers the dynamics of source areas of sediment and sediment-associated nutrient transport. Reliable modelling of hydrological special situations i.e. snowmelt is of high importance for the quality of erosion and sediment yield modelling. Data from the research catchment Schäfertal demonstrate the individuality of snowmelt events in terms of runoff coefficient and delivery ratio. This 1.44 km² low mountain catchment is characterised by a high portion of arable land with a winter grain/winter rape crop rotation. The integrated winter erosion and nutrient load model (IWAN) considers these dynamic aspects by coupling a hydrological model with a sediment load model. Cell size of this raster-based approach is 10 × 10 m². Additionally, snowmelt rill erosion is simulated with a newly developed physically based model that is firstly applied on a catchment scale. A sensitivity analysis of this model system component demonstrates the plausibility of the model approach and the overall robustness of the model system IWAN. The results of the long-term hydrological modelling from 1991 to 2003 are reliable and form the basis for the simulation of six snowmelt events which were observed in the Schäfertal catchment. The estimated total runoff volumes for these events match the observations well. The modelled overland runoff coefficients vary from 0.001 to 0.72. The mean values of cell erosion, which were modelled with one set of parameters for all six events range from 0.0006 to 0.96 t ha^− 1. The total modelled erosion for the events with unfrozen soil and low amount of surface runoff is of a factor 50 below those with partly frozen soil. In addition to these distinctions, the major differences are caused by flow accumulation in shallow depressions in variable parts of the catchment. However, the validation of these results on the single event scale is restricted due to limited spatial data. Total simulated sediment yield at the catchment outlet was as high as 13.84 t which underestimates the observed values, with the exception of one event. Oversimplification of the modelled channel processes may be a reason. The temporal variability and spatial heterogeneity of the surface roughness parameter, which was identified to be sensitive, also causes uncertainty in the parameter estimation. Despite these findings, the model system IWAN was applied successfully on the catchment scale and the simulated results are reliable.     

Using Cs measurements to validate the application of the AGNPS and ANSWERS erosion and sediment yield models in two small Devon catchments

Distributed erosion and sediment yield models are being increasingly used for predicting soil erosion and sediment yields in agricultural catchments. In most applications, validation of such models has commonly been restricted to comparison of the predicted and measured sediment output from a catchment, because spatially distributed information on rates and patterns of soil redistribution within the catchment has been lacking. However, such spatially distributed data are needed for rigorous model testing, in order to validate the internal functioning of a model and its applicability at different spatial scales. The study reported in this paper uses two approaches to test the performance of the agricultural non-point source pollution (AGNPS) and areal non-point source watershed environmental response simulation (ANSWERS) erosion and sediment yield models in two small catchments in Devon, UK. These involve, firstly, comparison of observed and predicted runoff and sediment output data for individual storm events monitored at the basin outlets and, secondly, information on the spatial pattern of soil redistribution within the catchments derived from ¹³⁷Cs measurements. The results obtained indicate that catchment outputs simulated by both models are reasonably consistent with the recorded values, although the AGNPS model appears to provide closer agreement between observed and predicted values. However, the spatial patterns of soil redistribution and the sediment delivery ratios predicted for the two catchments by the AGNPS and ANSWERS models differ significantly. Comparison of the catchment sediment delivery ratios and the pattern of soil redistribution in individual fields predicted by the models with equivalent information derived from ¹³⁷Cs measurements indicates that the AGNPS model provides more meaningful predictions of erosion and sediment yield under UK conditions than the ANSWERS model and emphasises the importance of using information on both catchment output and sediment redistribution within the catchment for model validation.     

Factors affecting soil loss at plot scale and sediment yield at catchment scale in a tropical volcanic agroforestry landscape

Tropical deforestation and land use change is often perceived as the major cause of soil loss by water erosion and of sediment load in rivers that has a negative impact on the functioning of hydropower storage reservoirs. The Sumberjaya area in Sumatra, Indonesia is representative for conflicts and evictions arising from this perception. The purpose of this study as part of a Negotiation Support System approach was to assess sediment yield both at plot and catchment scale and to relate it to a variety of possible clarifying factors i.e. land use, geology, soil and topography. Sediment yield at catchment scale per unit area, was found to be 3–10 times higher than soil loss measured in erosion plots. A stepwise regression showed that the dominant factors explaining sediment yield differences at catchment scale in this volcanic landscape were a particular lithology (Old Andesites) and slope angle followed by the silt fraction of the top soil. In lithologically sensitive areas soil loss at the plot scale under monoculture coffee gardens decreases over time from on average 7–11 Mg ha^? 1 yr^? 1 to 4–6 Mg ha^? 1 yr^? 1, mainly because of the development of surface litter layers as filters and top soil compaction in the areas without litter, but remains higher than under shade coffee systems or forest. The runoff coefficient under monoculture coffee remains on average significantly higher (10–15%) than under forest (4%) or under shade coffee systems (4–7%). In lithologically stable areas soil loss remained below 1.8 Mg ha^? 1 yr^? 1 and the runoff coefficient below 2.5% under all land use types, even bare soil plots or monoculture coffee gardens. Less than 20% of the catchment area produces almost 60% of the sediment yield. The reduction of negative off-site effects on e.g. the life time of a storage reservoir would benefit greatly from an improved assessment of the lithologies in volcanic landscapes and the consideration of potential sediment source and sink areas. In lithologically sensitive areas, a shift from sun to shade coffee systems may result in reducing surface runoff and soil loss, although water erosion at the plot scale is not the main contributor to sediment yield at the catchment scale. The quantification of land use effects on dominant erosive processes such as river bank and river bed erosion, landslides and the concentrated flow erosion on footpaths and roads can contribute to more targeted efforts and relevant incentives to reduce (or live with) sediment load of the rivers.     

Integrated assessment of catchment suspended sediment budgets: a Zambian example

In many developing countries, the management of sediment‐related environmental problems is severely hampered by a lack of information on sediment mobilization and delivery in river basins. The sediment budget concept represents a valuable framework for assembling such information, which can, in turn, be used to assist with the design and implementation of soil erosion and sediment control policies. However, the information necessary to construct a catchment sediment budget is difficult to assemble. Against this background, an integrated approach to establishing a catchment suspended sediment budget, involving a river monitoring station, the use of ¹³⁷Cs measurements to estimate soil erosion and deposition and floodplain accumulation rates within the catchment, and sediment source fingerprinting, has been developed and tested in the 63 km² catchment of the upper Kaleya River in southern Zambia. The approach developed not only provides detailed information on individual components of the suspended sediment delivery system, but also permits the establishment of the overall catchment sediment budget. A sediment budget for the upper Kaleya catchment is presented and both its key features and its wider implications for catchment management are discussed. Copyright © 2001 John Wiley & Sons, Ltd.     

Description of hydrological and erosion processes determined by applying the LISEM model in a rural catchment in southern Brazil

Purpose

Hydrosedimentological modeling is a tool that can be used to understand better important processes occurring at the catchment scale, such as runoff and sediment yield. The aim of this study was to use the Limburg Soil Erosion Model (LISEM) to describe the runoff and sediment yield during rainfall–runoff events in a small rural catchment in southern Brazil.

Materials and methods

The study was conducted in the Lajeado Ferreira Creek catchment (drainage area of 1.19 km²) where intense land use has caused a negative impact on water resources. Thirteen rainfall–runoff events that occurred in 2010 and 2011, including high-magnitude events, were used to model hydrosedimentological processes.

Results and discussion

Results

from the calibration and validation stages indicate that the model had a good performance when representing the hydrograph, including events with greater complexity. The use of a second soil layer in the model increased its efficiency, which is in accordance with the importance of subsurface flow in this catchment and its sensitivity to the physical properties of the soil, which are essential for controlling hydrosedimentological processes at the catchment scale. The simulation of sediment yield was overestimated by the model, constrained by the lack of sensitivity of the model to soil cohesion and the stability of soil aggregates. During the model calibration stage, these parameters had values different from those measured in the field.

Conclusions

The LISEM model performed well in representing runoff for events of different magnitudes. The discretization of the physical–hydrologic properties in the soil profile enabled the evaluation of the effect of subsurface impediment layers on water infiltration and runoff. The simulation was less accurate for suspended sediment concentration than for runoff. This indicates the need for further studies to either identify other factors controlling erosion and sediment yield that have not been identified by the model, or identify if the representation of the physical parameters is inadequate, especially the values of soil cohesion and aggregate stability.     

Soil erosion assessment of Lake Alemaya catchment,Ethiopia

Land degradation due to soil erosion is the major problem facing Ethiopia today. In the Lake Alemaya catchment soil erosion is caused by the intense rainfall, steep topography, and poor vegetation cover coupled with cultivation of steep lands, and inadequate conservation practices. Sediment from the catchment has affected the storage capacity of Lake Alemaya. This study has integrated the Agricultural Non‐point Source Pollution Model (AGNPS) and the technique of the Gographic Information System (GIS) to quantify soil erosion in the Lake Alemaya catchment. After application of the AGNPS, it appears that 66 per cent of the catchment has a soil erosion rate of 10 to more than 80 t ha⁻¹ y⁻¹. The annual soil loss is estimated at 31 t ha⁻¹, which is more than the permissible value of 1–16 t ha⁻¹ for different agro‐ecological zones of Ethiopia. The sediment yield of the catchment is about 10 148 ton with a delivery ratio of 6·82 per cent. Therefore, an effective management plan is needed for the conservation and rehabilitation of the catchment and to maintain the storage capacity of Lake Alemaya. Copyright © 2006 John Wiley & Sons, Ltd.     

ASSESSING THE PERFORMANCE OF A SPATIALLY DISTRIBUTED SOIL EROSION AND SEDIMENT DELIVERY MODEL (WATEM/SEDEM) IN NORTHERN ETHIOPIA

Most regional‐scale soil erosion models are spatially lumped and hence have limited application to practical problems such as the evaluation of the spatial variability of soil erosion and sediment delivery within a catchment. Therefore, the objectives of this study were as follows: (i) to calibrate and assess the performance of a spatially distributed WATEM/SEDEM model in predicting absolute sediment yield and specific sediment yield from 12 catchments in Tigray (Ethiopia) by using two different sediment transport capacity equations (original and modified) and (ii) to assess the performance of WATEM/SEDEM for the identification of critical sediment source areas needed for targeting catchment management. The performance of the two model versions for sediment yield was found promising for the 12 catchments. For both versions, model performance for the nine catchments with limited gully erosion was clearly better than the performance obtained when including the three catchments with significant gully erosion. Moreover, there is no significant difference (alpha 5 per cent) between the performances of the two model versions. Cultivated lands were found to be on average five times more prone to erosion than bush–shrub lands. The predicted soil loss values in most parts of Gindae catchment are generally high as compared with the soil formation rates. This emphasises the importance of implementing appropriate soil and water conservation measures in critical sediment source areas prioritising the steepest part of the catchment (i.e. areas with slope >50 per cent). The applicability of the WATEM/SEDEM model to environments where gully erosion is important requires the incorporation of permanent gully and bank gully erosion in the model structure. Copyright © 2011 John Wiley & Sons, Ltd.     

三峡库区小流域修正通用土壤流失方程适用性分析

土壤侵蚀量的定量研究可为国家生态环境建设和水土保持宏观决策的制定提供重要的依据。修正通用土壤流失方程(revised universal soil loss equation,RUSLE)是开展土壤侵蚀定量评价的主要手段。该文在地理信息系统(geographic information system,GIS)的支持下,依据中国土壤流失方程各因子的算法确定RUSLE模型各因子值,估算了三峡库区黄冲子小流域不同时期的土壤侵蚀量,并与基于泥沙平衡原理计算的土壤侵蚀量比较后分析RUSLE模型在库区小流域的适用性。结果表明,基于RUSLE模型估算的小流域1963-2000年(农地小流域)和2001-2014年(林地小流域)的年均土壤侵蚀模数分别为2246.09和868.3 t/(km2·a),其结果与采用137Cs和210Pb技术的塘库沉积物定年结果基本吻合,表明210Pb定年结果可靠。依据泥沙平衡原理计算的小流域1963-2000年和2001-2014年的年均土壤侵蚀模数分别为942.48和811.47t/(km2·a)。RUSLE模型估算小流域1963-2000年和2001-2014年的土壤侵蚀模数相对误差分别为138.32%和7.00%。因此RUSLE模型适用于库区林地小流域,而不适用于库区农地小流域;但是基于地形因子(LS因子)修正的RUSLE模型估算结果相对误差减少至8.14%,其适用于库区农地小流域。     

Quantifying farmland shelterbelt impacts on catchment soil erosion and sediment yield for the black soil region,northeastern China

As one part of the ‘Three Norths’ forest protection system, dense farmland shelterbelt networks in northeastern China could greatly modify water and sediment flows. In this paper, catchment soil erosion rate and sediment yield (SY) that are impacted by farmland shelterbelts were estimated using WaTEM/SEDEM model. The shelterbelts reduced catchment soil erosion and SY to some extent. The mean soil erosion rate and specific sediment yield (SSY; defined as the ratio of SY to catchment area; t km^?2 yr^?1) of the 25 reservoir catchments decreased from 351.6 and 93.9 t km^?2 yr^?1 under the supposed scenario without shelterbelts to 331.1 t km^?2 yr^?1 and 86.3% t km^?2 yr^?1 under the current situation with shelterbelts. The sediment trap efficiencies (STEs) varied from 0.01% to 23.6% with an average value of 7.6%. The STEs were significantly correlated with shelterbelt density, catchment perimeter, topographic factors, RUSLEP‐factor and land use patterns including patch density (PD), patch cohesion index (COHESION), Shannon's diversity index (SHDI) and aggregation index (AI). The multiple regression equation involving factors of catchment's topography and morphology and land use pattern has a satisfactory performance, and mean slope gradient (MSG) and AI explained most of the variability of shelterbelts’ STE. This information can help land managers to better design shelterbelts and to reduce water‐derived soil loss at catchment scale.     

A pragmatic approach to modelling soil and water conservation measures with a catchment scale erosion model

To reduce soil erosion, soil and water conservation (SWC) methods are often used. However, no method exists to model beforehand how implementing such measures will affect erosion at catchment scale. A method was developed to simulate the effects of SWC measures with catchment scale erosion models. The method was implemented by applying the LISEM model to an agricultural catchment on the slopes of Mt. Kenya. The method consisted of a field scale calibration based on P-factors, followed by application at catchment scale. This calibration included factors such as saturated conductivity, Manning's n, roughness and slope angle. It was found that using data on P-factors, such models can be calibrated to give acceptable predictions at pixel scale. However, P-factors were also found to vary with land use type and storm size. Besides, more data on the physical effectiveness of SWC measures are needed. At catchment scale, the effect of SWC was found to be different from that at pixel scale. Most SWC were simulated to be more effective at catchment scale, indicating additional infiltration during transport through the catchment to the outlet. However, slope corrections in case of terraces were found to be less effective at this scale. Nevertheless, a simulation for current land use with current SWC measures indicated that these SWC measures decrease runoff by 28% and erosion by 60%.     

Comparing empirical models for sediment and phosphorus transfer from soils to water at field and catchment scale under data uncertainty

Soils are important sources of sediment and phosphorus in rural catchments, necessitating the development of mathematical models for impact assessment. In this paper, multiple empirical models are tested on an event basis at four nested locations in an intensively managed grassland headwater catchment while accounting for parameter and data uncertainties using extended Generalized Likelihood Uncertainty Estimation (GLUE). The study provides the first template of model comparison under data uncertainty in soil erosion and phosphorus transfer modelling as well as hypotheses of soil and water processes in the study catchment. A fodder field, yielding large sediment and phosphorus concentrations in runoff, is characterized by inter‐event variation in sediment‐discharge relationship, mild intra‐event hysteretic behaviour and seemingly random erosion incidents. Sediment‐discharge variation is partly formalized by parameter variation as a function of antecedent soil moisture, indicative of a gradual shift from transport‐ to source‐limited behaviour, decreasing soil erodibility and/or decreasing initial flow erosivity and transport capacity with increasing antecedent wetness. The catchment outlet appears to be source‐limited while converging flows with different sediment concentrations, variable erosion processes and/or sporadic entrainment of near‐ or in‐stream sediments gain importance. Phosphorus dynamics are strongly linked to those of sediment. Non‐linearities can be explained by preferential transfer of phosphorus‐rich organic matter at small flows while there is no significant evidence of preferential transfer of phosphorus‐rich mineral fines. Iterating between collecting data, constraining uncertainties and rejecting and improving models is suggested as a consistent framework for understanding soil erosion and phosphorus movement.     

Application of catchment scale sediment delivery model INCA-Sed to four small study catchments in Finland

The novel catchment scale erosion and sediment delivery model INCA-Sed was applied to four small study catchments in Finland. Three of these, the Mustajoki, Haarajoki and Luhdanjoki, are headwater catchments located in central Finland. The associated rivers have differing morphological characteristics varying from a ditch to a small river. Soil textures in the area are derived from moraine deposits and are largely sand and gravel. The Mustajoki and Haarajoki catchments are forested and only 10% of the area is under cultivation. In the Luhdanjoki catchment agricultural fields cover 40% of the area. The fourth study site, the Savijoki catchment, represents an intensively cultivated area in south-western Finland. Cultivated fields cover 40% of the catchment area, and they are located on clay soils along the river. The INCA-Sed model was able to capture both the correct magnitude and seasonal behaviour of suspended sediment concentrations in the rivers, as well as the correct magnitude of the sediment load derived from different land use classes. Small differences in river morphology and soil textures between the catchments have a significant influence on suspended sediment concentration in the rivers. Correct timing of suspended sediment concentration peaks is not, however, captured by the INCA-Sed model, which may be due to the stochastic nature of erosion and delivery processes at the catchment scale which are not taken into account in the parameter values used in the modelling. Parameter values were estimated from previous researches based on average process loads. The INCA-Sed model was, however, generally found to be a suitable tool for evaluating effects of land use change on erosion and sediment delivery in Finland as it correctly reproduces spatial and seasonal variations in sediment delivery, in addition to annual averages with spatial and temporal variations.     

Comparison between rainfall simulator erosion and observed reservoir sedimentation in an erosion-sensitive semiarid catchment

Estimating catchment scale soil loss based on rainfall simulators is often hampered by the difficulty to scale up simulator results. Our objective was to develop and test a method for estimating catchment scale soil loss based on observed rainfall using a variable intensity rainfall simulator in an erosion-sensitive catchment in semiarid Tunisia. A 7-year period, 1992–1999, with observed sedimentation amounts in a downstream reservoir was chosen to test a methodology. The methodology was based on (1) energy adjustment for the used simulator due to the difference in kinetic energy of simulated and natural rainfall at equal intensities and (2) upscaling of simulated erosion in which rill erosion was estimated by adjusting the difference between slope lengths for the plots versus the catchment after onset of runoff. The comparison between calculated soil loss from rainfall simulator experiments and observed sedimentation in the downstream reservoir displayed good overall results. Calculated soil loss was found to be about 96%, 36%, and 80% for different observed subperiods, respectively. The observed low value for the second period was probably due an exceptionally intense rainfall event during this period, which appears to have led to gully erosion, soil slide, and riverbank collapse. Therefore, during this event, siltation in the reservoir may essentially be due to unaccounted erosion processes such as gully erosion. Overall, however, it appears that plot-scale variable intensity rainfall simulators can rather successfully estimate catchment scale soil losses.     

GIS支持下小流域农林复合经营的侵蚀控制模拟——以冀西北青边口河小流域为例

根据张家口郭家梁试验场的小区资料 ,建立了小流域侵蚀产沙模型以及植物篱侵蚀控制模型 ;并在 GIS的支持下 ,从小流域数字高程模型 (DEM)中提取基于地块间水沙汇流网络 ,生成基于地块的水沙运移网络图 ,并将地块间的水沙汇流过程引入小流域侵蚀产沙的模拟中 ,实现了侵蚀产沙模型与地理信息系统 (GIS)的深层次耦合 ,模拟出坡面水沙在小流域的空间运动过程。1995～ 1999年青边口河小流域模拟结果表明 ,坡耕地、荒坡地、低覆盖的天然草地平均侵蚀模数最大 ,是侵蚀控制的重点土地利用类型。在坡耕地、荒坡地、低覆盖的天然草地配置面积占流域总面积的 18.2 %的植物篱—农作系统 (10 m带间距 2年生紫穗槐植物篱 ) ,利用次降雨资料计算模拟结果表明 ,对径流的控制效果在 8.1%～ 46 .2 %之间 ,对侵蚀模数的控制效果在 42 .9%～ 5 0 .2 %之间 ;养分流失的模拟分析表明 ,在现有条件下有机质的损失较大 ,而通过配置植物篱—农作系统可以有效地减少养分流失。     

Application of the EROSION 3D model to the CATSOP watershed, The Netherlands

This paper describes the application of the EROSION 3D soil erosion model to the CATSOP catchment in the Netherlands using data of 10 storms from the period of 1987 to 1993. Based on observed runoff data the model was calibrated by adjusting initial soil moisture. The computations result in a raster map showing the spatial pattern of predicted erosion and deposition within the catchment. The map corresponds qualitively well with reality. For a quantitative evaluation, the model results were compared with observed sediment data measured at the channel when it leaves the catchment. The comparison shows that simulated soil loss was generally too high. This discrepancy might be caused by weak input data and the presence of processes—such as erosion and deposition within the channel—which are not covered by the model.     

A multi-scale approach of runoff generation in a Sahelian gully catchment: a case study in Niger

Runoff production conditions in a small gully catchment are studied at four different scales: the point scale (0.001 m²), the local scale (1 m²), the field scale (of the order of 100 m²) and the catchment scale (0.2 km²). At the point scale, infiltration measurements were conducted using a tension infiltrometer. At the local and the field scale, runoff plots were setup on typical soil surface conditions of the catchment (plateau bare soil, hillslope bare soil and fallow grassland). At the catchment scale, stream discharges were measured at two gauging stations.The overland flow yield is significantly nonuniform in space, due to the high spatial variability of infiltration capacities and the depressional storage of the soil surface. The runoff and the infiltration data collected confirmed the major role played by soil crusting on runoff generation in that part of Sahel. At the point scale, hydraulic conductivity measurements have shown that infiltration and runoff were driven by the hydraulic properties of the crust. At the field scale, microtopography and heterogeneity in the soil surface crusting decreased discharge volumes. The influence of vegetation growth on runoff yield was evident in the case of the fallow sites. Analysis of discharge data at the catchment scale highlights that infiltration through the bottom of the gully between two gauging stations leads to considerable runoff water transmission losses.     

Spatial variation and interaction of runoff generation and erosion within a semi-arid, complex terrain catchment: a hierarchical approach

Purpose

Closed erosion plots have been used extensively to investigate soil loss and its spatial variation within a watershed. However, erosion rates measured on closed plots at various locations within a watershed may not reflect the “real world” conditions due to plot boundary problems. The purpose of this study was to identify runoff and sediment sources in a semi-arid, complex terrain catchment by using the data collected from open plots, nested catchments, and tunnel systems.

Materials and methods

The study catchment, in the Loess Plateau of China, was partitioned into various-level geomorphic units. Runoff and sediment discharges were measured from 55 storm events between 1963 and 1968 on open plots and nested catchments. Storm flows were also monitored in 14 rainfall events from the tunnel systems between 1989 and 1990. This study combined the data collected from the two periods to investigate runoff and sediment sources from the different geomorphic units of the catchment.

Results and discussion

On the four open plots (S1, S2, S3, and S4) of the hill slope, total runoff depths of 128.5 mm (S1), 84.3 mm (S2), 101.92 mm (S3), and 141.73 mm (S4) were recorded from all the events over the first period, which correspondingly produced total sediment yields of 3.056 kg m^?2 (S1), 9.058 kg m^?2 (S1), 42.848 kg m^?2 (S3), and 97.256 kg m^?2 (S4). The number of runoff events also varied due to a non-uniformity in runoff generation among the different geomorphic units of the catchment. Tunnel flows generally had higher mean sediment concentrations than catchment outflows. Three nested catchments located from the headwaters (C1) to the mouth of the catchment (C3) generated total runoff depths of 120.02 mm (C1), 143.92 mm (C2), and 149.43 mm (C3), and correspondingly produced sediments yields of 62.01 kg m^?2 (C1), 144.02 kg m^?2 (C2), and 123.92 kg m^?2 (C3) for the first period.

Conclusions

Significant variations in runoff and erosion existed within the catchment. The spatial variation of runoff generation on the hill slopes resulted from the variation of soil infiltration. Sediment produced from the lower hill slope zone was disproportionally higher than that from the upper hill slope zone. Nevertheless, a significant portion of the sediment eroded on the lower slope zone was caused by runoff generated from the upper slope zone. Tunnel erosion also played a significant role in sediment production.     

A simple approach to soil loss prediction: a revised Morgan–Morgan–Finney model

A revised version of the Morgan–Morgan–Finney model for prediction of annual soil loss by water is presented. Changes have been made to the way soil particle detachment by raindrop impact is simulated, which now takes account of plant canopy height and leaf drainage, and a component has been added for soil particle detachment by flow. When tested against the same data set used to validate the original version at the erosion plot scale, predictions made with the revised model gave slopes of a reduced major-axis regression line closer to 1.0 when compared with measured values. The coefficient of efficiency, for sites with measured runoff and soil loss, increased from 0.54 to 0.65. When applied to a new data set for erosion plots in Denmark, Spain, Greece and Nepal, very high coefficients of efficiency of 0.94 for runoff and 0.84 for soil loss were obtained. The revised version was applied to two small catchments by dividing them into land elements and routing annual runoff and sediment production over the land surface from one element to another. The results indicate that, when used in this way, the model provides useful information on the source areas of sediment, sediment delivery to streams and annual sediment yield.