Spatial scale dependency issues in the application of the Modified Universal Soil Loss Equation (MUSLE)

ABSTRACT

The MUSLE is used within hydrological models to estimate sediment yields from catchments of various sizes, but the spatial scale dependency issues associated with estimating the MUSLE parameters have not been adequately addressed. In the absence of detailed observed data on both hydrological response and sediment yield, some analytical approaches and hypothetical examples are presented to identify the key issues. The results suggest that methods used to estimate both the erosivity and topographic factors are scale dependent, particularly if a lumped or semi-distributed modelling approach is used. The conclusion is that spatial scale dependencies will add to the uncertainties inherent in the use of the MUSLE if not carefully understood and appropriately addressed. One suggested approach is to apply the erosivity equation to a fixed (small) representative area and then scale up to the total catchment, an approach that recognizes the variability of averaged parameters across different spatial scales.     

A comparative analysis of sediment yield simulation by empirical and process-oriented models in Thailand / Une analyse comparative de simulations de l'exportation sédimentaire en Thaïlande à l'aide de modèles empiriques et de processus

Abstract

Although soil erosion has been recognized worldwide as a threat to the sustainability of natural ecosystems, its quantification presents one of the greatest challenges in natural resources and environmental planning. Precise modelling of soil erosion and sediment yield is particularly difficult, as soil erosion is a highly dynamic process at the spatial scale. The main objective of this study was to simulate soil erosion and sediment yield using two fundamentally different approaches: empirical and process-oriented. The revised form of the Universal Soil Loss Equation (RUSLE), along with a sediment delivery distributed model (SEDD) and the Modified Universal Soil Loss Equation (MUSLE), which are popular empirical models, were applied in a sub-basin of the Mun River basin, Thailand. The results obtained from the RUSLE/SEDD and MUSLE models were compared with those obtained from a process-oriented soil erosion and sediment transport model. The latter method involves spatial disaggregation of the catchment into homogeneous grid cells to capture the catchment heterogeneity. A GIS technique was used for the spatial discretization of the catchment and to derive the physical parameters related to erosion in the grid cells. The simulated outcomes from the process-oriented model were found to be closer to observations as compared to the outcomes of the empirical approaches.     

Evaluation of the SEDD model for predicting sediment yield at the Sicilian experimental SPA2 basin

In this paper a spatially distributed model of the hillslope sediment delivery processes, named the sediment delivery distributed (SEDD) model, is initially reviewed; the model takes into account the sediment delivery processes due to both the hillslope sediment transport and the effects of slope curvature. Then the rainfall and sediment yield events measured at the experimental SPA2 basin, in Sicily, are used both to calibrate the SEDD model and to verify the predictive capability of the distributed sediment delivery approach at event scale. For the SPA2 basin discretized into morphological units and stream tubes, the SEDD model is calibrated at event scale using the measurements carried out at the outlet of the experimental basin in the period December 2000–January 2001. The model calibration is used to determine a relationship useful for estimating the unique coefficient β_e of the model by rainfall erosivity factor R_e at event scale. To test the predictive capability of the β_e = f(R_e) relationship, 20 events measured in the period September 2002–December 2005 are used; the comparison between measured sediment yield values and calculated ones for all monitored events shows that the sediment delivery distributed approach has a good predictive ability at event scale. The analysis also shows that estimating β_e by the relationship β_e = f(R_e) gives a better agreement between measured and calculated sediment yields than obtained with the median value β_e,m of all 27 calculated β_e values. Finally the analysis at annual scale, for the period 2000–2005, allows the estimation of the median value β_a,m representative of the annual behaviour. This analysis shows that the sediment delivery distributed approach also has a good predictive ability at annual scale. Copyright © 2006 John Wiley & Sons, Ltd.     

Comparative application of two erosion models to a basin

Abstract

Two mathematical models were used to estimate the annual sediment yield resulting from rainfall and runoff at the outlet of the Nestos River basin (Toxotes, Thrace, Greece). The models were applied to that part of the Nestos River basin (838 km²) which lies downstream of three dams. Both models consist of three submodels: a simplified rainfall-runoff submodel, a physically-based surface erosion submodel and a sediment transport submodel for streams. The two models differ only in the surface erosion submodel: that of the first model is based on the relationships of Poesen (1985) for splash detachment and splash transport, while the corresponding submodel of the second model is based on the relationships of Schmidt (1992) for the momentum flux exerted by the droplets and the momentum flux exerted by the overland flow. The degree of conformity between the annual values of sediment yield at the basin outlet according to both models is satisfactory.     

A transport‐distance approach to scaling erosion rates: 1. Background and model development

The process basis of existing soil‐erosion models is shown to be ill‐founded. The existing literature builds directly or indirectly on Bennett's (1974) paper, which provided a blueprint for integrated catchment‐scale erosion modelling. Whereas Bennett recognized the inherent assumptions of the approach suggested, subsequent readings of the paper have led to a less critical approach. Most notably, the assumption that sediment movement could be approximated by a continuity equation that related to transport in suspension has produced a series of submodels that assume that all movement occurs in suspension. For commonly occurring conditions on hillslopes, this case is demonstrably untrue both on theoretical grounds and from empirical observations. Elsewhere in the catchment system, it is only partially true, and the extent to which the assumption is reasonable varies both spatially and temporally. A second ground‐breaking paper – that of Foster and Meyer (1972) – was responsible for subsequent uncritical application of a first‐order approximation to deposition based on steady‐state analysis and again a weak empirical basis. We describe in this paper an alternative model (Mahleran – Model for Assessing Hillslope‐Landscape Erosion, Runoff And Nutrients) based upon particle‐travel distance that overcomes existing limitations by incorporating parameterizations of the different detachment and transport mechanisms that occur in water erosion in hillslopes and small catchments. In the second paper in the series, we consider the sensitivity and general behaviour of Mahleran , and test it in relation to data from a large rainfall‐simulation experiment. The third paper of the sequence evaluates the model using data from plots of different sizes in monitored rainfall events. From this evaluation, we consider the scaling characteristics of the current form of Mahleran and suggest that integrated modelling, laboratory and field approaches are required in order to advance the state of the art in soil‐erosion modelling. Copyright © 2008 John Wiley & Sons, Ltd.     

A comprehensive analysis of Universal Soil Loss Equation-based models at the Sparacia experimental area

Improving Universal Soil Loss Equation (USLE)-based models has large interest because simple and reliable analytical tools are necessary in the perspective of a sustainable land management. At first, in this paper, a general definition of the event rainfall- runoff erosivity factor for the USLE-based models, REF_e = (Q_R)^b1(EI₃₀)^b2, in which Q_R is the event runoff coefficient, EI₃₀ is the single-storm erosion index, and b₁ and b₂ are coefficients, was introduced. The rainfall-runoff erosivity factors of the USLE (b₁ = 0 and b₂ = 1), USLE-M (b₁ = b₂ = 1), USLE-MB (b₁ ≠ 1 and b₂ = 1), USLE-MR (b₁ = 1 and b₂ ≠ 1), USLE-MM (b₁ = b₂ ≠ 1), and USLE-M2 (b₁ ≠ b₂ ≠ 1) can be defined using REF_e. Then the different expressions of REF_e were simultaneously tested against a data set of normalized bare plot soil losses, A_eN, collected at the Sparacia (south Italy) site. As expected, the poorest A_eN predictions were obtained with the USLE. The observed tendency of this model to overestimate small A_eN values and underestimate high A_eN values was reduced by introducing in the soil loss prediction model both Q_R and an exponent for the erosivity term. The fitting to the data was poor with the USLE-MR as compared with the USLE-MB and the USLE-MM. Estimating two distinct exponents (USLE-M2) instead of a single exponent (USLE-MB, USLE-MR, and USLE-MM) did not appreciably improve soil loss prediction. The USLE-MB and the USLE-MM were recognized to be the best performing models among the possible alternatives, and they performed similarly with reference to both the complete data set and different sub-data sets, only including small, intermediate, and severe erosion events. In conclusion, including the runoff coefficient in the soil loss prediction model is important to improve the quality of the predictions, but a great importance has to be paid to the mathematical structure of the model.     

Sediment transport capacity and erosion processes: model concepts and reality

Sediment transport capacity, T_c, defined as the maximum amount of sediment that a flow can carry, is the basic concept in determining detachment and deposition processes in current process-based erosion models. Although defined conceptually and used extensively in modelling erosion, T_c was rarely measured. Recently, a series of laboratory studies designed to quantify effects of surface hydrologic conditions on erosion processes produced data sets feasible to evaluate the concept of T_c. A dual-box system, consisting of 1·8 m long sediment feeder box and a 5 m long test box, was used. Depending on the relative magnitudes of sediment delivery from feeder and test boxes, five scenarios are proposed ranging from deposition-dominated to transport-dominated sediment regimes. Results showed that at 5 per cent slope under seepage or 10 per cent slope under drainage conditions, the runoff from the feeder box caused in the additional sediment transport in the test box, indicating a transport-dominated sediment regime. At 5 per cent slope under drainage conditions, deposition occurred at low rainfall intensities. Increases in slope steepness, rainfall intensity and soil erodibility shifted the dominant erosion process from deposition to transport. Erosion process concepts from the Meyer–Wishmeier, Foster–Meyer and Rose models were compared with the experimental data, and the Rose model was found to best describe processes occurring during rain. A process-based erosion model needs to have components that can represent surface conditions and physical processes and their dynamic interactions. Copyright © 1999 John Wiley & Sons, Ltd.     

Sensitivity analysis and calibration of the Modified Universal Soil Loss Equation (MUSLE) for the upper Malewa Catchment, Kenya

Simulation models are widely used for studying physical processes such as surface runoff, sediment transport and sediment yield in catchments. Most models need case-specific empirical data for parameterization before being applied especially in regions other than the ones they have been developed. Sensitivity analysis is usually performed to determine the most influential factors of a model so that they can be prioritized for optimization. In this way uncertainties in model outputs can be reduced considerably. This study evaluates the commonly used modified universal soil loss equation (MUSLE) model used for sediment yield simulation for the case of the upper Malewa catchment in Kenya. The conceptual factors of the model are assessed relative to the hydrological factors in the model. Also, the sensitivity of the model to the choice of the objective function in calibration is tested. The Sobol' sensitivity analysis method was used for evaluating the degree of sensitivity of the conceptual and hydrological factors for sediment yield simulations using the MUSLE model. Nash-Sutcliffe Efficiency (NSE) and the modified Nash-Sutcliffe Efficiency (NSEm) are used to test the sensitivity of the model to the choice of the objective function and robustness of model performance with sediment data measured from upper Malewa catchment, Kenya. The results indicate that the conceptual factors are the most sensitive factors of the MUSLE model contributing about 66% of the variability in the output sediment yield. Increased variability of sediment yield output was also observed. This was attributed to interactions of input factors. For the upper Malewa catchment calibration of the MUSLE model indicates that the use of NSEm as an objective function provides stable results, which indicates that the model can satisfactorily be applied for sediment yield simulations.     

A conceptual model for determining soil erosion by water

Current estimates of rates of soil erosion by water derived from plots are incompatible with estimates of long‐term lowering of large drainage basins. Traditional arguments to reconcile these two disparate rates are ?awed. The ?ux of sediment leaving a speci?ed area cannot be converted to a yield simply by dividing by the area, because there is no simple relationship between ?ux and area. Here, we develop an approach to the determination of erosion rates that is based upon the entrainment rates and travel distances of individual particles. The limited available empirical data is consistent with the predictions of this approach. Parameterization of the equations to take account of such factors as gradient and sediment supply is required to proceed from the conceptual framework to quantitative measurements of erosion. However, our conceptual model solves the apparent paradox of the sediment delivery ratio, resolves recent discussion about the validity of erosion rates made using USLE erosion plots, and potentially can reconcile erosion rates with known lifespans of continents. Our results imply that previous estimates of soil erosion are fallacious. Copyright © 2004 John Wiley & Sons, Ltd.     

Storm event sediment fingerprinting for temporal and spatial sediment source tracing

Sediment fingerprinting has been widely used to distinguish discrete sediment sources; however, application to intra-storm sediment source variability has received relatively little focus despite the benefit being long recognized. In this investigation, sediment fingerprinting was applied to a 53-hr storm event sampled hourly to determine sediment source dynamics throughout the event. Sediment sources were differentiated using 16 variables, and source contribution determined using Bayesian and Frequentist mixing models for comparison. Both models provided comparable source predictions for the dominant source estimates and the general temporal pattern. The Frequentist model appeared to exhibit some unreliable values coinciding with low GOF and attributed to inherent model structure. The Bayesian model showed higher uncertainty, likely due to the “process error” utilized associated with single sample mixtures. High variability in sediment source contribution was observed between hourly time steps; however, local smoothing reveals temporal trends during the event. A higher average proportion of mudstone is found in the falling limb (0.544) compared with the rising limb (0.464), and the reverse is observed for mountain range (0.218 vs. 0.283) and unconsolidated (0.073 vs. 0.055). In the initial hours of the storm, mudstone source contribution significantly drops, whereas mountain range and unconsolidated contributions peak. The SSC-Q clockwise hysteresis indicates proximal sediment sources, suggesting the mudstone sediment is stored channel sediment and easily entrained. This sediment flushes through, coinciding with a drop as the distal mountain range and unconsolidated sources arrive to peak contribution. The wider Manawatū discharge and sediment load then arrive, delivering increasing levels of mudstone throughout the remainder of the event while mountain range sediment diminishes. Spatial representation of the sediment source contribution was derived from distributing sediment source loads to the spatial extent of the source material according to sub-catchment sediment loads and was weighted according to slope. This provided an effective means to visualize the origin of the sediment and a better spatial interpretation of sediment fingerprinting results, which is typically limited by poor spatial resolution.     

Applicability of the Modified Universal Soil Loss Equation for prediction of sediment yield in Khanmirza watershed,Iran

Abstract

The present study aims to estimate the sediment yield due to storm rainfall and runoff at the outlet of the Khanmirza watershed (395 km²) located in western Iran. The estimation was made for six storm events using the Modified Universal Soil Loss Equation (MUSLE). All the inputs required for the application of the model were determined through runoff and sediment concentration monitoring at the time of storm events, and field surveys in the study area. The applicability of the model to the study area was then evaluated by comparison of its estimates with those calculated using the measured sediment data. The results of the study demonstrated the efficiency of the MUSLE in estimating storm-associated sediment yield except one storm event in the study area with a high level of agreement and non-significant differences between mean estimated and measured values in the study storm events.     

Comparative analysis of sediment yield estimations using different empirical soil erosion models

The study aims to evaluate the performance of different empirical soil erosion models (EPM, USLE, Koutsoyiannis and Tarla, RUSLE) in mountainous Mediterranean-type catchments. The study area comprises the Arachthos, Kalamas, Upper Acheloos and Venetikos river basins, located in northwestern Greece. The methodology followed includes both qualitative and quantitative analyses. The former refers to the specific attributes of the models and the latter to the estimated sediment yield results. The results were initially validated against observed sediment yield values. The ambiguous reliability of such measurements led to their replacement by simulated ones, estimated using the sediment rating curve methodology. In the latter analysis, the models performed better, with more accurate results. Overall, the RUSLE corresponded best to such basins. Finally, the performance of seven empirical equations (Syvitski, Avendano Salas et al., Dendy and Bolton, Lu et al., Webb and Griffiths, Zarris et al.) was assessed, yielding relatively poor results.     

Physically-based distributed soil erosion and sediment yield model (DREAM) for simulating individual storm events

Abstract

A relatively simple process-oriented, physically-based distributed (PBD) hydrological model, the distributed runoff and erosion assessment model (DREAM), is described, and a validation study conducted in the semi-forested watershed of Pathri Rao, in the Garhwal Himalayas, India, is reported. DREAM takes account of watershed heterogeneity as reflected by land use, soil type, topography and rainfall, measured in the field or estimated through remote sensing, and generates estimates of runoff and sediment yield in spatial and temporal domains. The model is based on simultaneous solution of flow dynamics, based on kinematic wave theory, followed by solution of soil erosion dynamics. As the storm rainfall proceeds, the process of overland flow generation is dependent on the interception storage and infiltration rates. The components of the soil erosion model have been modified to provide better prediction of sediment flow rates and sediment yields. The validation study conducted to test the performance of the model in simulating soil erosion and sediment yield during different storm events monitored in the study watershed showed that the model outputs are satisfactory. Details of a sensitivity analysis, model calibration and the statistical evaluation of the results obtained are also presented and discussed. It is noteworthy that the distributed nature of the model combined with the use of geographical information system (GIS) techniques permits the computation and representation of the spatial distribution of sediment yield for simulated storm events, and a map of the spatial distribution of sediment yield for a simulated storm event is presented to highlight this capability.

Citation Ramsankaran, R., Kothyari, U.C., Ghosh, S.K., Malcherek, A., and Murugesan, K., 2013. Physically-based distributed soil erosion and sediment yield model (DREAM) for simulating individual storm events. Hydrological Sciences Journal, 58 (4), 872–891.     

A new approach for linking event‐based upland sediment sources to downstream suspended sediment transport

In this study, we proposed a new approach for linking event sediment sources to downstream sediment transport in a watershed in central New York. This approach is based on a new concept of spatial scale, sub‐watershed area (SWA), defined as a sub‐watershed within which all eroded soils are transported out without deposition during a hydrological event. Using (rainfall) event data collected between July and November, 2007 from several SWAs of the studied watershed, we developed an empirical equation that has one independent variable, mean SWA slope. This equation was then used to determine event‐averaged unit soil erosion rate, Q_S/A, (in kg/km²/hr) for all SWAs in the studied watershed and calculate event‐averaged gross erosion E_ea (in kg/hr). The event gross erosion E_t (in kilograms) was subsequently computed as the product of E_ea and the mean event duration, T (in hours) determined using event hydrographs at the outlet of the studied watershed. Next, we developed two linear sediment rating curves (SRCs) for small and big events based on the event data obtained at the watershed outlet. These SRCs, together with T, allowed us to determine event sediment yield SY_e (in kilograms) for all events during the study period. By comparing E_t with SY_e, developing empirical equations (i) between E_t and SY_e and (ii) for event sediment delivery ratio, respectively, we revealed the event dynamic processes connecting sediment sources and downstream sediment transport. During small events, sediment transport in streams was at capacity and dominated by the deposition process, whereas during big events, it was below capacity and controlled by the erosion process. The key of applying this approach to other watersheds is establishing their empirical equations for Q_S/A and appropriately determining their numbers of SWAs. Copyright © 2011 John Wiley & Sons, Ltd.     

Sediment delivery from hillslopes and the Universal Soil Loss Equation: some perceptions and misconceptions

The Universal Soil Loss Equation (USLE) or the revised USLE (RUSLE) are often used together with sediment delivery ratios in order to predict sediment delivery from hillslopes. In using sediment delivery ratios for this purpose, it is assumed that the sediment delivery ratio for a given hillslope does not vary with the amount of erosion occurring in the upslope area. This assumption is false. There is a perception that hillslope erosion is calculated on the basis that hillslopes are, in effect, simply divided into 22·1 m long segments. This perception fails to recognize the fact the inclusion of the 22·1 m length in the calculation has no physical significance but simply produces a value of 1·0 for the slope length factor when slopes have a length equal to that of the unit plot. There is a perception that the slope length factor is inappropriate because not all the dislodged sediment is discharged. This perception fails to recognize that the USLE and the RUSLE actually predict sediment yield from planar surfaces, not the total amount of soil material dislocated and removed some distance by erosion within an area. The application of the USLE/RUSLE to hillslopes also needs to take into account the fact that runoff may not be generated uniformly over that hillslope. This can be achieved by an equation for the slope length factor that takes account of spatial variations in upslope runoff on soil loss from a segment or grid cell. Several alternatives to the USLE event erosivity index have been proposed in order to predict event erosion better than can be achieved using the EI₃₀ index. Most ignore the consequences of changing the event erosivity index on the values for the soil, crop and soil conservation protection factors because there is a misconception that these factors are independent of one another. Copyright © 2007 John Wiley & Sons, Ltd.     

The impact of typhoons on sediment connectivity: lessons learnt from contaminated coastal catchments of the Fukushima Prefecture (Japan)

Sediment connectivity characterizes the physical transfer of sediment through different geomorphic compartments in catchments due to sediment detachment, transport and deposition. Quantifying and modelling sediment connectivity is therefore a key prerequisite to improving our understanding of the dispersion of particle‐borne contaminants, especially in catchments exposed to highly erosive climates. The objective of this study is to provide novel insights into typhoon impacts on sediment connectivity from hillslopes to rivers. The dispersion of particle‐bound caesium‐137 (¹³⁷Cs) was investigated in two coastal catchments draining the main contamination plume from the Fukushima Daiichi Nuclear Power Plant accident. Five sampling campaigns were carried out from November 2011 to November 2015, after each typhoon season. The spatial and temporal evolution of ¹³⁷Cs contamination was investigated through the calculation of ¹³⁷Cs enrichment ratios in sediment relative to nearby soils. Rainfall erosivity (EI₃₀) associated with the main typhoons that occurred prior to each sampling campaign were computed, mapped, and finally used to improve a topographic‐based index of connectivity. From 2011 to 2015, mean contamination levels in Mano and Niida catchments decreased from 11.9 kBq kg^?1 to 3.3 kBq kg^?1 and from 34.1 kBq kg^?1 to 8.0 kBq kg^?1, respectively. Regional mean EI₃₀ ranged from 262 MJ mm ha^?1 h^?1 for typhoon Jelawat (in 2012) to 1695 MJ mm ha^?1 h^?1 for typhoon Roke (in 2011). Typhoons Roke (2011) and Etau (2015) showed the highest connectivity from contaminated sources to the rivers, and induced a significant export of sediment to the ocean. In 2013 a slight increase in ¹³⁷Cs levels in river sediments occurred, likely resulting from initial decontamination works and the occurrence of two consecutive typhoons. Importantly, this research provides new insights into the connectivity of the main sources of sediments contaminated with radiocaesium in Fukushima Prefecture and their temporal evolution, which will help with ongoing decontamination efforts. Copyright © 2016 John Wiley & Sons, Ltd.     

A revised soil erosion budget for India: role of reservoir sedimentation and land‐use protection measures

Among the different controls of erosion budget at basin level, the relative impact of dams and land management is yet to be investigated. In this paper, the impact of dams on sediment yield has been assessed by using a conceptual modelling framework which considers the gross erosion and the cascade of dams constructed on a river network. The sediment budget has been estimated based on the gross erosion, deposition of sediment in reservoirs, and sediment yields of 23 mainland river basins of India. The gross erosion of the country is estimated as 5.11 ± 0.4 Gt yr^?1 or 1559 t km^?2 yr^?1, out of which 34.1 ± 12% of the total eroded soil is deposited in the reservoirs, 22.9 ± 29% is discharged outside the country (mainly to oceans), and the remaining 43.0 ± 41% is displaced within the river basins. The river basins of northern India contribute about 81% of the total sediment yield from landmass while the share of southern river basins is 19%. The components of revised sediment budget for India are prominently influenced by the sediment trapped in reservoirs and the treatment of catchment areas by soil and water conservation measures. Analysis of sediment deposition in 4937 reservoirs indicated the average annual percentage capacity loss as 1.04% though it varies from 0.8% to >2% per year in smaller dams (1–50 Mm³ capacity) and from <0.5% to 0.8% per year in larger dams (51 to >1000 Mm³ capacity). Siltation of smaller dams poses a serious threat to their ecosystem services as they cater to a wider population for domestic, agricultural, and industrial purposes. Amongst the environment controls, land use significantly impacts the gross erosion rate and specific sediment yield as compared to climatic and topographic parameters. However, to analyse their integrated effect on the complex processes of sediment fluxes in a basin, further research efforts are needed. Copyright © 2016 John Wiley & Sons, Ltd.     

Evaluation of a dynamic multi‐class sediment transport model in a catchment under soil‐conservation agriculture

Soil erosion models are essential tools for the successful implementation of effective and adapted soil conservation measures on agricultural land. Therefore, models are needed that predict sediment delivery and quality, give a good spatial representation of erosion and deposition and allow us to account for various soil conservation measures. Here, we evaluate how well a modified version of the spatially distributed multi‐class sediment transport model (MCST) simulates the effectiveness of control measures for different event sizes. We use 8 year runoff and sediment delivery data from two small agricultural watersheds (0·7 and 3·7 ha) under optimized soil conservation. The modified MCST model successfully simulates surface runoff and sediment delivery from both watersheds; one of which was dominated by sheet and the other was partly affected by rill erosion. Moreover, first results of modelling enrichment of clay in sediment delivery are promising, showing the potential of MCST to model sediment enrichment and nutrient transport. In general, our results and those of an earlier modelling exercise in the Belgian Loess Belt indicate the potential of the MCST model to evaluate soil erosion and deposition under different agricultural land uses. As the model explicitly takes into account the dominant effects of soil‐conservation agriculture, it should be successfully applicable for soil‐conservation planning/evaluation in other environments. Copyright © 2008 John Wiley & Sons, Ltd.     

Dynamics of soil erosion rates and controlling factors in the Northern Ethiopian Highlands – towards a sediment budget

This paper analyses the factors that control rates and extent of soil erosion processes in the 199 ha May Zegzeg catchment near Hagere Selam in the Tigray Highlands (Northern Ethiopia). This catchment, characterized by high elevations (2100–2650 m a.s.l.) and a subhorizontal structural relief, is typical for the Northern Ethiopian Highlands. Soil loss rates due to various erosion processes, as well as sediment yield rates and rates of sediment deposition within the catchment (essentially induced by recent soil conservation activities), were measured using a range of geomorphological methods. The area‐weighted average rate of soil erosion by water in the catchment, measured over four years (1998–2001), is 14·8 t ha^?1 y^?1, which accounts for 98% of the change in potential energy of the landscape. Considering these soil loss rates by water, 28% is due to gully erosion. Other geomorphic processes, such as tillage erosion and rock fragment displacement by gravity and livestock trampling, are also important, either within certain land units, or for their impact on agricultural productivity. Estimated mean sediment deposition rate within the catchment equals 9·2 t ha^?1 y^?1. Calculated sediment yield (5·6 t ha^?1 y^?1) is similar to sediment yield measured in nearby catchments. Seventy‐four percent of total soil loss by sheet and rill erosion is trapped in exclosures and behind stone bunds. The anthropogenic factor is dominant in controlling present‐day erosion processes in the Northern Ethiopian Highlands. Human activities have led to an overall increase in erosion process intensities, but, through targeted interventions, rural society is now well on the way to control and reverse the degradation processes, as can be demonstrated through the sediment budget. Copyright © 2007 John Wiley & Sons, Ltd.