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Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 10/24/2016 Publication Date: 12/29/2016 Citation: Zhang, X.J. 2016. Evaluating water erosion prediction project model using Cesium-137-derived spatial soil redistribution data. Soil Science Society of America Journal. doi:10.2136/sssaj2016.06.0172. Interpretive Summary: The lack of spatially distributed soil erosion data has been the bottle neck for fine tuning and applying the process-based erosion prediction computer models. The traditional soil erosion measurements at watershed outlet does not yield information on erosion distribution inside the watershed. Sediment tracers such as the isotope cesium-137 have been widely used to generate spatial soil redistribution data. The objectives are to compare the soil redistribution patterns predicted using the cesium-137 tracer and the Water Erosion Prediction Project (WEPP) computer model, and to evaluate and validate the WEPP model. An experimental hillslope (80 m wide by 200 m long with 1 to 4% slope) was used. The plot was sampled for cesium-137 measurement. The measured cesium-137 inventories were converted to soil erosion rates. Measured runoff and soil loss data were used to calibrate the WEPP model. Soil redistribution patterns predicted by the cesium-137 and the WEPP model were vastly different along the study hillslope. Compared to the cesium-137 method, the average soil loss rates predicted by WEPP were overly sensitive to slope lengths exceeding 100 if the default 1-m rill spacing is used. The spatial erosion patterns were well matched if three different rill spacings along the hillslope were used. This study shows that rill spacing is a key input parameter for correctly simulating soil redistribution patterns with WEPP. Knowledge of rill distribution along a hillslope is critical to satisfactorily predicting rates of soil erosion and sediment deposition on a hillslope. The conclusion needs further testing on more hillslopes having various gradients and shapes. This result will be useful to erosion modelers and soil conservationists for improving the model or for conservation planning. Technical Abstract: The lack of spatial soil erosion data has been a major constraint on the refinement and application of physically based erosion models. Spatially distributed models can only be thoroughly validated with distributed erosion data. The fallout cesium-137 has been widely used to generate spatial soil redistribution data. The objectives are to compare the soil redistribution patterns predicted using the cesium-137 technique and the Water Erosion Prediction Project (WEPP) model, and to diagnose potential shortcomings in the configuration and assumptions of the WEPP model, if any. An experimental hillslope (80 m wide by 200 m long with 1 to 4% slope) was used. The plot was sampled in 10-m grid for cesium-137 measurement. Measured rainfall, runoff, soil loss, and crop managements were used to calibrate WEPP. Soil redistribution patterns predicted by the two methods were vastly different on the study hillslope. Compared to cesium-137, the average soil loss rates predicted by WEPP were overly sensitive to slope lengths exceeding 100 m, resulting from the assumption of the 1-m rill spacing made in WEPP. The spatial erosion patterns were well matched if different rill spacings along the hillslope were used. This study shows that rill spacing is a key input parameter for correctly simulating soil redistribution patterns with WEPP. Knowledge of rill distribution along a hillslope is critical to satisfactorily predicting rates of soil erosion and sediment deposition on a hillslope. The conclusion needs further testing on more hillslopes having various gradients and shapes. |
