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Title: Does WEPP meet the specificity of soil erosion in steep mountain regions?

item Nearing, Mark

Submitted to: Hydrology and Earth System Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/2/2009
Publication Date: 3/1/2009
Citation: Konz, N., Banninger, D., Nearing, M.A., Alewell, C. 2009. Does WEPP meet the specificity of soil erosion in steep mountain regions? Hydrology and Earth System Sciences. 6: 2153–2188.

Interpretive Summary: This USDA-ARS-WEPP model application was the first comparison between high temporal resolute field installations (erosion, soil moisture, and surface flow measurements) and WEPP simulations in subalpine areas. The study was done in the framework of seeking adequate methods for the prediction of erosion under different grassland conditions in alpine systems. We distinguished between short-term erosion prediction for a single vegetation period and long-term erosion prediction, compared with Cs-137 data. Because of the uncertainties of winter processes, including snow height and development, temporal snowmelt and water amount distribution that were discussed above, we conclude that WEPP is not a useful tool for alpine regions where winter processes have a great influence on the water balance and erosion processes. The uncertainty of winter process simulation results in erroneous description of the hydrology during the whole year. However, relative differences between erosion rates of land use types were simulated successfully compared to measured values. The implications of the study are that WEPP may be used to estimate the influence of land use change of different land use types within the vegetation period but is not a useful tool to predict long-term annual erosion rates or winter processes in alpine regions.

Technical Abstract: We chose the USDA-ARS-WEPP model (Water Erosion Prediction Project) to describe the soil erosion in the Urseren valley (Central Switzerland) as it seems to be one of the most promising models for steep mountain environments. Crucial model parameters were determined in the field (slope, plant species, fractional vegetation cover, initial saturation level), by laboratory analyses (grain size, organic matter) or by the WEPP manual (rill and interrill erodibility, effective hydraulic conductivity, cation exchange capacity). The quantification of soil erosion was performed on hill slope scale for three different land use types: meadows, pasture with dwarf shrubs and pasture without dwarf shrubs. Erosion rates for the vegetation period were measured with sediment traps between June 2006 and November 2007. Long-term soil erosion rates were estimated by measuring Cs-137 redistribution, deposited after the Chernobyl accident. In addition to the erosion rates, soil moisture and surface flow was additionally measured during the vegetation period in the field and compared to model output. We found that short-term erosion rate simulations for the vegetation period in 2007 are in agreement with measured erosion rates. However, modeled soil moisture is up to two times higher than measured field data. Additionally, simulated soil moisture has a converse dynamic (increasing in springtime) compared to measured field data and surface flow is not simulated correctly. Snow cover melting is modeled too late compared to field observations and thus water from snowmelt is available till summer time. We assume that these differences lead to the general overestimation of erosion rates for long-term rate erosion predictions for all three land use types. Thus, the WEPP model may be a useful tool for alpine regions during the vegetation period to appraise the influence of different land use conditions but should be applied carefully to winter time processes or snow covered regions.