Adapting WEPP (Water Erosion Prediction Project) for forest watershed erosion modeling

Shuhui Dun

There has been an increasing public concern over forest stream pollution by excessive sedimentation resulting from human activities. Adequate and reliable erosion simulation tools are urgently needed for sound forest resources management. Computer models for predicting watershed runoff and erosion have been developed in the past. These models, however, are often limited in their application due to inappropriate representation of the hydrological processes involved. The Water Erosion Prediction Project (WEPP) watershed model has proved useful in certain forest applications such as modeling erosion from a segment of insloped or outsloped road, harvested units, and burned units. Nevertheless, when used for modeling water flow and sediment discharge from a forest watershed of complex topography and channel systems, WEPP consistently underestimates these quantities, in particular, the water flow at the watershed outlet. The main purpose of this study was to improve the WEPP watershed model so that it can be applied to adequately simulate forest watershed hydrology and erosion. The specific objectives were to: (1) identify and correct WEPP algorithms and subroutines that inappropriately represent forest subsurface hydrologic processes; and (2) assess the performance of the modified model by applying it to a conceptual forest setting as well as a real forested watershed in the Pacific Northwest, USA. In modifying the WEPP model, changes were primarily made in the approach to, and algorithms for modeling deep percolation of soil water and subsurface lateral flow. The modified model was verified using a conceptual data set, with model predictions from both the new and original codes compared. Additionally, the adequacy of the modified routines was evaluated by applying WEPP to Hermada watershed, a representative forest watershed located in the Boise National Forest in central Idaho, and comparing the WEPP-predicted and field-observed runoff and erosion. Conclusions of this study included: (1) compared to the original model, the modified WEPP more realistically and properly represents the subsurface hydrologic processes in a forest setting; and (2) application of the modified model produced satisfactory results, demonstrating the adequacy of the model modifications.

Adapting WEPP (Water Erosion Prediction Project) for forest watershed erosion modeling

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