Submitted to: ASAE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 1/16/2003
Publication Date: 7/27/2003
Citation: LIN, C., MCCOOL, D.K., FLANAGAN, D.C. WINTER RUNOFF PREDICTION BY WEPP WITH AN ENERGY BUDGET APPROACH TO SIMULATING SNOW AND SOIL FROST. ASAE ANNUAL INTERNATIONAL MEETING. 2003. Interpretive Summary: Water erosion models are used to design crop and land management systems to prevent soil degradation by erosion and runoff pollution by the resulting sediment. In cold regions, proper simulation of winter hydrology is crucial to satisfactory performance of water erosion models. Evaluation of WEPP (Water Erosion Prediction Project) of USDA-ARS indicated inadequate simulation of soil freezing periods and frost depths, resulting in poor simulation of winter runoff and resulting erosion and sediment production. Thus it became necessary to modify or replace the current winter subroutines in order to improve performance of WEPP. An energy budget approach was selected and tested for its potential in replacing the winter subroutines in WEPP. This approach has been used to model snow pack development and snow melting and the occurrence of soil freezing. However, this approach had not been used previously to predict soil frost depths. Thickness of the soil frost layer was then used to estimate frost effect on infiltration. Snow depth, frost depth and runoff simulation was improved with the new winter subroutines. These modifications to WEPP showed improved winter runoff simulation for areas as diverse as eastern Washington and Minnesota, making WEPP more universally applicable for runoff prediction.
Technical Abstract: In cold regions, the occurrence of snow and soil frost influences hydrology and, in turn, the mechanisms of soil erosion processes. For these regions, modeling the dynamics of snow and soil frost is necessary to estimate runoff and erosion accurately under different management practices. With better winter hydrology simulation, schemes for predicting the rates and amounts of soil erosion by water can be established on a firm hydrological footing. This project examined the potential of an energy budget approach to simulating the magnitude and variations of snow and soil frost depths. It was assumed that the net sum of all energy components in the environment is consumed or compensated by water phase changes that occur near or under the ground surface, such as snow melting or soil freezing-thawing. After testing, this energy budget approach showed potential to simulate winter hydrology and for adaptation to erosion prediction models. It was then incorporated into an operational erosion prediction model, the Water Erosion Prediction Project (WEPP) of USDA-ARS. Earlier evaluation of WEPP indicated the winter hydrology subroutines in this model predict longer soil freezing periods and deeper frost depths than actually observed in many areas. After incorporating the source code of the energy budget approach, not only can WEPP simulate winter hydrology better than before, but with the new adjustment in hydraulic conductivity, the prediction performance of WEPP for runoff events under the circumstances with snow and frost was significantly improved. We also suggest directions for future research and model revision.