ARS | Publication request: Integrating WEPP into the WEPS infrastructure

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: September 16, 2011
Publication Date: September 18, 2011
Citation: Fox, F.A., Frankenberger, J.R., Flanagan, D.C., Wagner, L.E. 2011. Integrating WEPP into the WEPS infrastructure. In: Proceedings International Symposium on Erosion and Landscape Evolution (ISELE), 18-21 September 2011, Anchorage, Alaska. ISELE Paper No. 11142. D.C. Flanagan, J.C. Ascough II, and J.L Nieber (eds.). St. Joseph, MI ASABE.

Technical Abstract: The Wind Erosion Prediction System (WEPS) and the Water Erosion Prediction Project (WEPP) share a common modeling philosophy, that of moving away from primarily empirically based models based on indices or "average conditions", and toward a more process based approach which can be evaluated using actual site conditions or simulations. Initial development of both models began with some auxiliary sub-models based upon similar code and concepts, e.g. crop growth (EPIC-based) and residue decomposition for example. However, separate development teams and the unique requirements of modeling wind erosion versus water erosion have resulted in additional divergence of the two models at the process level. In an effort to unite the two models so that both wind and water erosion can be estimated with a single simulation, the differences at the process level are clearly exposed. WEPP hydrology routines were previously integrated into WEPS to reduce the computational requirements for WEPS simulations. Testing has shown that the runoff, infiltration, evaporation, and winter hydrology between this code and the original WEPP code are significantly different. Using a fallow soil test scenario, the factors causing these differences have been isolated to include random roughness, hydraulic conductivity, soil matric potential, reference evapotranspiration, soil layering, snowfall and snow melt processes. It is expected that test scenarios with growing crops will lead to an additional set of difference factors. The random roughness created by tillage implements is set in the operations database. For identically named implements, these values differ. In WEPP, the value is unchanged on input, while in WEPS, it is modified based on soil texture and residue amounts. Random roughness decay processes are differentially affected by the soil texture, affecting the depression storage and consequently the runoff volume. Random roughness also directly affects the erosion processes in both models, increasing the importance of the parameter. The hydraulic conductivity is the primary factor affecting the infiltration process. WEPP uses an effective hydraulic conductivity including the effects of surface sealing which is estimated from the soil texture and conditions since the last tillage. In the incorporation of WEPP hydrology into WEPS, this factor was not identified correctly and the saturated hydraulic conductivity was used instead. The causes of differences seen in soil matric potential are tied to the water content of the soil in the infiltration zone and the soil water functions relating matric potential to water content. The infiltration differences are self compensating for small infiltration amounts. Evaporation differences are twofold. Different methods are used to calculate the reference evapotranspiration with WEPP biased to higher values. The bare soil evaporation processes are also different with WEPS biased to higher values. In WEPS thin soil layers are used near the surface to represent the surface drying. In WEPP, two thicker layers are used to represent the infiltration zone in tilled soil profiles. These compound the differences. Winter hydrology differences can be seen by differences and snow fall dates and differences in snow melt dates. WEPP, based on daily maximum and minimum temperatures, estimates the hourly temperature, randomly assigns a time of day to the precipitation event and determines whether the precipitation is rain or snow according to the hourly air temperature. This allows both rain and snow to occur on the same day. WEPS uses the daily average temperature to determine whether the precipitation is rain or snow. Snow melt times are generally within several days of each other. The two models were developed to model very different types of erosion processes. Resolving the differences requires retaining the process sub-models from both that are critical to modeling their specific erosion type.