WEPP simulations of dryland cropping systems in small drainages of northeastern Oregon

Computer simulation models are essential tools for evaluating soil erosion potential over large areas of cropland. Small-plot and field-scale evaluations are commonly conducted for federal farm program compliance, but producers are now faced with off-farm water quality concerns. Predicting the potential contribution of upland sediment is of interest to producers and state and federal agencies. The purpose of this study was to evaluate the applicability of the Water Erosion Prediction Project (WEPP) model for quantifying hydrological and erosion processes in the semiarid croplands of the Columbia Plateau. Two headwater drainages managed using conventional inversion tillage (CT) or no-tillage (NT) management techniques were monitored from 2001 through 2007 in the dryland cropping region of northeastern Oregon. The WEPP model was parameterized primarily from field data, including management and weather data. Crop parameters (above-ground biomass and crop yield), water balance components (volumetric soil water, evapotranspiration [ET], and surface runoff), and soil loss were observed and subsequently used to evaluate WEPP simulations. This detailed dataset allowed for a unique opportunity to evaluate not only the WEPP routines for runoff and erosion but also the routine for crop growth, which greatly influences the erodibility and hydraulic conductivity of top soil layers. Graphical and goodness-of-fit analyses indicate that WEPP generated satisfactory estimates for volumetric soil water and crop yields in NT and CT and above-ground biomass production in NT. Gross patterns of ET simulated by WEPP were compatible with those determined using observed precipitation and soil water data. Observed annual runoff and erosion values from both drainages were low (NT: 0.1 mm [0.004 in], 2.5 kg ha−1 [0.001 tn ac−1]; CT: 0.9 mm [0.04 in], 72.0 kg ha−1 [0.03 tn ac−1]). On average only 0.3% and 0.03% of total precipitation left the catchment as runoff during the six-year study period for CT and NT, respectively. No runoff was predicted by WEPP when default input values for a Walla Walla silt loam soil were used in the model. Simulated runoff and erosion agreed well with field observations after the effective surface hydraulic conductivity Keff and rill erodibility Kr were calibrated. With minimal calibration, the WEPP model was able to successfully represent the hydrology, sediment transport, and crop growth for CT and NT cropping systems in northeastern Oregon during years of below normal precipitation, mild weather, and little runoff.