|Robertson, David - Dave|
Submitted to: Journal of Soil and Water Conservation Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/8/2009
Publication Date: 1/15/2010
Citation: Williams, J.D., Dun, S., Robertson, D.S., Wu, J.Q., Brooks, E., Flanagan, D.C., Mccool, D.K. 2010. WEPP Simulations of Dryland Cropping Systems in Small Drainages of Northeastern Oregon. Journal of Soil and Water Conservation Society. 65(1):22-33.
Interpretive Summary: Soil erosion from croplands is of interest to producers, and state and federal agencies. Computer simulation models, such as the Water Erosion Prediction Project (WEPP) model, are essential tools for evaluating soil erosion potential over large areas of cropland. We tested WEPP’s ability to predict hydrological and erosion processes in the semiarid croplands of the Columbia Plateau in northeastern Oregon. Surface runoff, soil erosion, soil water, and crop yields were measured in fields farmed from 2001 through 2006 using conservation (no-till) or conventional (plowing) farming practices and compared to the output from the WEPP model. Model simulations were conducted using climate files derived from weather data collected at the research site. WEPP adequately simulated annual runoff, ET and biomass, and soil water. Soil erosion from the fields was very low, less than 0.1 ton/acre from the conventionally farmed field and less than two percent of that value from the no-till farmed field, while WEPP predicted no erosion for both systems. Adjustments to effective surface hydraulic conductivity, a key parameter in WEPP, improved the model’s ability to simulate the timing of runoff and erosion events. With minimal calibration, WEPP can be considered for use in calculating sediment contributions of croplands to regional streams and rivers, based on the relatively short time span of data used in this evaluation.
Technical Abstract: 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. WEPP 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 influence 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, 2.5 kg ha-1 (0.004 in, 0.001 t ac-1) ; CT: 0.9 mm, 72.0 kg ha-1 (0.04 in, 0.03 t 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.