Runoff and Soil-Loss Responses to Changes in Precipitation: A Computer Simulation Study

Changes in precipitation have occurred over the past century and are expected to continue over the next century. These changes will have significant implications for runoff, soil erosion, and conservation planning. This study was undertaken to investigate how runoff and soil erosion by water can be expected to be altered as a function of changes in the average number of days of precipitation per year and changes in the amount and intensity of the rain that falls on a given day. The Water Erosion Prediction Project (WEPP) model was used to simulate erosion for three locations, three soils, three slopes, and four crops. Average annual precipitation was changed ±10% and ±20% by changing either a) the number of wet days per year, b) the amount and intensity of precipitation per day, or c) a combination of the two. Results indicated that, on average, each 1% change in average annual precipitation induced a 1.28%, 2.50%, and 1.97% change in runoff and a 0.85%, 2.38%, and 1.66% change in soil loss for the three types of precipitation changes, respectively. Comparisons of the results of the soil-loss simulations to published relationships for Revised Universal Soil Loss Equation (RUSLE) R-factors in the United States suggest that the third option of changing both the number of wet days per year and the amount and intensity of precipitation per day is the most realistic scenario for representing changes in precipitation for hydrologic studies.