TEMPORAL VARIATION IN RUNOFF AND SOIL LOSS FROM SIMULATED RAINFALL ON CORN AND SOYBEANS

Runoff and soil loss were measured from simulated rainfall applied to a Catlin silt loam soil at two crop stages during four growing seasons in a com soybean rotation implemented on the contour and up-and-down slope. Runoff depth, runoff rate, soil loss/ha, and soil loss/ha-mm of runoff varied significantly by crop stage and by year. Runoff depth and runoff rate were correlated with variations in antecedent rainfall, soil moisture, residue, and canopy. Soil loss/ha from soybeans and soil loss/ha-mm of runoff from com varied by as much as a factor of four from one year to another. Much of the variation in soil loss appeared to be related to variations in runoff, slope steepness, and antecedent rainfall. Soil loss/ha-mm of runoff 30 to 40 days after planting com was significantly different across years for both row directions. In six of eight comparisons for soybeans, soil loss/ha-mm of runoff from simulated rainfall was not statistically different than from an eight-year study under natural rainfall. Nevertheless, this study indicates that results from one-time rainfall simulation may not represent the long term average for a soil.