Surface runoff along two agricultural hillslopes with contrasting soils

The targeting of critical surface runoff-producing zones should account for the influence of subsurface soil characteristics. In this study we assessed the runoff response of contrasting colluvial and residual soils. The study was conducted along two hillslopes within a 39.5-ha mixed land use watershed in Pennsylvania. Six sites (four colluvial, two residual) were monitored for runoff, hydraulic head, water table depth, and soil water content. A total of 111 rainfall events were monitored during the periods of July to December 2000, April to December 2001, and April to December 2002. Two high-intensity (5-min peak > 8 cm h(-1)) events had return periods of 2.5 and 4 yr. The colluvial soils are somewhat poorly and moderately well drained with fragipans and high clay content (37-44%) argillic horizons (fine, mixed, semiactive, mesic Aquic Fragiudalfs); the residual soils are well drained with moderate clay content (24%) argillic horizons (fine-loamy, mixed, semiactive, mesic Typic Hapludults). Across all events, overall runoff yields averaged 2.4% from the four colluvial sites and 0.01% from the two residual sites. The two colluvial sites with the greatest runoff production were located at the base of a primarily colluvial hillslope. The largest events at these sites occurred during periods of surface saturation (soil surface to a depth of at least 30 cm). These results suggest that nonwinter P management for these residual soils should focus on rare, large events. Nutrient management planning could be improved if runoff estimation methods were to better integrate information on subsurface and upslope soil hydrologic properties.