Runoff Curve Numbers For The Southern High Plains

ABSTRACT Rainfall produces significant amounts of surface runoff from cultivated drylands of the western Great Plains. Currently, engineers must extrapolate runoff data from more humid regions to estimate runoff from farm fields. Runoff data are now available from a 32-year record in the semiarid, western Great Plains. We derived curve numbers (CN) for the Soil Conservation Service curve number method for three field-sized watersheds that were dryland-farmed in a wheat-sorghum-fallow crop sequence. The watershed soils are classified in hydrologic group D. The CNs for wheat and sorghum should be 79 and 82, respectively (handbook CN are both 80). The single handbook CN for fallow (90) is too high; it should be 77 for fallow after wheat and 82 for fallow after sorghum. These new CNs should improve computer model predictions of runoff, erosion and water quality, and provide more accurate estimates of runoff volume for use in designing conservation structures, water supply, flood control, and numerous other uses.