Runoff Curve Number variation with drainage area, Walnut Gulch, Arizona

Runoff Curve Numbers (a measure of a watershed’s runoff response to a rainstorm) were determined using three different methods for 18 semiarid watersheds in southeastern Arizona. Each of the methods produced similar results. A relationship was then developed between optimum Curve Number and drainage area of the watershed used. Curve Numbers decreased with increasing drainage area. This response is a reflection of spatial variability in rainfall and infiltration losses in the coarse-textured material of the channels associated with larger drainage basins.