Thunderstorm runoff in southeastern Arizona.

Air-mass thunderstorm rainfall produces the major floods on small [100 sq mile (259 km² or less)] rangeland watersheds in the southwest. The intense central volume of thunderstorm rainfall, referred to as the core of runoff-producing rainfall, is correlated to peak discharge. The core was represented by the maximum 30-min. depth of rainfall at each gage as estimated from a dense rain gage network on the ARS Walnut Gulch Experimental Watershed. Physical limits are considered for thunderstorm rainfall and runoff in the southwest. These limits are important because of the scarcity of records and the uncertainties involved in extrapolating short records to longer recurrence intervals. Some upper limits for peak discharges are suggested for different sized Walnut Gulch watersheds, and the variances of the individual predictions are estimated through least squares fitting of the sample data.