Long‐term runoff database, Walnut Gulch Experimental Watershed, Arizona, United States

Runoff measurement at the semiarid Walnut Gulch Experimental Watershed began in the middle 1950s with five critical depth flumes. Since that time, the measurement network has evolved to include measurement structures on 11 large watersheds (2.27–149 km 2 ), 8 medium watersheds (0.35–1.60 km 2 ), and 11 small watersheds (0.0018–0.59 km 2 ). The ephemeral nature of runoff, high‐flow velocities, and high‐sediment concentrations in the flow led to the development of the Walnut Gulch supercritical flume used on the large watersheds and the Smith supercritical flume used on the small watersheds. The period of record considered good to excellent ranges from 26 to 47 years. In 1999, the original analog recording systems were augmented with digital recorders. Runoff occurs at Walnut Gulch primarily as a result of convective thunderstorms during the months of July through September. Runoff volume and flow duration are correlated with drainage area as a result of the limited areal extent of runoff producing rainfall and transmission losses or infiltration of the flood wave into the channel alluvium. Runoff records including hydrographs and summary data are available in several formats via a Web interface at http://www.tucson.ars.ag.gov/dap/ .