The Walnut Gulch Experimental Watershed (WGEW) runoff database has the longest period of record of runoff in the world for a semiarid location, with data collection beginning in 1953. 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.