Runoff is the amount of water left from precipitation that can be measured as streamflow after losses to evaporation, transpiration by plants, and the replenishment of storage within the aquifers. The areal distribution of runoff from 1951 to 1980 (fig. 5) closely followed the areal distribution of precipitation for the same period (fig. 3), but the relative amounts of runoff varied as a result of climatic conditions. Runoff is greatest in the mountains, where the majority of precipitation falls as snow, which melts in the spring and runs off with minimal evapotranspiration (the process by which liquid water is converted to water vapor either by evaporation or by transpiration from plants). Runoff is greater than 40 inches per year in many mountainous areas. The basins in the arid parts of Nevada and southeastern California have virtually zero runoff because most precipitation that falls is evaporated almost immediately. However, high-intensity storms or rapid snowmelt in the mountains that border the basins may cause flash floods that reach the floors of the basins. Coastal areas have a direct relation between the amount of precipitation and runoff.
