The annual runoff estimate for California is the quantity of water that is discharged - or "runs off" - from the state in one year. Runoff occurs as the result of precipitation (both rainfall and snowfall) that is in excess of the demands of evaporation from land surfaces, transpiration from vegetation, and infiltration into soils. The water that remains available, or runoff, is the amount of water that makes its way to streams, rivers, and, possibly, to the ocean. The USGS stream gaging network provides accurate stream flow records and enables the compilation of annual runoff data for California. It is important to note that because the number of stream gages in operation can vary from year to year, more accurate estimates of annual runoff are made during years when a larger number of gages are operational. Periods of sparse stream gage data, such as the years prior to 1930, may have unreliable runoff estimates. Runoff data are from USGS Water Watch.