The surface water network is shown in figure 1. The average annual precipitation is 12 inches. The other mountainous parts in the north and east have cold, wet winters with major amounts of snow providing considerable runoff for the summer water supply. The snowmelt is the major source of freshwater for the rivers of the study unit. These higher mountainous areas may receive rainfall during any month of the year.
Summers are usually mild.
Precipitation totals from 21 to 41 inches. The Sacramento Valley, the south-central part of the region, has mild winters with less precipitation. Precipitation usually takes place from October through May and virtually no precipitation occurs from June to September. The average annual precipitation in the city of Sacramento is 18 inches. Of the 7 physiographic provinces, the Sacramento Valley will be given the most attention because it is there that the greatest amount of water use, and potential impacts on water use occur.