The South Coast hydrologic region of California is one of the 10 major hydrologic regions of the state as identified by the California Department of Water Resources. The South Coast hydrologic region has a surface area of about 11,100 sq mi (29,000 km2), and includes all of Orange County, major portions of Los Angeles, Riverside, San Bernardino, San Diego, and Ventura counties, and a small portion of Santa Barbara County.[1] The state-defined South Coast hydrologic region is roughly equivalent to the federally-defined Southern California Coastal water resource subregion. The South Coast hydrologic region has 78 state-recognized alluvial groundwater basins and subbasins.
Significant geographic features include the Transverse Ranges and the Peninsular Ranges. Major rivers in the region include the Los Angeles River, San Gabriel River, San Diego River, San Luis Rey River, Santa Ana River, Santa Clara River, Santa Margarita River, and Ventura River.
The climate in the region ranges from Mediterranean to subtropical steppe. Annual precipitation in the region ranges from more than 40 in (1,000 mm) in the mountains to less than 10 in (250 mm) in some valleys, with an overall average of 17.6 in (450 mm) for the region.