Hydrologic Regions California For water planning and conservation purposes, the California Department of Water Resources (DWR) and the State Water Resources Control Board (SWRCB) have divided the state into 10 Hydrologic Regions, also referred to as a Hydrologic Study Area (HSA), and are based on the Watershed or Water Basin concept. These California HSAs include: 1 North Coast Region Comprises all of the California area tributary to the ocean from the mouth of Tomales Bay north to the Oregon border and east along the border to a point near Goose Lake, consisting of 19,590 square miles (12 percent of the state’s total area), 571,750 persons (1.9 percent of the state’s total population all populations as of 1990), with average annual precipitation of 53 inches (range: 15 to over 100 inches), and average annual runoff of 28,886,000 acre-feet (40.8 percent of total state runoff), 2 San Francisco Bay Region Extends from Pescadero Creek in southern San Mateo County to the mouth of Tomales Bay in the north and inland to the confluence of the Sacramento and San Joaquin rivers near Collinsville, consisting of 4,400 square miles (3 percent of the state’s total area), 5,484,000 persons (18 percent of the state’s total population), with average annual precipitation of 31 inches (range: 14 to almost 48 inches), and average annual runoff of 1,245,500 acre-feet (1.8 percent of total state runoff), 3 Central Coast Region Encompasses the area adjacent to the Pacific Ocean including Santa Cruz County in the north through Santa Barbara County in the south to the Diablo and Temblor mountain ranges on the east, consisting of 11,280 square miles (7 percent of the state’s total area), 1,292,900 persons (4 percent of the state’s total population), with average annual precipitation of 20 inches (range: 14 to 45 inches), and average annual runoff of 2,477,000 acre-feet (3.5 percent of total state runoff), 4 South Coast Region Extending eastward from the Pacific Ocean, the region is bounded by the Santa Barbara–Ventura county line and the San Gabriel and San Bernardino mountains on the north, the Mexican border on the south, and a combination of the San Jacinto Mountains and low-elevation mountain ranges in central San Diego County on the east, consisting of 10,950 square miles (7 percent of the state’s total area), 16,292,800 persons (54 percent of the state’s total population), with average annual precipitation of 18.5 inches (range: 10 to 45 inches), and average annual runoff of 1,227,000 acre-feet (1.7 percent of total state runoff), 5 Sacramento River Region Contains the entire drainage area of the Sacramento River and its tributaries and extends almost 300 miles from Collinsville in the Sacramento–San Joaquin Delta north to the Oregon border to the crest of the Sierra Nevada and Cascade Ranges which form the eastern border to the crest of the Coast Range forming the western side, consisting of 26,960 square miles (17 percent of the state’s total area), 2,208,900 persons (7 percent of the state’s total population), with average annual precipitation of 36 inches (range: 10 to 80 inches), and average annual runoff of 22,389,700 acre-feet (31.6 percent of total state runoff), 6 San Joaquin River Region Located in the heart of California bordered on the east by the crest of the Sierra Nevada and on he west by the coastal mountains of the Diablo Range, extending from the Delta and the Cosumnes River drainage south to include all of the San Joaquin River watershed, consisting of 15,950 square miles (10 percent of the state’s total area), 1,430,200 persons (5 percent of the state’s total population), with average annual precipitation of 13 inches (range: 9 to 35 inches), and average annual runoff of 7,933,300 acre-feet (11.2 percent of total state runoff), 7 Tulare Lake Region Including the southern San Joaquin Valley from the southern limit of the San Joaquin River watershed to the crest of the Tehachapi Mountains, stretching from the Sierra Nevada Crest in the east to the Coast Range in the west, consisting of 16,520 square miles (10 percent of the state’s total area), 1,554,000 persons (5 percent of the state’s total population), with average annual precipitation of 14 inches, and an average annual runoff of 3,313,500 acre-feet (4.7 percent of total state runoff), 8 North Lahontan Region Comprises the eastern drainages of the Cascade Range and the eastern Sierra Nevada, north of the Mono Lake drainage, consisting of 3,890 square miles (less than 3 percent of the state’s total area), 78,000 persons (less than 0.3 percent of the state’s total population), with average annual precipitation of 32 inches (range: 4 to 70 inches), and average annual runoff of 1,842,000 acre-feet (2.6 percent of total state runoff), 9 South Lahontan Region Encompassing the area from the mountain divide north of Mono Lake to the divide south of the Mojave River, which runs through the Mojave Desert, bordered on the east by the Nevada state line and on the west by the crest of the Sierra Nevada, consisting of 29,020 square miles (18 percent of the state’s total area), 599,900 persons (2 percent of the state’s total population), with average annual precipitation of 8 inches (range: 4 to 10 inches, with extremes of 1.9 inches in Death Valley and over 120 inches at Mammoth Lakes), and an average annual runoff of 1,334,000 acre-feet (1.9 percent of total state runoff), 10 Colorado River Region Encompassing the southeastern corner of California with the region’s northern boundary, a drainage divide, beginning along the southern edge of the Mojave River watershed in the Victor Valley area of San Bernardino County and extending northeast across the Mojave Desert to the Nevada state line. The southern boundary is the Mexican border while a drainage divide forms the jagged western boundary through the San Bernardino, San Jacinto, and Santa Rosa mountains and the Peninsular ranges and the Nevada state line and the Colorado River form the region’s eastern boundary, consisting of 19,730 square miles (12 percent of the state’s total area), 464,200 persons (less than 2 percent of the state’s total population), with average annual precipitation of 5.5 inches (range: 3 to 36 inches), and an average annual runoff of 178,700 acre-feet (less than 0.3 percent of total state runoff),