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California Water Science Center - Santa Ana Basin, National Water Quality Assessment Program

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Study Unit Description

Illustration of the Santa Ana Surface Water BoundaryThe Santa Ana NAWQA study unit covers an area of about 2,700 square miles in parts of Orange, San Bernardino, Riverside, and Los Angeles Counties. The Santa Ana Basin is substantially urbanized: about 32 percent of the land use is residential, commercial, or industrial, and the area is home to more than 4 million people. Agricultural land use accounts for about 10 percent of the watershed.

The Santa Ana River is the largest stream system in southern California, beginning in the San Bernardino Mountains, which reach altitudes exceeding 10,000 feet, and flowing more than 100 miles to the Pacific Ocean near Huntington Beach. The San Jacinto River is a tributary of the Santa Ana River, but it normally terminates at Lake Elsinore.  

The climate is Mediterranean with hot, dry summers, and cooler, wetter winters. Average annual precipitation ranges from 12 inches per year in the coastal plain, to 18 inches per year in the inland alluvial valleys, and  40 inches per year in the San Bernardino Mountains. Most of the precipitation occurs between November and March.  Consequently, under natural conditions, the Santa Ana River would be intermittent with little or no flow in the summer months.  

Ground water is the main source of water supply in the watershed, providing about 66 percent of the consumptive water demand. Inland aquifers, upstream from Prado Dam, underlie about 1,200 square miles of the study unit. Coastal aquifers, downstream from Prado Dam, underlie about 400 square miles. Thickness of these aquifers ranges from several hundred to more than 1,000 feet. Depth to ground water ranges from several hundred feet near the flanks of mountains to near land surface along rivers and wetlands, and in the coastal plain.  

Imported  water from northern California and the Colorado River is also an important source of water supply, accounting for 27 percent of the consumptive demand. Other sources of supply include surface water derived from precipitation within the basin (4 percent) and recycled water (3 percent).  

Enhanced recharge of ground water is an important component of the hydrologic cycle in the Santa Ana watershed. The volume of water recharged is 37 percent of the volume pumped, with most of  the enhanced recharge consisting of surface water derived from precipitation within the basin.  Discharge from wastewater treatment facilities is also an important component of the hydrologic cycle, providing base flow in many parts of the drainage network. These activities are among the many factors affecting water quality in the watershed.

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