The Santa Ana Region continues to be one of the most rapidly growing areas of the State. While the region is geographically the smallest (nearly 3,000 square miles), it boasts one of the largest populations (almost five million people). Extensive groundwater basins underlie much of the region, but local recharge provides only a fraction of the area's water needs, which are primarily met by imported water. The Santa Ana River, the region's main surface water body, transports more than 125 million gallons per day of reclaimed water from Riverside and San Bernardino Counties for recharge into the Orange County Groundwater Basin. This satisfies approximately 40 percent of the county's water demand. This semiarid region is known for its temperate climate and relatively low rainfall -- about 15 inches per year. The Regional Boards office is located in Riverside.