The Santa Maria River Valley Groundwater Basin (Santa Maria Basin), DWR Bulletin 118 Basin No. 3-12 , underlies the coastal portion of the northern Santa Barbara and southern San Luis Obispo Counties. The DWR-defined Santa Maria Basin encompasses approximately 184,000 acres (288 square miles), of which approximately 61,220 acres (95.7 square miles) are within San Luis Obispo County. Annual precipitation for the basin ranges from 13 to 17 inches with an average of 15 inches. Groundwater is found in alluvium, dune sands, and the Orcutt, Paso Robles, Pismo, and Careaga Formations of the basin. Groundwater is unconfined throughout most of the basin except in the coastal portion where it is confined.