The Southern Sierra Nevada study unit is approximately 1,800 square miles; it includes six groundwater basins (California Department of Water Resources, 2003) and parts of the hard-rock watersheds surrounding the basins (Fram and Belitz, 2007). The primary aquifers in the Tehachapi-Cummings Valley basins in the southern part of the study unit consist of alluvial fan and floodplain sediments (mixtures of sand, silt, clay, gravel, cobbles, and boulders). The primary aquifers in the Kern River Valley basin in the northern part of the study unit are composed of fluvial sediments from the Kern River. Outside of and beneath the basins, the primary aquifers are fractured granitic and metamorphic rocks.

The primary aquifers in the study unit are defined as those parts of the aquifers corresponding to the screened or open intervals of wells listed in the California Department of Public Health database. In the Tehachapi-Cummings basins, these wells typically are drilled to depths between 300 and 500 feet, consist of solid casing from land surface to a depth of about 100 to 225 feet, and are screened or open below the solid casing. In the Kern River Valley basin, these wells typically are 50 to 175 feet deep, and are screened or open below 25 to 100 feet. Water quality in the shallower and deeper parts of the aquifer system may differ from that in the primary aquifers. The areas outside of the basins have both wells and developed springs.

The Southern Sierra Nevada study unit has warm, dry summers and cold, wet winters. Average annual precipitation ranges from about 11 inches in the basins to over 50 inches in the surrounding mountains, and the majority of precipitation falls as snow. Land use in the Southern Sierra Nevada study unit is approximately 90 percent (%) undeveloped (grasslands and scrublands), 8% cultivated agriculture, and 2% urban. The undeveloped lands are used mostly for recreation and open-range grazing. The largest urban areas are the city of Tehachapi and the small towns surrounding Lake Isabella.

Recharge to the groundwater flow system is mainly from mountain-front recharge at the margins of the basins, stream-channel infiltration, and direct infiltration of precipitation. Groundwater leaves the aquifer system mainly as pumpage for irrigation and drinking-water supply, flow into streams and lakes, and evaporation from areas with a shallow depth to groundwater.