The groundwater aquifer system underlying Borrego Valley currently represents the sole source of water to the town of Borrego Springs and the surrounding community for municipal, agricultural and recreational demands. Groundwater has been extracted from the Borrego Valley aquifer since the early part of the 20th century. Beginning in the late 1940's and occurring throughout much of the period of groundwater development in Borrego Valley, groundwater extraction has exceeded natural groundwater recharge, resulting in an apparent overdraft condition. The net depletion of groundwater from storage within the aquifer system was approximately 510,000 acre-feet during the period 1945 through 2000. Overdraft of the aquifer has resulted in a decline of groundwater levels in the majority of monitored wells. Recent monitoring has indicated that water levels are currently declining an average of approximately 2 feet per year. Continued overdraft of the aquifer will inevitably lead to continued decline in groundwater levels, resulting in increasing water costs as water lifts increase and dry wells need to be replaced with successively deeper ones. In addition, continued drawdown of groundwater levels could increase the risk of upconing of deeper poor quality water. The Borrego Valley aquifer system is comprised of four hydrogeologic units of Quaternary and Tertiary age. The uppermost three units are the Quaternary Alluvium, designated as younger, intermediate and older. The oldest and lowermost unit is the Tertiary Palm Spring formation. The hydrogeologic units are underlain by the Cretaceous and older crystalline basement rocks. The Quaternary older alluvium is the principal water-bearing unit of the aquifer. It is relatively coarse grained and is thickest in the northern portion of the basin. Within the alluvium, soil texture is generally coarse grained in the northern portion and along the margins of the basin, where it is closer to the source areas. Soil texture within the alluvium typically grades finer towards the center of the valley at Borrego Sink. Aquifer tests indicate hydraulic conductivity in the older alluvium in the northern portion of Borrego Valley on the order of 300 to 350 feet per day, in the distal portions of the older alluvium in central Borrego Valley at 17 feet per day, and in the relatively shallow portions of the Palm Spring formation in southern Borrego Valley at 10 feet per day. Groundwater in the Borrego Valley appears to be isolated from Lower Borrego Valley. Isolation occurs in southern Borrego Valley due to the thick sequence of Palm Spring formation, which is at or near the surface in the vicinity of Desert Lodge and the Sleepy Hollow folds; and may also be due to the geometry of the basement complex, which is relatively shallow in the area from Yaqui Ridge to Borrego Mountain. The primary source of recharge to the Borrego Valley aquifer is from infiltration of runoff from the several creeks and intermittent streams that drain to the valley from the mountains of the surrounding watershed. This stream recharge has been estimated to have ranged from approximately 600 acre-feet to approximately 26,000 acre-feet annually, and average 3,860 acre-feet per year during the period 1945 through 2000. Bedrock recharge is another important source of recharge to the Borrego Valley aquifer, and is estimated to average nearly 1,800 acre-feet per year. Bedrock recharge occurs as subsurface seepage into the aquifer from the fractured crystalline basement rocks surrounding the basin. Total recharge, calculated as the sum of stream and bedrock recharge, is highly variable, ranging from approximately 600 acre-feet in 1975 to approximately 46,000 acre-feet in 1980, a range of almost two full orders of magnitude within a time period of only 5 years. Total recharge to the Borrego Valley aquifer has been estimated to average 5,670 acre-feet per year. Estimates of irrigation return flow were made by applying the chloride mass balance technique to soil samples collected from a citrus orchard and a golf course fairway. An estimated 22 percent of applied irrigation water in citrus orchards is returned to groundwater, while an estimated 14 percent of water applied to golf courses returns to groundwater. Other irrigation in the Valley is assumed to return at the 14 percent rate measured for golf courses. Net total groundwater extraction from the Borrego Valley aquifer was estimated based on well production records, where available, and depicted land use from aerial photographs. Net total groundwater extraction is estimated to have ranged from approximately 100 acre-feet in 1945 to approximately 17 ,000 acre-feet in 1959. Net total groundwater extraction was estimated at 15,300 acre-feet in 2000. A net water budget was calculated as the difference between total recharge and net total groundwater extraction. Groundwater extraction has exceeded recharge in all but the wettest years since 1947, averaging an overdraft of approximately 4, 100 acre-feet per year during the period 1945 to 2000. For the year 2000, the net overdraft was estimated at approximately 12,800 acre-feet.