Groung Water Supply 6

| Home | The District | Drainage | Links | Reference Library | Water Conservation | Water Supply |

CVP Contract Supplies

Transfers

Groundwater
DIP Program

Groundwater Manage-
ment Program

Groundwater Maps

Deep Groundwater Conditions

GROUNDWATER SUPPLY

Farming in the Westlands area was originally developed through the use of groundwater for irrigation. The groundwater basin underlying Westlands is comprised generally of two water-bearing zones: (1) an upper zone above a nearly impervious Corcoran Clay layer containing the Coastal and Sierran aquifers and (2) a lower zone below the Corcoran Clay containing the Sub-Corcoran aquifer. The location of these water-bearing zones is depicted on a generalized cross section of the District shown on below.

These water-bearing zones are recharged by subsurface inflow from the east and northeast, the compaction of water-bearing sediments, percolation of pumped groundwater, and percolation from imported and natural surface water. Land subsidence due to groundwater overdraft ranged from 1 to 24 feet between 1926 and 1972 (U.S. Geological Survey (USGS), 1988). Surface water deliveries from the San Luis Unit (SLU) began in 1968 and largely replaced groundwater for irrigation. However, extensive pumping occurred in 1977, a drought year when deliveries of CVP water amounted to only 25 percent of the District's entitlement. In response to the surface water shortfall, farmers reactivated old wells and constructed new wells, pumping groundwater to irrigate their crops. During 1977, groundwater pumping rose to nearly 500,000 AF and the piezometric surface declined about 90 feet, resulting in localized subsidence of about 4 inches according to USGS officials.

Groundwater pumping increased to about 300,000 AF in 1989-90 because of decreased CVP water supplies caused by the drought. Pumping during 1990-91 and 1991-92 is estimated to be about 600,000 AF annually. This increase in pumping has resulted in a piezometric water surface decline of about 91 feet from 1988 through 1991, but had recovered by 1997.

A study by the Bureau, USGS, and Westlands estimated the safe yield of the deep confined aquifer underlying Westlands to be between 100,000 and 135,000 AF annually (Westlands Water District, 1980). Westlands does not supply groundwater to District farmers nor does the District regulate or control groundwater pumping; individuals pump their own groundwater. The District does, however, survey the static water levels in the wells and the water quality and quantity of the pumped groundwater, as part of the Groundwater Management Plan completed under provisions of AB3030 in 1996. More recent district analysis of these data indicate that a better estimated safe yield may be between 135,000 and 200,000 AF. Contour maps showing the depth and elevation to deep groundwater in the District are compiled annually. Bulletin 118 -Groundwater Basin Map- is a DWR publication for more general information about groundwater in California.


\| Home \| The District \| Drainage \| Links \| Reference Library \| Water Conservation \| Water Supply \|
CVP Contract Supplies Transfers Groundwater DIP Program Groundwater Manage- ment Program Groundwater Maps Deep Groundwater Conditions	GROUNDWATER SUPPLY
	Farming in the Westlands area was originally developed through the use of groundwater for irrigation. The groundwater basin underlying Westlands is comprised generally of two water-bearing zones: (1) an upper zone above a nearly impervious Corcoran Clay layer containing the Coastal and Sierran aquifers and (2) a lower zone below the Corcoran Clay containing the Sub-Corcoran aquifer. The location of these water-bearing zones is depicted on a generalized cross section of the District shown on below. These water-bearing zones are recharged by subsurface inflow from the east and northeast, the compaction of water-bearing sediments, percolation of pumped groundwater, and percolation from imported and natural surface water. Land subsidence due to groundwater overdraft ranged from 1 to 24 feet between 1926 and 1972 (U.S. Geological Survey (USGS), 1988). Surface water deliveries from the San Luis Unit (SLU) began in 1968 and largely replaced groundwater for irrigation. However, extensive pumping occurred in 1977, a drought year when deliveries of CVP water amounted to only 25 percent of the District's entitlement. In response to the surface water shortfall, farmers reactivated old wells and constructed new wells, pumping groundwater to irrigate their crops. During 1977, groundwater pumping rose to nearly 500,000 AF and the piezometric surface declined about 90 feet, resulting in localized subsidence of about 4 inches according to USGS officials. Groundwater pumping increased to about 300,000 AF in 1989-90 because of decreased CVP water supplies caused by the drought. Pumping during 1990-91 and 1991-92 is estimated to be about 600,000 AF annually. This increase in pumping has resulted in a piezometric water surface decline of about 91 feet from 1988 through 1991, but had recovered by 1997. A study by the Bureau, USGS, and Westlands estimated the safe yield of the deep confined aquifer underlying Westlands to be between 100,000 and 135,000 AF annually (Westlands Water District, 1980). Westlands does not supply groundwater to District farmers nor does the District regulate or control groundwater pumping; individuals pump their own groundwater. The District does, however, survey the static water levels in the wells and the water quality and quantity of the pumped groundwater, as part of the Groundwater Management Plan completed under provisions of AB3030 in 1996. More recent district analysis of these data indicate that a better estimated safe yield may be between 135,000 and 200,000 AF. Contour maps showing the depth and elevation to deep groundwater in the District are compiled annually. Bulletin 118 -Groundwater Basin Map- is a DWR publication for more general information about groundwater in California.