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CONJUNCTIVE USE Conjunctive use of groundwater and surface water is rapidly becoming an important component of water marketing plans. Under a conjunctive use program, surface water is relied upon for irrigation, urban use, and to recharge groundwater basins in wet years, Excess water is stored, or banked, in underground aquifers so that it is available when surface supplies are low i.e. dry years. Storing water underground can have several advantages over surface water storage. It can be less damaging to the environment than the construction of reservoirs and dams and usually does not require an extensive distribution system. Water banked underground has a lower evaporation rate. Surface water also can be substituted for groundwater through in-lieu techniques in areas that historically have relied either solely on groundwater or on a combination of ground and surface water. There are numerous examples of large-scale conjunctive use in California. However, according to a 1995 analysis by the Natural Heritage Institute (NHI), California has not realized the full potential of conjunctive use because of the difficulties caused by state laws and institutions and because of the state's emphasis on surface water development. The NHI study advocates exploring the potential of using large groundwater basins to store water to increase the state's surface water supply. The water would be captured for environmental, municipal and agricuitural use through a voluntary state level conjunctive use program. In the NHI exploratory study, an operations model was used to identify the potential for a groundwater storage and retrieval system in the Central Valley yielding approximately 1 million acre-feet of water in an average year. Like other marketing examples, NHI believes urban water supply agencies are likely to be the partner most able to absorb costs of a voluntary conjunctive use program in exchange for water supply benefits. Conjunctive use programs are being implemented by some local water entities including Kern Water Bank, SemitropicWater Storage District (Semitropic) and MWD. In the 1980s, the SWP purchased 20,000 acres along the Kern River west of Bakersfield in order to develop 16 an underground water bank. During renegotiation of contracts with SWP contractors in 1994, the bank was placed under the control of the Kern County Water Agency. In 1998, Kern County recharged about 300,000 acre-feet of water into its aquifer and recently has proposed a transfer of up to 100,000 acre-feet of SWP water to the SDCWA. The Kern County Board of Supervisors also passed an ordinance in 1998 requiring all out~of~county water transfers to obtain a county conditional use permit. The new ordinance is intended to curb groundwater overdraft caused by pumping and selling too much groundwater. The Semitropic groundwater banking program located in western San Joaquin Valley has been in operation since 1990. Semitropic has the capacity to eventually store up to 2-million acre-feet of water - 1 million of which is earmarked for banking partners. As of 1999, about 577,000 acre-feet had been banked, primarily by SWP contractors - includ- ing MWD, Santa Clara Valley Water District, Alameda County Water District and Zone 7 Water Agency - at a cost of about $175 an acre-foot for storage, pumping, delivery, and maintenance costs. Much of the operation is done through in-lieu recharge. In 1998, Vidler Water Co, a private marketer of water, agreed to pay Semitropic $23 million over 10 years to bank about 185,000 acre-feet of water on the premise that the water can be withdrawn and sold during critically dry years. MWD also has arranged a water banking agreement with Arvin-Edison Water Storage District. located in southern California. In 1998, Arvin-Edison completed work on a 200-acre (eventually to be expanded to 550 acres) spreading basin in order to bank up to 250,000 acre-feet of water for MWD. About 150,000 acre-feet of water has been banked thus far. MWD will be able to extract up to 40,000 acre-feet of water in dry years. Ratherthan a direct exchange between Arvin-Edison and MWD, which would violate service area contract regulations, the CVP water will be wheeled through the Kern County Water Agency and exchanged for SWP water, Consequentiy, water will only be available to MWD when flood releases are being made from Friant Dam and when capacity is available within the Friant-Kern Canal. MWD says it will pay around $25 million for the storage with about half of the money being used to build a new canal connecting the Arvin-Edison bank with the Califor- nia Aqueduct. 1