In an average year, about 40 percent of the urban and agricultural applied water use or over 20 percent of total applied water in California is provided by ground water extraction. In drought years, when surface supplies are reduced, ground water provides an even larger percentage of applied water. This shift from surface to ground water supplies in drought years is an indication of the sheer magnitude of ground water storage versus surface storage. Surface water and ground water are really one source of supply that originates with precipitation and runoff.
DWR's Bulletin 118, California's Ground Water, September 1975, identified 450 ground water basins in the state. The statewide total amount of ground water stored in these ground water basins is estimated to be about 850 million acre-feet, about 100 times the annual net ground water use in California. Probably less than half of this total, under present circumstances, is usable because:
The large quantity of good quality ground water in storage makes it an extremely important component of California's total water resource that must be managed in conjunction with surface water supplies to ensure sustained availability. This chapter presents a definition of ground water and covers the history of ground water development in California, statewide ground water use, ground water overdraft, management of ground water, the effect of the 1987-92 drought on ground water, and conjunctive use.
Ground water is subsurface water occurring in a zone of saturation. In that zone, water fills the pore spaces or openings in rock and sediments. Large basins in southern California and the Central Valley can contain thousands of vertical feet of sediments washed in over millions of years by runoff. The sediments are a randomly interfingered mixture of fine-grained material that can restrict movement of ground water and coarse-grained material that constitutes the aquifers within a zone of saturation. An aquifer is a geologic formation that stores, transmits, and yields significant quantities of water to wells and springs. Ground water also occurs in limited quantities in fractured hard rock and is an important source for domestic supplies in foothill and mountain communities. However, the following discussion will focus on the ground water in basins with abundant ground water storage and high well yields.
Ground water basins in California have been defined on the basis of geologic and hydrologic conditions in DWR Bulletin 118, Ground Water Basins in California, January 1980. In Bulletin 118-80, some basin boundaries were modified to reflect political or water district boundaries that constitute potential ground water management units. Figure 4-1 illustrates components of ground water use and sources of ground water recharge.
When Europeans first arrived in California, essentially all of the ground water basins in the state were full of water. Marshes existed in many parts of California and many flowing streams were supplied from overflowing ground water basins. As California settlers began to use water for crop irrigation and for industrial and domestic purposes, readily available and reliable ground water was used to augment surface water supplies.
As the amount of ground water extraction increased, ground water levels in many basins began to decline as more of the aquifer in the basin was emptied each year. The empty portion of the aquifers provided available storage space for any water that was available for recharge. Some ground water recharge was provided by direct rainfall, but most recharge resulted from infiltration of surface water runoff directly into the sediments in the bottoms of stream channels, or by infiltration of a portion of the water applied to irrigate agricultural crops.
The amount of water flowing in many streams gradually decreased as more water infiltrated into stream bottoms and recharged depleted aquifers. In some basins, the amount of ground water extracted greatly exceeded the amount of runoff available in the streambed to recharge the basins, resulting in no surface flows out of some basins. In other years when flood flows occurred, surface water would again flow down the river channels. This process continues today.
Extensive ground water use during California's early development led to establishment of vigorous agricultural and urban economies. These sectors were later able to pay the costs of developing and importing surface water by building dams and conveyance systems to meet the growing demand for water; reduce ground water overdraft; and, in some instances, increase ground water storage.
In a year of average precipitation and runoff, an estimated 15 maf of ground water is extracted and applied for agricultural, municipal, and industrial use. There is a significant amount of ground water recharge from surface water and ground water used to irrigate agricultural crops. Some of the irrigation water flowing in unlined ditches and some of the water that is applied to irrigate crops infiltrates into the soil, percolates through the root zone and recharges the ground water basins. The annual net use of ground water is ground water extraction minus deep percolation of applied water. The 1990 statewide average annual net ground water use was about 8.4 maf. The use of prime supply from ground water basins for 1990 was about 7.1 maf, and the remaining 1.3 maf was overdrafted from the basins. (Ground water prime supply is the long-term average annual percolation into major ground water basins from precipitation and from flows in rivers and streams.) Table 4-1 shows use of ground water (excluding overdraft) by hydrologic region.
In an average year, the amount of deep percolation from applied surface and ground water supplies that recharges the aquifers is an estimated 6.5 maf. In addition, over 7.0 maf recharges naturally from rainfall and streambed seepage. Still more water is recharged deliberately through artificial means. Statewide, the average amount of ground water extracted exceeds the average recharge by about 1.3 maf--a considerable reduction from former estimates of nearly 2 maf--and is largely the result of changes in water management. Implementation of agricultural water conservation and urban landscape conservation will decrease deep percolation of applied water, thereby reducing future ground water recharge and perennial yield of ground water basins. In areas like San Joaquin and Tulare regions, where deep percolation of applied water is a major contributor of ground water perennial yield, this process could exacerbate ground water overdraft in the future.
In wet years, when more surface water is available, less ground water is extracted, more recharge occurs, and ground water levels can recover. Conversely, in years of low runoff, such as the 1987-92 drought, much less surface water is available for recharge, and much more ground water is extracted. Ground water use also varies in different areas of the State; ground water may provide as little as a few percent or as much as 90 percent of the total applied water in an area during an average year.
Table 4-2 shows the normalized 1990 level of development for ground water. The perennial yields include the benefits of imported surface supplies that have occurred historically. In areas that rely on SWP or CVP imports from the Delta, future perennial yields may be reduced because of changes in the amount of surface water that is imported.
In areas where water demands exceed available surface water and sustainable ground water supplies, a portion of the difference between supply and demand is often made up by extracting ground water, thereby decreasing the amount of ground water in storage in those basins. Where the ground water extraction is in excess of inflow to the ground water basin over a period of time, the difference provides an estimate of overdraft. Such a period of time must be long enough to produce a record that, when averaged, approximates the long-term average hydrologic conditions for the basin. Bulletin 118-80 defines "overdraft" as the condition of a ground water basin where the amount of water extracted exceeds the amount of ground water recharging the basin "over a period of time." It also defines "critical condition of overdraft" as water management practices that "would probably result in significant adverse overdraft-related environmental, social, or economic effects." Water quality degradation and land subsidence are given as examples of two such adverse effects. Table 4-3 shows 1990 estimated ground water overdraft by hydrologic region.
During the 1987-92 drought, ground water, where available, was extracted to make up for reductions in surface water deliveries. The result was that ground water levels and the amount of ground water in storage declined considerably. Such a decline is not considered overdraft, rather it is considered as removal of ground water from storage, similar to removal of water from a surface reservoir. In the past, such declines have been reversed during wet years when surface water reservoirs refilled and ground water aquifers were recharged.
Ground water quality degradation reduces usable ground water storage in ground water basins. Ground water overdraft in a basin can produce a gradient that induces movement of water from adjacent areas. If the adjacent areas contain poor quality water, degradation can occur in the basin. There is a west-to-east water gradient in the San Joaquin valley from Merced County to Kern County. Poor quality ground water moves eastward along this gradient, displacing good quality ground water in the trough of the valley. The total dissolved solids in the west side of the valley generally range from 2,000 to 7,000 milligrams per liter, the east side water from 300 to 700 milligrams per liter. This adverse effect of overdraft and possible degradation of ground water quality in the San Joaquin Valley has been evaluated and included in ground water overdraft estimates.
In the short term, those areas of California that rely on Delta exports for all or a portion of their supplies face great uncertainty in terms of water supply reliability due to the uncertain outcome of a number of actions undertaken to protect aquatic species in the Delta. For example, in 1993, an above-normal runoff year, environmental restrictions limited CVP deliveries to 50 percent of contracted supply for federal water service contractors from Tracy to Kettleman City. Because ground water is used to replace much of the shortfall in surface water supplies, limitations on Delta exports will increase ground water overdraft in the San Joaquin River and Tulare Lake regions, and in other regions receiving a portion of their supplies from the Delta.
The ground water basins in small coastal areas of the Central Coast Region have limited storage capacity. During drought periods, water levels in most of these basins sometimes decline to a point where ground water basins are not usable. However, during wet periods, most of these basins recover, thus making evaluation of overdraft or perennial yields difficult. Overdraft amounts shown for the Central Coast Region were estimated by reviewing previous studies and could be overestimated. In addition, the Central Coast presently receives USBR water through San Felipe and will soon receive SWP water through the Coastal Branch of the California Aqueduct. These imported supplies could reduce overdraft in the region. A more comprehensive study of the ground water use in this region is needed to more accurately estimate the overdraft.
Estimated overdraft amounts are based on overdraft being defined as the amount of ground water extracted for the 1990 level of development that is in excess of the current perennial yield. "Current perennial yield" is the amount of ground water that can be extracted without lowering ground water levels over the long-term. Perennial yield in basins where there is hydraulic continuity between surface and ground water depends in part on the amount of extraction that occurs. Perennial yield can increase as extraction increases, as long as the annual amount of recharge is equal to, or greater than, the amount of extraction. Extraction at a level that exceeds the perennial yield for a short period does not result in an overdraft condition. In basins with an adequate ground water supply, increased extraction may establish a new hydrologic equilibrium with a new perennial yield. The establishment of a new and higher perennial yield requires that adequate recharge be induced. The methods used to estimate perennial yield and ground water overdraft assume that the amount of ground water extracted for the 1990 level of development is the amount of extraction that has taken place, or could take place, without lowering ground water levels over a long period of time. These estimates must include evaluation of the existing water management program in the basin.
Changes in surface water deliveries will undoubtedly change the perennial yield and overdraft conditions in the future. For example, delivery of surplus surface water supplies from the SWP and CVP will probably occur much less frequently in the future. Such decreases in delivery of surface water will probably decrease perennial yields in basins that receive SWP and CVP water.
Along some parts of the coast, declining ground water levels allow sea water to intrude into fresh water aquifers. Los Angeles County operates sea water intrusion barrier projects in West Basin and Dominguez Gap. Los Angeles and Orange counties jointly operate a sea water intrusion barrier in Los Alamitos Gap, which straddles the border between the two counties. In most of these barriers, water from water recycling facilities or from MWDSC imported deliveries is injected and flows down gradient in both directions--toward the ocean as well as inland where it mixes with ground water in the aquifer and can be extracted by irrigation and municipal wells. In some basins, a sea water intrusion barrier may be a cost-effective management tool that would allow greater use of the basin's ground water storage capacity.
In Salinas Valley, sea water intrusion was occurring before the drought began. During the drought, the rate of intrusion accelerated because of decreased ground water recharge and increased ground water extraction. Monterey County Water Resources Agency has formulated long-term plans to construct and operate facilities to substitute surface water for ground water to alleviate the sea water intrusion problem. The SWRCB is putting pressure on the Agency to start action immediately to stop the intrusion, which is now almost 5 miles inland and threatens to contaminate municipal wells in Salinas. MCWRA is dealing with overdraft and sea water intrusion in the coastal areas of the Salinas Basin and is in the process of preparing the Salinas River Basin Management Plan. Under this plan, MCWRA will screen management alternatives for preparation of an EIR/EIS. The agency has also adopted eight ordinances including requiring the metering of all wells with a discharge size greater than three inches, agricultural and urban conservation measures, establishing upper pumping limits, and ground water management charges with penalties for use exceeding the pumping limits. Sea water intrusion is also occurring in the area of the Pajaro River. Pajaro Valley Water Management Agency and the City of Watsonville are formulating plans to address the problems in that area.
In Ventura County, elevated chloride levels have been measured in much of the Oxnard Plain since the 1950s. Recent studies have concluded that there are three sources of chloride: sea water intrusion in a relatively small area; a larger area into which saline water has migrated from adjacent marine formations; and leakage of chloride from an upper perched aquifer through failed well casings into an underlying aquifer. The sea water does not appear to be moving inland. Local agencies are developing programs to address the migration of saline water and the wells that have been improperly destroyed. Fox Canyon Ground Water Management Agency, United Water Conservation District, and City of Ventura are all formulating plans to address the problems in that area.
In some parts of California, ground water extraction has caused subsidence of the land surface. Accurate prediction of subsidence is generally not possible with our present level of knowledge or current data about the extent and properties of aquifer sediments in subsidence areas. In some areas subsidence occurs when ground water levels decline below a certain level. Data collected from six extensometers in Westlands Water District indicate that subsidence occurred in 1990, 1991, and 1992, with the highest amount of subsidence occurring in 1991. Land subsidence can change canal gradients, damage buildings, and require repair of other structures. In some instances, local water management agencies may determine that a certain amount of land subsidence is allowable as a part of their ground water management program.
In areas where ground water extraction is proceeding or where such programs are planned, the potential for subsidence should be evaluated. Water managers may wish to include extensometer and land surface surveying if subsidence is a real potential.
A change in ground water gradient may accelerate movement of contaminants toward water-producing wells. (See Chapter 5 for an explanation of contaminant movement and levels.) This accelerated movement of contaminants may be particularly true where ground water levels have been lowered significantly because of increased extraction during droughts. However, a ground water monitoring program for water levels and water quality is necessary to evaluate such changes.
Ground water basin management is defined as: protection of natural recharge and use of intentional recharge; planned variation in amount and location of extraction over time; use of ground water storage conjunctively with surface water from local and imported sources; and, protection and planned maintenance of ground water quality. If the basin is managed to achieve these goals, ground water overdraft will be reduced and water supplies of good quality will be sustainable.
Initial use of ground water in California considered only one aspect--building a well and extracting ground water. It was only when ground water levels began to decline, or landowners could not extract enough water from their wells, that consideration was given to the second aspect of ground water use--recharge. In contrast, no one would think of building a dam for water supply purposes before first identifying and quantifying a source of water to fill the reservoir behind the dam. Water managers in many areas where ground water was depleted realized that action was required and requested legislation to provide authority to manage the ground water basins.
The type of management structure and the extent of management of ground water basins in California vary considerably. In part, this variety arose because ground water was treated as a property right while surface water was treated under a complex system of riparian and appropriative rights. The result is that ground water is regulated both by statute and by case law from court decisions. As might be imagined, the combination of the two makes for great complexity in managing this resource.
Management of ground water in California has generally been considered a local responsibility. This view is strongly held by landowners and has been upheld by the Legislature (in a number of statutes that have established local ground water agencies) and by the courts (in decisions). State agencies have encouraged local agencies to develop effective ground water management programs to maximize their overall water supply and to avoid lengthy and expensive lawsuits resulting in adjudicated basins. The end result of either local agency ground water management programs or adjudication may be similar. Effective management can be achieved through either method.
Thirteen ground water basins have been adjudicated and are operated in accordance with court settlements. A fourteenth watershed has been adjudicated in federal court, but water users are not limited in their ground water extraction.
The California Water Code provides for management and distribution of surface water and in many instances provides some limited authority to deal with ground water through a number of types of local water agencies and districts, formed either by general or special legislation. Nine ground water management agencies have been authorized by the State Legislature. These agencies can enact ordinances affecting ground water extraction, establish zones of benefits, and charge a ground water extraction fee or levy taxes for actions that benefit the extractors. "Zone of benefit" means an area, including but not limited to, subbasins within a district which will benefit from planning, studies, or any management program undertaken by that district in a manner different from other areas or subbasins within the district (Water Code, Appendix 119-322 and 135-833).
Many water agencies have statutory authority from the Legislature to levy charges for ground water extraction when it is shown that the surface water conveyed to the area recharges the aquifer, thereby benefiting the ground water extractors. Not all of these agencies have exercised that authority. Some of those that have are Orange County Water District, Rosedale-Rio Bravo Water Storage District, Santa Clara Valley Water District, Monterey Peninsula Water Management District, and recently, Monterey County Water Resources Agency.
Such charges are colloquially called a "pump tax," although the term "water replenishment assessment" is used in the Water Code. The water replenishment assessment may consist of a water charge, a general assessment, a replenishment assessment, or a combination of two or more of the above.
In 1992, the Water Code was amended (Water Code Section 10750, et seq.) to provide authority and define procedures to allow certain local agencies to produce and implement a ground water management plan. To date, more than 40 local agencies have expressed interest in using that section of the Water Code provision to adopt a ground water management program. A number of those agencies have adopted resolutions of intent in accordance with Water Code Section 10750 to adopt a ground water management plan. Adoption of such a resolution allows the agency two years to adopt a plan. If no plan is adopted in that time frame, the agency must start the process over again. The Water Code encourages coordination between agencies in the same basin. Early indications are that some agencies that share a basin are interested in formulating their own plans, while some other agencies that share a basin intend to develop one coordinated cooperative plan for the entire basin. In addition, several mutual water companies have expressed interest in developing ground water management plans. However, such local entities are not included in the legal definition of "local agency" but can sign Memorandums of Understanding with local agencies to develop a ground water management plan under Section 10750.
In 13 adjudicated ground water basins, ground water extraction is regulated by a watermaster that has been appointed by the court. Twelve of these adjudicated basins are in Southern California and one is in Northern California (Figure 4-2). Ground water extraction in each of these basins was adjudicated with concern only for ground water quantity. Ground water quality was not a part of the original court decisions.
The amount of ground water that each well owner can extract is determined by the court decision and is based on the amount of ground water that is available each year, as determined by the watermaster. While each court decision may be slightly different, the goal is to avoid ground water overdraft by providing sustainable yield. Adjudication of these ground water basins has generally resulted in additional imports of surface water supplies to make up for reduced extraction.
The thirteen adjudicated ground water basins and watermasters in California are:
Ground water and surface water in a fourteenth basin, Santa Margarita River Watershed in Riverside and San Diego Counties, has also been adjudicated by the federal court. Water users are required by the court decision to report to the court-appointed water master the amount of surface water they divert from the river, canals, or ditches, and the amount of ground water they extract from the aquifer. However, the amount of water they are entitled to is not limited by the decision.
The watermaster for Main San Gabriel Basin in Southern California has since returned to the court and obtained approval of regulations to control extraction for the purpose of protecting ground water quality. Ground water underflow from Puente Basin, a part of Main San Gabriel Basin, was addressed in a court decision separate from the Main San Gabriel adjudication. The court named two individuals to act in the capacity of watermaster.
The Legislature has enacted several specific statutes establishing ground water management agencies that can enact ordinances to regulate the amount of ground water that is extracted and limit its place of use within the district's boundaries. Nine ground water management agencies have been formed by such special legislation. (See Figure 4-3 for their locations.)
While these agencies have the authority to pass ordinances, such ordinances limiting extraction are not popular with landowners within the agency's boundaries. In addition, the funding for studies that are required to establish zones of benefit to ensure equitable assessments has not been readily available. Therefore, it is not yet clear whether these agencies will become viable and effective at managing ground water in a manner that conserves quantity and preserves good quality.
The nine ground water management agencies are:
A number of water districts have obtained Legislative authority to levy a pump charge on wells that extract a certain amount of ground water. Two of these districts manage their surface water and ground water in a conjunctive operation. The third is moving in the same direction. These water districts are:
Desert Water Agency and Coachella Valley Water District are authorized to levy replenishment assessment charges to fund certain programs. Many other flood control and water conservation districts, water storage districts, water replenishment districts, irrigation districts, community services districts, water agencies, and others either manage surface water only or may be involved in some minor ground water management. Management of surface water can affect the timing and location of ground water extraction, use, and recharge.
The large amount of ground water available in California's ground water basins provided a reliable source of water during the 1987-92 drought. During previous droughts ground water extraction has provided as much as 60 percent of urban and agricultural applied water statewide. The following sections describe the effects of drought on ground water levels and storage and potential impacts from overdrafting basins.
The depth of water in wells in California's ground water basins differs considerably among basins and even in different parts of the same basin. The water levels are affected by many factors, including the amount of recharge that has occurred in previous years, the ratio of surface water to ground water used, the total number and location of wells extracting ground water from the basin, the amount of ground water that flows out of the basin, and the total amount of ground water extracted from the basin.
While smaller surface water reservoirs can refill in a single year if the precipitation and runoff are above normal, it can take several years of above normal precipitation before ground water levels in a basin recover to pre-drought levels. The increase in ground water storage is a function of the amounts of pumping and natural recharge, as well as the contribution to recharge from applied irrigation water or direct recharge operations.
The amount of ground water currently in storage in the San Joaquin Valley has decreased considerably since 1987 because of the low amount of recharge from spring 1987 through spring 1992, combined with the large amount of ground water that was extracted during that time.
As a result of the drought, it was expected that the extraction of ground water through spring 1992 would be much higher than normal. In Kern County, more ground water was extracted between spring 1991 and spring 1992 than during the previous four years. However, the amount of ground water extracted between spring 1991 and spring 1992 in Stanislaus, Merced, Madera, Fresno, Tulare, and Kings counties was significantly less than the amount of ground water extracted during the previous few years. The reasons for the unexpected decreases in ground water extractions are still being investigated. Possible factors include rainfall variations, fallowed land, changes in crops, a high intensity-long duration rainfall in some parts of California in March 1991, and somewhat better runoff amounts in 1991 than in 1990 for the southern Sierra Nevada. The change in ground water in storage in the San Joaquin Valley is shown in Figure 4-4.
Ground water levels in most basins rose as a result of ground water recharge from the storms that passed over California in December 1992 and January through March 1993 which provided large amounts of precipitation and runoff. Such recovery of ground water levels in many basins occurs during wet years, primarily as a result of two factors:
The net change in the amount of ground water storage during summer 1993 will not be known until spring 1994 water level measurements are evaluated. The spring measurements of any year reflect events that occurred during the previous 12 months. Thus, spring 1993 water level measurements reflect the recharge that occurred in winter 1992-93 and the extraction that took place in summer 1992.
In the Sacramento Valley, ground water levels and storage did not decline significantly in Glenn and Colusa counties during the 1987-92 drought. In Butte and Tehama counties, ground water levels declined, but some remained higher than they were after the 1976-77 drought. The change in ground water storage in the Sacramento Valley is shown in Figure 4-5.
In coastal areas, some ground water basins have limited storage. Ground water levels in such basins are often lowered to near critical levels each fall, thus making evaluation of overdraft or sustainable yield difficult. These basins require relatively little time to recharge to return to a full condition. As a result, ground water levels in these basins can rise rapidly due to high rainfall such as occurred in March 1991, December 1992, and January through March 1993.
The ground water basins surrounding Clear Lake in Lake County also have limited storage capacity. Each year ground water levels in these shallow ground water basins decline to a point where ground water quality starts to deteriorate. But each winter these basins normally refill. In these areas of limited storage, ground water has very little capacity to support additional development.
Ground water levels in the adjudicated basins and managed basins in Southern California vary. In Main San Gabriel Basin and the coastal plain of Orange County, water levels are about at the middle of their court-approved operating range. Ground water levels in San Fernando Valley range from high to low, depending on location. Levels in Central and West Coast Basins are fairly high.
Reduction of surface supplies during drought increases ground water extraction while recharge remains significantly below normal. As ground water levels decline, more energy is required to lift the water to the surface, adding to the cost of water for urban and agricultural use. Furthermore, existing wells often become unusable, requiring deepening or, in some cases, replacement of wells. (Figure 4-6 shows the number of well completion reports filed, by year, from 1974 through 1992.) Upon the return of normal or above normal precipitation, such as that occurring in late 1992 and 1993, ground water extraction decreases markedly as surface water becomes more available. The shift from using ground water to using surface water results in significant ground water recharge.
The number of new wells reported as drilled during the 1987-92 drought peaked in 1990 after increasing during the earlier years of the drought. Slightly over one-third of the wells reported in 1990 were monitoring wells and many others were either replacement or deepening of existing wells.
Conjunctive use is the operation of a ground water basin in coordination with a surface water system to increase total water supply availability, thus improving the overall reliability of supplies. The basin is recharged, both directly and indirectly, in years of above-average precipitation so that ground water can be extracted in years of below-average precipitation when surface water supplies are below normal. In some instances conjunctive use is employed for annual regulation of supplies. These programs involve recharge with surface water or reclaimed water supplies and same-year extraction for use. Aquifer storage and recovery programs are a good example of conjunctive use. Following is a discussion of effective conjunctive use programs and the types of programs in-place today.
Conjunctive use programs are designed to increase the total usable water supply by jointly managing surface and ground water supplies as a single source. As such, they are widespread in California but differ greatly in their intensity and degree of planning. Management can vary from recharging a limited amount of sporadically available surface water to a comprehensive management program that coordinates surface water use, delivery, recharge, and ground water extraction and use.
In the future, carefully planned conjunctive use will increase and become more comprehensive because of the need for more water and the generally higher cost of new surface water facilities. Conjunctive use programs generally promise to be less costly than new traditional surface water projects because they increase the efficiency of water supply systems and cause fewer negative environmental impacts than new surface water reservoirs.
Various local agencies have implemented programs and coordinated with other agencies to recharge surface water, when it is available, so that ground water will be stored in the aquifer until it is needed. These agencies have effectively secured or implemented some or all of the following components of a conjunctive use program:
Carefully planned and implemented conjunctive use programs can be developed without causing significant adverse impacts. However, the effect of such programs on native vegetation and wetland habitat, fish and wildlife resources, third parties, land subsidence, and degradation of water quality in the aquifer must be evaluated. Phreatophytic vegetation may be stressed when ground water levels are lowered because less water is available in root zones. Similar processes can also affect wetlands. Potential adverse effects on third parties include lowering of ground water levels below the bottom of wells, or raising ground water levels so that local flooding occurs. Subsidence caused by extraction of ground water can affect canals, wells, buildings, tanks, bridges, and levees that require costly repair. Ground water quality can be degraded if ground water gradients induce movement of lower quality water into the aquifer.
Interest in conjunctive use as a means of augmenting supplies that may then be exported to areas outside the basin has led to questions about the feasibility and legal complexity of water transfers involving ground water. Both the State Water Code and the recently passed Central Valley Project Improvement Act of 1992 specify that any water transfers under their respective jurisdictions cause "no significant long-term adverse impact on ground water conditions in the transferor's service area." The CVPIA requirement will affect water districts that receive water from the CVP and seek to transfer either surface or ground water.
A broad range of conjunctive use activities have been undertaken in California, although many of them probably were not thought of as conjunctive use when developed. The range of conjunctive use activities in California is illustrated by the following partial list of examples of programs in place today.
Alameda County Water District. The district is located near the mouth of the Niles Cone area of Alameda County, adjacent to San Francisco Bay. Historically, extraction of ground water from the basin lowered ground water levels and allowed sea water from the Bay to intrude. In response, the district has developed an extensive program to recharge local supplies from Alameda Creek and imported supplies from other surface sources.
Kern County. In Kern County, a mix of local, regional, and State conjunctive use projects are operating or are under development. The Kern County Ground Water Basin is in overdraft although changes in storage vary considerably depending on the surface water availability to local agencies. Several districts have responded by building and operating recharge projects that take advantage of imported and/or local surface water when available. For example, the Rosedale-Rio Bravo Water Storage District purchases surface water from three sources and recharges ground water via Goose Lake Slough. Essentially all water use within the district is supplied by ground water.
On an interregional scale, the Arvin-Edison Water Storage District and the Metropolitan Water District of Southern California are developing a cooperative water banking project. In this complex program, Arvin-Edison will provide MWDSC water during dry years from Arvin-Edison's CVP supply and will replace this water by pumping ground water from a basin previously recharged with surface water supplies made available by MWDSC from its SWP supply. (See Chapter 11 for more details about the program.)
The Department of Water Resources, in cooperation with local agencies in Kern County, is developing the Kern Water Bank project to augment the supplies available to SWP contractors in drought years. (See Chapter 11 for more details.)
Metropolitan Water District of Southern California. In 1989, MWDSC implemented a seasonal ground water storage program utilizing both direct and in lieu recharge and storage in local ground water basins to increase emergency supply and provide carryover storage for droughts.
Orange County Water District. This district has one of the most elaborate conjunctive use programs. It purchases imported surface water from MWDSC for ground water recharge, manages runoff and recycled water in the Santa Ana River, manages extraction from the basin, operates a sea water intrusion barrier, is contemplating additional barriers to allow use of even more ground water storage capacity, is improving ground water quality in areas where it has been degraded, and recharges a large quantity of recycled water.
Santa Clara Valley Water District. The district provides and operates treatment and distribution facilities for surface water imported from the SWP and the CVP and recharge sites for local surface and imported water supplies. The basin is managed to provide an adequate supply of ground water annually, eliminate land subsidence, and provide carryover ground water storage as a buffer against dry years when local and imported surface water supplies are reduced.
South Sutter Water District. Irrigated agriculture in this area has relied on ground water for many years. As a result, a regional ground water depression developed as local pumping exceeded recharge. In response to the declining ground water levels, the district constructed Camp Far West reservoir on the Bear River to develop a partial surface water supply for the district. This has been successful in reducing demand on the ground water basin, which has since recovered. During extended dry periods, increased ground water use causes ground water levels to fall. The district is investigating ways to further develop the conjunctive use potential of the basin.
United Water Conservation District. The district captures winter runoff in Lake Piru and releases the water each fall down the Santa Clara River to replenish the ground water basins along the river. These basins have limited storage capacity and are generally operated on an annual cycle that largely uses the entire capacity. United also operates two spreading areas to recharge the Oxnard Plain ground water basin in coastal Ventura County.
Westlands Water District. The early development of irrigated agriculture in Westlands was based on extraction of ground water from a deep, confined aquifer system. This development resulted in extensive land subsidence. To alleviate this problem, Westlands obtained an imported surface water supply from the CVP that allowed it to largely eliminate ground water pumping in most years. In years with deficient surface water supplies, water users revert to ground water pumping.
Yolo County Flood Control and Water Conservation District. This district operates Clear Lake and Indian Valley reservoirs to provide a surface water supply for irrigated agriculture. The district does not have the capability of extracting ground water, but local farmers maintain the capability to largely offset dry year surface water shortages by pumping additional ground water. The district has undertaken a program to artificially recharge ground water in its service area.
In the future, conjunctive use is expected to increase and become more comprehensive if California's water needs are to be met in a cost effective and efficient manner while resolving conflicts with other resources. Conjunctive use programs generally promise to be less costly than new traditional surface water projects as they increase the efficiency of existing systems and are expected to cause fewer negative environmental impacts.
The State should encourage efforts to develop ground water management programs at the local and regional levels and to remove legal, institutional, financial, and other barriers that limit conjunctive use of ground water basins. The programs should be focused on solutions to clearly identified problems, such as overdraft, and natural and human-caused contamination so as to optimize the use of surface and ground water resources. Specific recommendations are as follows: