Within California, groundwater satisfies about 35% of fresh water demand, under “normal” climate and current land use conditions, but reliance on groundwater is highly variable across the state. The Colorado River hydrologic region (in the southwestern corner of the state) uses groundwater to satisfy <10% of normal freshwater demand, whereas the Central Coast hydrologic region (extending from Santa Cruz to Santa Barbara) uses groundwater to satisfy >80% of normal demand. During drought periods, there is a commonly a significant increase in groundwater use, both in absolute terms and as a fraction of total freshwater demand.

During years where surface water is not available and rainfall is scarce, groundwater may provide up to 100 percent of irrigation water for certain areas and, where surface supplies are not accessible or economically feasible, groundwater can provide 100 percent of a community’s public water.

Most groundwater originates as precipitation (rain or snow) that falls to the ground, infiltrates, and is eventually recharged into an aquifer. The age of groundwater can be difficult to determine, in part because many aquifers contain a mixture of water having a range of ages. In general, shallower groundwater is younger than deeper groundwater. Groundwater tends to become saltier naturally as it ages and interacts with surrounding rock, but groundwater chemistry can also be influenced by human activities. Groundwater management focuses on both water supply and water quality.

Water Savings

The most obvious way to save groundwater is to not pump it out of the ground. This can also be the cheapest way to improve groundwater resource conditions, if there has been excess extraction from a groundwater basin. Of course, reducing groundwater pumping generally requires finding an alternative supply. One obvious approach is to use surface water when it is available, and then switch to using groundwater when there is an inadequate surface supply. When this approach is practiced on a broad scale, it is referred to as “conjunctive use.” Resting groundwater wells by using alternative supplies can allow aquifer levels to recover, and can be considered a form of “in lieu” recharge.

But resting wells will not assure that water not pumped is “saved” for future use. For example, pumping of a groundwater well adjacent to a stream could result in capture of stream flow. In this case, pumping causes recharge, the flow of water from the surface into an aquifer. If all of the water pumped from a well comes from a nearby stream, then resting that well might not help to restore groundwater supplies. Also, pumping is not the only means by which water is removed from a groundwater basin. Groundwater outflow can occur by evapotranspiration (ET, water loss by evaporation and use by plants), baseflow to streams, flow into adjacent basins, or discharge to the ocean (for basins in coastal areas). In some cases, pumping of groundwater captures one or more of these flows.

Many groundwater managers and stakeholders are concerned about limiting damage that can be done to groundwater resources if extractions exceed the “sustainable yield” of an aquifer. There are many definitions for “sustainable yield,” but the most widely accepted by practicing hydrologists is the amount of water that can be pumped over the long term without causing unacceptable harm. In general, sustainable yield is less than natural recharge. Of course, what is “unacceptable” is open to discussion, and harm can be measured using a number of physical, economic, political, legal, or social metrics. There are similar terms like “safe yield” and “basin yield” that are also commonly used.

Groundwater overdraft is a condition within a developed groundwater basin in which the amount of water pumped over the long term exceeds the sustainable yield of the basin. Overdraft can lead to numerous problems, including lowering of water levels, ground subsidence, loss of aquifer storage, seawater intrusion, and a reduction in groundwater quality.

What might be sustainable in a basin at one time may not be sustainable at another time, for example if inputs (like recharge) are reduced. Thus overdraft can occur even when there is relatively little pumping, or when pumping has stayed the same. The most accurate way to tell if there is overdraft is by monitoring water levels in aquifers over the long term. If water levels go down year after year, and do not rebound robustly when pumping stops or recharge occurs, there may be a problem with overdraft. In addition, overdraft can occur even if there is no net imbalance between annual inflows and outflows, or if pumpage is distributed in such a way as to lead to problems (for example, being concentrated along the coast).

Increasing Supply

The primary input into an aquifer is called “recharge.” Not all water that infiltrates into the ground becomes recharge; some of this water goes into soil pores (above the water table), and some is lost to evaporatranspiration (ET). Groundwater recharge occurs in different parts of each basin, but exactly where recharge occurs, how recharge occurs, and how quickly recharge occurs remains unknown in many aquifers. Of all the primary flows in the hydrologic cycle, recharge is the hardest to measure and (arguably) the only primary flow that can’t be measured remotely.

In many groundwater basins, recharge occurs around the basin edge, where aquifer units are exposed at the surface or present below a thin soil layer. Recharge also commonly occurs through the beds of streams and rivers, particularly where they have incised through fine-grained layers that sometimes form the top of an aquifer. Measuring groundwater recharge from streams is relatively easy, requiring mainly accurate measurements of stream flow at two or more locations and accounting for additional losses and gains. Measuring recharge more broadly in a basin is challenging. In many basins, recharge is estimated over a year or more by conducting a water balance analysis, accounting for all other source or sink terms, and the change in water level (storage), and then calculating the recharge term. This common approach can lead to large errors in the calculation of recharge.

Groundwater recharge can be enhanced through a variety of techniques, collectively known as artificial recharge. The two most common types are managed aquifer recharge (MAR) and aquifer storage and recovery (ASR). MAR involves the use of natural streams and rivers, or constructed infiltration basins or trenches, to introduce excess surface water to a location where it can recharge underlying aquifers. In some basins, irrigation returns comprise a significant fraction of annual recharge, and may be considered a form of MAR, particularly if intentional over-irrigation is planned. MAR can also be accomplished using injection wells. ASR is a specialized form of artificial recharge where water is injected into an aquifer using one or more wells, and subsequently recovered using the same wells. Individual forms of MAR and ASR systems offer distinct advantage and challenges, and all must be carefully designed, installed, and operated; following a careful evaluation of site conditions and processes. MAR and ASR systems that work well in one region may not perform well in another.

Water Quality

Water supply and water quality are closely linked, because supply is defined based on water being usable. Some groundwater is naturally of low quality, particularly if that water is close to a source of salt (for example, immediately adjacent to the ocean, below a beach). Human activities can also lead to a reduction in water quality, either directly (such as pollution) or because of changes in hydrologic and geochemical conditions caused by pumping, limitations to recharge, or other processes. In fact, water quality includes a wide variety of characteristics, including temperature, sediment load (turbidity), solutes, gases, and biological constituents.

Contamination of groundwater resources can occur as point source or non-point source pollution. Point sources have a known location (and often time) of application, for example a landfill or a leaking underground tank. Non-point source pollution often is caused by widespread practices, such as salting roads or applying pesticides. As such, it can be difficult to both identify the proximal cause of non-point source pollution and to stop it from occurring.

Water quality standards are regulations that indicate how much of a particular substance can be in water in order for it to be used for a specific purpose. These can be set at federal, state, regional, or local levels. Standards can be set in terms of the concentration or sample-specific magnitude of a substance or condition that is permitted, or standards can be established to assess how much of a substance can be transported over a period of time. For example, many basins have established total maximum daily load (TMDL) standards for the transport of salts or nutrients, emphasizing the total amount that can be moved in a day rather than the instantaneous concentration at a moment in time.

There are many potential contaminants in groundwater, but some of the most common ones in California include salts, nitrates and other nutrients, bacteria, metals, organic solvents, and pesticides. Sources for these compounds include septic systems, landfills, leaking underground tanks, seawater intrusion, and common agricultural practices such as application of fertilizers and pesticides. Some toxic compounds occur naturally in groundwater, and these may become more concentrated or diluted as a result of human activity. In general, it is more expensive and complicated to clean up (remediate) an aquifer than it would be to prevent initial contamination. Additional challenges can arise when flow occurs down and around or through groundwater wells, for example, allowing contamination from one aquifer to leak into another.

Regulation

California is unique among western states in how it manages groundwater resources, with three primary methods of management: local agencies, local governments, and court adjudication. Which of these methods is applied in a particular basin depends on local conditions and legal precedent. Local government entities manage groundwater largely on the basis of land use planning and police powers, often through elected county boards. Adjudicated basins are managed through court decree, often developed on the basis of lawsuits when stakeholders cannot agree how resources will be used. There is no statewide permit system for groundwater (in contrast to rules for managing surface water), and most property owners can exercise an “overlying right” to groundwater by drilling a well and extracting water.

All water use in California is subject to the requirement of “reasonable and beneficial use.” Water rights in California are usufructuary, which is a right to use water, but not to own it outright. Groundwater rights in California are classified according to water category (as defined in the state constitution): underflow of a surface stream, underground stream, and percolating water. The first two of these categories are managed as surface water, whereas the third is subject to distinct rules and considerably less statewide statuary control. Absent evidence to the contrary, groundwater is assumed to be percolating groundwater and not an underground stream.

The primary forms of groundwater rights are overlying, appropriative, and prescriptive. The first is associated with land ownership. The second allows water to be used off property, for example, by a municipality, is commonly defined on the basis of historical use, and generally requires that appropriative extraction be applied to surplus of overlying needs. A prescriptive right is one that is acquired by someone who openly uses water for which another party has an existing prior right.are successfully using more dense plantings (9×4.5 for example). Depending on the rootstock and variety, vigor and production can vary greatly. In addition, dry farm systems don’t have the costs associated with irrigated systems. Combined with buyers who prefer dry farm fruit, these systems can be profitable, although some regions with lower price per ton averages may not be feasible.