- What is “groundwater?”
- Should I spell it “ground water” or “groundwater?”
- How much of California’s water comes from the ground?
- What is the “sustainable yield” of a groundwater basin?
- What is “groundwater overdraft?
- What is the annual groundwater overdraft in California, and how has it changed in recent years?
- How does the DWR think California is going to meet its water needs in coming year?
- Why can’t California build more dams to capture winter rains?
- Won’t climate change result in more rain?
- Can we close the statewide water deficit just by increasing groundwater recharge?
- But if groundwater recharge is not “the answer,” then why have an initiative dedicated to this topic?
- Why don’t we stop farming so much, won’t that solve the problem?
What is “groundwater?”
Groundwater is subsurface water that occurs below the water table within geologic formations that are saturated (all pore spaces filled with water rather than air). Most groundwater exists in tiny pore spaces between grains of rock or sediment that comprise aquifers, geologic formations that can store and transmit water in usable quantities. Actual “underground rivers” are rare, although there are some on the UCSC campus within the cave network.
Should I spell it “ground water” or “groundwater?”
It can be spelled either way, but spelling it as two words makes it easier to talk about “surface and ground water,” and for many years the U.S. Geological Survey (USGS) used two words for the noun, hyphenating when “ground” was used as a modifier (i.e., “ground-water recharge”). However, USGS personnel eventually decided to transition to using one word, so I have done the same. You will likely find some documents and figures that use the two words, but over time you can expect one-word usage to become more standard.
How much of California’s water comes from the ground?
It varies year by year and region by region across the state. Current estimates are that about 40% of annual fresh water demand in California is satisfied by groundwater during an “average” year. During “dry” years, groundwater satisfies more than 60% of California’s fresh water needs. Interestingly, the Central Coast hydrologic region, which extends from Santa Cruz south to Santa Barbara, gets a larger percent of its fresh water from the ground than any other hydrologic region in the state (>80%). At the same time, the Central Coast hydrologic region is more “off the grid” with respect to statewide water transfers than any other part of the state. Thus we are both uniquely dependent on groundwater and particularly mindful of needing to live within our means.
What is the “sustainable yield” of a groundwater basin?
This is the amount of water that can be pumped from an aquifer over the long term without causing unacceptable harm. Of course, “unacceptable” is someone’s definition (physical, economic, political, legal, sociological); this is not a hydrologic term. Some people use similar terms like “safe yield” or “basin yield;” it is important to define these terms when they are used. Note that the sustainable yield is not equal to recharge. In general the sustainable yield from an aquifer or basin will be less (often a lot less) than recharge.
What is “groundwater overdraft?”
This 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. Note that what might be sustainable 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. 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, if pumpage is distributed in such a way as to lead to problems (for example, being concentrated along the coast).
What is the annual groundwater overdraft in California, and how has it changed in recent years?
DWR’s California Water Update (Bulletin 160) from 1998 indicated that fresh water demand exceeded supply during “normal” years by 1.6 million acre-feet (Maf, an acre foot is enough water to cover an area of an acre, 200 x 200 feet, with one foot of water). Although they did not call this “overdraft” in Bulletin 160-98, that is what most of it must be. How else can the state produce water in excess of supply year after year (surface reservoirs and rivers can be “overdrafted” for a little while, but not for many years)? DWR’s 1998 estimate of the “dry” year water deficit was 5.1 Maf, enough water for 10 million California families. Note that this is overdraft, not use; actual water usage was much greater. DWR projected that these water deficits would plummet by 2020, at the same time that California’s population went up by 50% (equivalent to the total populations of the nearest eight adjacent states). In the 2005 DWR California Water Update (Bulletin 160-05), the “normal” year deficit was estimated to be 5.8 Maf, an increase from 1998 of 260%. The “dry” year deficit was estimated to be 14.3 Maf (up by 180%). The 2009 and 2013 Water Updates showed that conditions are getting worse, and this was before the recent drought. It does not look like we are going to achieve DWR’s 1998 projections for improvements in the state-wide water balance.
How does the DWR think California is going to meet its water needs in coming years?
The 2005 DWR California Water Update shows that the greatest new “supply” will come from improvements to urban water use efficiency, e.g., conservation. Improvements to groundwater management and conjunctive use comprise the next greatest water source. It should be noted that urban water users in the Central Coast hydrologic region already use less water per capita than residents in other regions. Obtaining additional large urban conservation benefits in this region will become increasingly difficult as we continue to make improvements. There are other ways to save water, like restricting agricultural use, but we should be very careful about entertaining ideas about taking a “command and control” approach to managing water. Certain water rights are enshrined in the California constitution, including an overlying right to pump groundwater when it is put to constructive use.
Why can’t California build more dams to capture winter rains?
Governor Schwarzenegger proposed construction of two new dams in Fall 2007, comprising <1 Maf of additional storage (less than 20% of what is needed during “normal” years to overcome annual shortages), and there was a firestorm of protest. The Governor stopped talking about dams very quickly. The era of big dam projects is probably behind us in California. And even if we did build more dams, we would have to move this stored water around the state where it is needed. Current estimates are that ~8-10% of all the energy expended in California is used to move water. That conveyance infrastructure is rickety, and may not survive the next major earthquake or the coming rise in sea level. Also, some hydrologic regions are not strongly connected to the Central Valley Project or the State Water Project, including the Central Coast region, and do not stand to benefit from dam construction unless it occurs on a massive scale. This is unlikely, particularly because people are more aware today of the environmental impact of dams than they were 40-50 years ago. Finally, conservative climate projections suggest that 60-90% of the winter snow pack in the Sierra Nevada will be gone by 2100. We will never build enough dams to keep up with this loss of snow pack, never mind the growing state water deficit based on current hydrologic conditions. Although increased surface storage could help to address ongoing and future shortages, it is unlikely that this approach will provide major new sources of fresh water.
Won’t climate change result in more rain?
What the models suggest, in addition to massive loss of snow pack, higher mean temperatures, and more extremely warm days, is greater variability and intensity of precipitation. Most likely this will result in less opportunity for groundwater recharge. Some studies have suggested that climate change could increase recharge in particular areas, but these studies did not include a full suite of new hydrologic and climatic conditions. Sea level rise will increase seawater intrusion into coastal basins, where much of the population lives. In addition, global and regional climate models are notoriously poor in representing precipitation and related processes, so there is considerable uncertainty as to how precipitation will be distributed following large changes in the atmosphere that are anticipated during the rest of the 21st century. The best approach we can take is to maintain flexibility in supply, and to get as much water into the ground as possible when we have the chance. This is not going to get easier or cheaper moving forward.
Can we close the statewide water deficit just by increasing groundwater recharge?
Probably not. Conservation, recycling, artificial recharge, conjunctive use and other approaches are all likely to contribute to solutions. Keep in mind: this problem will need to be solved basin by basin, district by district, on the basis of local conditions and political and economic realities. Given how contentious water projects are, those projects with a strong local component, with the most local control, are most likely to be the most politically feasible. Protecting and enhancing groundwater recharge is about as local as it gets. Certainly in the Central Coast hydrologic region, enhancing groundwater supplies is going to be profoundly important. We have storage to spare, and we should get busy filling that space with water.
But if groundwater recharge is not “the answer,” then why have an initiative dedicated to this topic?
There are many excellent organizations working on reducing urban and agricultural demand, so it is relatively harder to have an impact that area. Other groups are focusing on desalinization and recycling, both of which are likely to be important in individual basins. But I’ve not seen any other group working specifically on enhancing recharge. In addition, groundwater recharge is a scientific and technical frontier, one of the most cryptic of the major hydrologic flows. Other flows, like evapotranspiration and runoff, are being evaluated from space by remote sensing, but recharge is not as amenable to spatial monitoring in this way. Recharge is important, a technical frontier, not well understood, a form of hydrologic system services, and a basis for connecting people to their groundwater basins. This has a benefit all its own.
Why don’t we stop farming so much, won’t that solve the problem?
It is clear that some low-value crops (like cotton and rice) probably should not be grown where there is not enough surface water, and many high-value crops can be a particular drain on resources. But it is politically difficult to force farmers to grow or not grow certain crops. Growing strawberries can generate more money than growing apples; who should decide what gets grown? We also need to be careful about fallowing land if that land gets used for construction of housing. It used to be conventional wisdom that changing from farming to housing saved water, but studies suggest that there is often little or no water savings associated with this shift. We can be sure that large, rapid shifts in farming practices or farmed acreage would lead to significant economic problems. The best approach may be to shift practices slowly over time, but it can be challenging to sustain these kinds of shifts. Now let me ask you a question: don’t you like having all the wonderful local produce? Would it really be better if we trucked it all in from elsewhere? One goal of the Recharge Initiative is to facilitate the sustainable transformation of our water supply system, to make it more flexible and increase the capacity to benefit from natural and anthropogenic variability. Some agricultural changes are inevitable, but hopefully they will not be so extreme so as to lose the benefits from living in such a productive region. We include encouragement of sustainable agricultural practices as a fundamental goal of The Recharge Initiative.