Home » Climate Change » Drought in California Part 3: California’s Total Water Deficit

Drought in California Part 3: California’s Total Water Deficit


This post is Part 3 of my series on Drought in California. In Drought and the California Climate, published 2 weeks ago, I reviewed the drought California is experiencing, the state’s historical climate, and projections for the state’s future climate. I discovered that drought is expected to become the “new normal” for the state. In The Status of California’s Current Water Resources, published last week, I surveyed California’s current water resources, discovering that California is already experiencing deficits in both groundwater and surface water. Both groundwater and surface water supplies are being depleted. Unless changes are made, the depletion is projected to continue.

This post attempts to construct a summary total of California’s overall water deficit. The estimate is going to be a very rough, “back-of-the-envelope” exercise, that relies on a number of assumptions that may, or may not, be correct. Nevertheless, such an estimate is needed in order to evaluate potential measures to procure additional water supplies and to conserve existing ones. There are surprisingly few published estimates out there. They are uneven in what they include and omit. Indeed, much of the data and the analysis used for this series have problems and limitations. You can read about them in the introduction to the series.

California’s Total Water Deficit

As noted in the previous post, the total water supply for California has been estimated for wet, average, and dry years. Because climate change is expected to cause drought to be the “new normal” in California, the supply during dry years is the one that should be of most interest to us.

In addition, either the total supply or the dedicated supply can be estimated. The total supply is an estimate of every drop of water that falls on the state as precipitation, every drop that is imported in aqueducts from other states, every drop that is desalinated, and every drop that is pumped out of the ground. The dedicated supply is the portion available for human consumption. The not-dedicated water is not surplus; it recharges aquifers, and supports the plants, animals, and fish that live in California. Thus, it is not an untapped water supply available for humans to grab. The dedicated supply is the estimate that is relevant to these calculations. As noted in the previous post, the dedicated supply of water during a dry year is 65.1 million acre-feet. (California Department of Water Resources (b)).

Table 3: California Dedicated Water Supply. Source: California Department of Water Resources (b).

Table 3: California Dedicated Water Supply. Source: California Department of Water Resources (b).

Table 3 at right shows how the dedicated water supply is used in wet, average, and dry years. The table shows that in dry years, the amount of water distributed by the California State Water Project to agriculture and to urban uses actually increases. But the amount distributed to environmental uses decreases by almost 1/3. This is not difficult to understand: they close the sluice gates. More water is held in the reservoirs and diverted into the aqueducts, less flows into the rivers.

(Click on graphics for larger view.)

California currently derives its water from two primary sources: groundwater and surface water. Groundwater comes from aquifers. The largest and most important fresh water aquifer in California is the Central Valley Aquifer. Its status was reviewed in the previous post. As it is the most important aquifer, I will let its deficit stand in for the total groundwater deficit, though it is surely an underestimate. Water storage in the Central Valley Aquifer declined by at least 114 million acre-feet between 1962 and 2014. That is about 2.2 million acre-feet per year. Withdrawals would have to be curtailed by that much to stabilize the aquifer. (May, 2015)

An analysis by the Pacific Institute and the National Resources Defense Council estimates that California diverts 5 million more acre-feet from the Sacramento-San Juan watershed than can be sustained, and depletes up to 2 million acre-feet from that watershed’s groundwater supplies. It is not clear how this study’s estimated groundwater deficit overlaps with the Central Valley Aquifer decline estimated above, but I will assume that half of it does, and the Pacific Institute estimate should be adjusted by that amount. Thus, the overall estimated deficit is 6 million acre-feet. (Pacific Institute and the NRDC, 2014)

The EPA estimates that California’s groundwater deficit in 2010 was 2 million acre-feet per year. Given current trends, population growth in California will result in an additional 6.2 million acre-feet deficit in a drought year. The current deficit of 2 million acre-feet appears to overlap with the estimate by the Pacific Institute, and will not be included further in this calculation. The future estimate appears to be an estimate of increased demand caused by the anticipated increase in population. For many years, California’s booming economy has depended on a growing population, and it seems unlikely that economic interests will want to freeze or reduce it. It seems equally unlikely, however, that the trend can continue unchanged. Thus, because population growth may slow, it may be reasonable to reduce the EPA’s estimated deficit. By how much is not clear, but by 50% may be reasonable. That would be 3.1 million acre-feet. (EPA, 2010)

Neither the Pacific Institute estimate nor the EPA estimate attempts to assess the effects of reservoirs going dry, or of a significant decline in the snowpack. Were Lake Mead to go dry, it would interrupt water deliveries from the Colorado River System. Although the intakes for the aqueducts that deliver the water are in Lake Havasu, if Lake Mead were to go dry, it would disrupt managers’ ability to manage the water level in Lake Havasu, uncovering the intakes. The system provides about 4.4 million acre-feet per year to the state, and this would represent an immediate loss of that supply. For Southern California’s metro areas, it would represent a loss of about 23% of their water supply. The loss of one or more of California’s instate reservoirs would be only slightly less catastrophic. I don’t have an estimate of when that might occur, however, so it will not be included in this calculation. Recall from the previous post that even a 25% reduction in water withdrawals still left a 50% chance that Lake Mead would go dry by 2060. Thus, the draw from the Colorado River would need to be reduced by more than that. How much more is unclear, but again, a 50% reduction seems not unreasonable. That would be 2.2 million acre-feet. (Barnett and Pierce, 2008)

In 2015 the California snowpack was only 5% of its historical average. If this were to become the normal occurrence, it would represent an immediate and existential threat to the state. Not only would runoff into the streams that charge California’s man-made reservoirs be reduced, but water seeping into the ground to recharge the aquifers would, also. However, the 2015 snowpack may be a temporary extreme, and it may be better to accept the National Climate Assessment’s projection of an average 40% decline in the water content of the snowpack (discussed in Part 1). That would amount to 11.6 million acre-feet.

It is difficult to know how to combine this information into an overall estimate of California’s looming water shortage. It seems that only the National Climate Assessment anticipates a decline in water supply based on the decline in the snowpack, and none of the estimates discuss the need to reverse the already-occurring depletion of essential reservoirs.

The estimated deficit in the Central Valley Aquifer is 2.2 million acre-feet per year. The Pacific Institute’s estimate for this same region overlaps with this estimate by about 1 million acre-feet per year, but adds surface water deficits. Adjusting the Pacific Institute estimate to reflect the overlap, the deficit is estimated at 6 million acre-feet per year. These two estimates pertain to only part of the state, the Central Valley. However, that watershed is by far the state’s largest and most important. The aquifer and the two rivers there are the largest within the state. A large part of the excluded area is very dry, and water from the two Central Valley rivers is delivered far and wide across the state. Therefore, these two estimates appears to be a minimum, but reasonable starting point. The EPA’s estimate (adjusted to remove overlap with the Pacific Institute and to anticipate a reduced rate of population growth) represents the increase in demand that would come from an increase in population. It would be 3.1 million acre-feet per year. To prevent Lake Mead from going dry would require a 2.2 million acre-feet reduction. Finally, by far the largest of these potential water deficits would be the decline in the snowpack, which would be 11.6 million acre-feet. (U.S. Global Change Research Program, 2014)

Combined, they sum to a total water deficit of 25.1 million acre-feet, about 39% of California’s dedicated water supply in dry years. Thus, I estimate that to remove its future water deficit, California will need to increase supply and/or reduce demand by more than 1/3.

This estimate is much larger than currently published estimates. The primary reason is that I have included both population growth and the projected effects of climate change. If population growth and climate change trends were to change, it would impact the estimate. In addition, though I remarked on the limitations of this analysis at the beginning, I feel I should restate that the analysis relies on a number of assumptions that may or may not be correct, and on data that is far from perfect.

California can contribute to minimizing the impacts of climate change, but no single state is in control of climate change. California will have to adjust. The estimate constructed above provides one perspective on just how much adjusting they will have to do. In coming posts, I will explore potential strategies for increasing water supply and/or reducing demand.


Barnett, Tim, and David Pierce. 2008. “When Will Lake Mead Go Dry?” Water Resources Research, 44, W03201. Retrieved online at http://www.image.ucar.edu/idag/Papers/PapersIDAGsubtask2.4/Barnett1.pdf.

California Department of Water Resources (b). California State Water Project Water Supply. Web page accessed 5/21/2015 at http://www.water.ca.gov/swp/watersupply.cfm.

EPA. 2010. “California Water Fact Sheet.” EPA WaterSense. EPA832-F-10-016. http://www.epa.gov/watersense/docs/california_state_fact_sheet.pdf.

May, John. 2015. “The Status of California’s Current Water Resources.” Blog post in the blog MoGreenStats. http://www.mogreenstats.com.

Pacific Institute and the National Resources Defense Council (NRDC). 2014. The Untapped Potential of California’s Water Supply: Efficiency, Reuse, and Stormwater. http://pacinst.org/publication/ca-water-supply-solutions/#issuebriefs.

U.S. Global Change Research Program. 2014. Climate Change Impacts in the United States: The Third U.S. National Climate Assessment. http://www.nca2014.globalchange.gov.

1 Comment

  1. […] valid. As time passes, California will face increasing challenges meeting the demand for water (see here). The state will be unable to secure large new sources of surface water or ground water (see here), […]

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