Home » Climate Change » Drought in California Part 2: The Status of California’s Current Water Resources

Drought in California Part 2: The Status of California’s Current Water Resources


This is Part 2 of a series Drought in California. Part 1, Drought and the California Climate was published last week. The data and the analysis used for this series have some limitations. You can read about them in the introduction to the series.

California’s Overall Water Resources

It is difficult to estimate the total amount of water that could theoretically be available to California. One would have to count every raindrop, snowflake, and droplet of mist that fell on every inch of the state. One would have to include every pond and rivulet across the state, and every pocket of underground water, no matter how hidden. And one would have to include the water imported via aqueduct from outstate.

In addition, the amount available for human consumption, called dedicated supply, is only a portion of the total. The remainder is absorbed by the ground, runs off into streams that flow to the ocean, or evaporates. Historically, the water not used by humans has been considered lost or wasted. However, we now know better. The water that is absorbed by the ground recharges underground aquifers and it is used by California’s plants and animals. The water that flows into streams supports the life along the streams, including fisheries: the Sacramento River used to be one of the larger salmon fisheries on the West Coast. These are all valid and important uses, and you cannot deprive them of water without damaging them.

The California Department of Water Resources estimates California’s water supply in wet, average, and dry years:

Table 1: Summary of California’s Water Supply


1998 (Wet Year) (million acre-feet)

2000 (Avg Year) (million acre-feet)

2001 (Dry Year) (million acre-feet)

Total Supply (Precipitation & Imports)




Dedicated Supply (Includes Reuse)




Source: California Department of Water Resources.



Figure 5b: Distribution of the Dedicated Water Supply. Data source: California Department of Water Resources (b).

Figure 5b: Distribution of the Dedicated Water Supply. Data source: California Department of Water Resources (b).

Because drought is projected to be the “new normal” for California, dedicated supply during dry years is the supply in which we should be interested. That supply is distributed to three broad use categories: urban uses, agricultural uses, and environmental uses (Figure 5b).

(Click on graphics for larger view.)

The first two are self-explanatory. Environmental uses are those uses where water is returned to the environment to restore or prevent damage caused by water withdrawal. An example might be the release of water into a river where the ecosystem has been damaged by water withdrawal.

California’s Groundwater Resources

Groundwater is one of California’s most important water sources. In average years, it supplies about 40% of California’s water, in drought up to 60%. (Pacific Institute and NRDC, 2014) The state is divided into 10 large hydrologic regions, defined by surface water runoff patterns. Underlying each region are dozens of individually named groundwater basins, or aquifers. These vary widely in size.

Figure 6: Map of Central Valley Aquifer. Source: USGS California Water Science Center.

Figure 6: Map of Central Valley Aquifer. Source: USGS California Water Science Center.

Only a few of California’s groundwater basins are well enough understood to have estimates of overall storage capacity associated with them. For the most part, the health of the groundwater supply is studied using changes in the water level in the aquifer, and comparing it to the aquifer’s overall depth.

The Central Coast of California, roughly from Santa Barbara to Monterey, is the region most dependent on groundwater: about 83% of demand is met from groundwater resources. It is not, however, the region that uses the most groundwater.

California’s Central Valley is a broad, flat valley about the size of West Virginia (see Fig. 6). It runs roughly from Redding in the North to Bakersfield in the south, and from the Sierra Nevada Mountains on the east to the Coastal Ranges on the west. It is the most important agricultural area in the USA, and water from the Central Valley Aquifer is used to irrigate crops. It consists of 3 hydrologic regions: Tulare Lake, the San Juan River, and the Sacramento River. These 3 hydrologic regions comprise by far the largest, most abundant, most used, and most studied groundwater supply in California. Approximately 70% of all groundwater extraction occurs in these 3 basins, 35% in the Tulare Lake basin alone. (California Department of Water Resources, 2003)


Land Subsidence in the Central Valley. Source: USGS Water Science School

Figure 7. Land Subsidence in the Central Valley. Source: USGS Water Science School

Withdrawals have greatly exceeded recharge, and the level of the aquifer has dropped. This has resulted in water shortages and land subsidence (see Figure 7). Aquifers are like lakes – they have irregular bottoms, so if the level drops too low, some regions are left high and dry. If subsidence occurs, it can cause several problems. Roads and foundations may buckle, irrigation canals through which the water flows downhill may suddenly find themselves going uphill instead, and worst of all, the ground may compact. If the ground compacts, the aquifer may lose some or all of its ability to recharge with water. Where this occurs, the result is a permanent loss of the aquifer.









Change in Groundwater Storage, Central Valley. Source: USGS, 2009.

Figure 8a. Change in Groundwater Storage, Central Valley. Source: USGS, 2009.

For a period during the late 20th Century, this trend was arrested, but it is recurring now, with the land subsiding a foot a year between 2003-2010. Figure 8 shows the change in the amount of water stored in the Central Valley Aquifer through 2003. The dark blue line shows the total aquifer, the other colors show various parts of it. Through 2003, the overall decrease was almost 50 million acre-feet. (Faunt, 2009)

The current drought has forced the California Water System to restrict deliveries of surface water to agriculture, so well pumping has accelerated. One source estimates that the Sacramento and San Joaquin River Valleys, about 2/3 of the Central Valley, lost 30 cu. km. of water between 2003 and 2010, years not included in the chart. (Farmigleietti, 2014) These regions had not lost significant water storage prior to 2003.

Yet another study suggested that between 2011 and 2014 another 34 million acre-feet of groundwater has been lost in the Sacramento and San Joaquin Valleys. Thus, the total loss is over 114 million acre-feet. For comparison sake, the storage capacity of Lake Mead is about 28 million acre-feet, so the total amount is equal to more than 4 Lake Meads. Wells are having to be deepened; permits for well drilling tripled in one county. Well depths are now commonly over 1,000 feet. (Howard, 2014)

I have been unable to find estimates of how much water is left in the Central Valley Aquifer. If unlimited withdrawals continue, at some point the aquifer will be exhausted. For most of its history, California did not limit the amount of water that a farmer could withdraw (most other states do). However, on 9/18/14, California adopted the Sustainable Groundwater Management Act of 2014, and it went into effect 1/1/2015. The act does not require groundwater sustainability until 2040. Will the aquifer last that long? How much damage will it sustain? I don’t think anybody knows.

Note that this aquifer supplies not only agriculture, but also many of the cities in the Central Valley, home to about 6.5 million people.

California Surface Water Resources

In far Northern California rainfall occurs much of the year. The rest of California depends on snowpack for its surface water. Snowmelt drains into rivers, and the rivers run into huge reservoirs, from which water is distributed as needed. There are several water distribution systems. For instance, San Francisco brings water from the Hetch-Hetchy Reservoir inside Yosemite National Park through the Hetch-Hetchy Aqueduct to the Bay Area. Los Angeles brings water in the Los Angeles Aqueduct from the Owens River (on the east side of the Sierra Nevada Mountains, as far north as Mono Lake) all the way to Southern California. And a consortium of Southern California water systems bring water from the Colorado River through the Colorado River Aqueduct to the large metropolitan areas of Southern California.

Figure 8a. Water Systems in California. Source: California Department of Water Resources, 2003.

Figure 8b. Water Systems in California. Source: California Department of Water Resources, 2003.

The largest two water systems, however, are the Central Valley Project and the California State Water Project (shown on the map at right, the Central Valley System in purple, the California State Water Project in orange). The Central Valley Project, owned by the Federal Government, stores water in the Trinity and Lake Shasta reservoirs in far northern California (top purple arrow on the map), and distributes it throughout the Central Valley, as far south as the town of Mendota (bottom purple arrow). The Central Valley Project was intended to prevent flooding along the Sacramento and San Juan Rivers, and to provide irrigation for agriculture. The California State Water Project collects its water in the Feather River watershed in northeastern California, stores it in Lake Oroville Reservoir (top orange arrow on map), discharges it into the Sacramento River, transfers it to the California Aqueduct, glides it 250 more miles south, lifts it over the Tehachapi Mountains, and finally distributes  it to the Los Angeles Metropolitan Area (bottom orange arrow on map).

The amount of water delivered by the systems is limited by the amount of water available for diversion and by the capacity of the aqueducts. On the California Water Project, the limit is the lift capacity over the Tehachapi Mountains, which is 1,926 cubic feet per second (California Department of Water Resources (c)) The Central Valley Project is more distributed around the state, but on the Delta-Mendota Canal, the canal that distributes northern California water to the southern Central Valley, capacity is 4,600 cubic feet per second.

Fig. 9. Sources of California Surface Water. Source: Kalusmeyer and Fitzgerald, 2012.

Figure 9. Sources of California Surface Water. Source: Kalusmeyer and Fitzgerald, 2012.

The drainage area that provides surface water to California covers much of the western United States. It includes the entire state of California, of course. But it also includes the area drained by the Colorado River, as shown in Figure 9 at right.

There are some basic problems here. The first is sharing. Water from the Colorado River is used by people in Colorado, New Mexico, Utah, Nevada, Arizona (all the way to Tucson), Mexico, and California. The water allocation scheme was developed in 1922. California gets the largest share. Suffice it to say that those other people don’t always like doing without water so California can have more.

The sharing problem is not unique to the Colorado River. The Owens River Valley (the eastern slope of the Sierra Nevada as far north as Mono Lake) supplies virtually all of its water to Los Angeles. Water from Northern California (think Lake Shasta) is transported all the way to Southern California. Hetch Hetchy Reservoir was built inside Yosemite National Park to provide water to San Francisco, flooding a valley that was said to rival its more famous sister to the south. All are controversial. Still, these are political problems and could theoretically be solved by people willing to compromise.

More difficult to solve is the fact that tremendous development has occurred, and the demand for water outstrips the supply. Since 1922, when the Colorado River Compact was established, California’s population has grown from about 3.5 million to about 37 million. Arizona’s has grown from about 0.3 million to about 6.4 million. Las Vegas has grown from about 2,000 to about 2 million. There just isn’t enough water, at least given current usage patterns.

Data source: Climate-At-A-Glance, National Center for Environmental Information

Figure 4. Data source: Climate-At-A-Glance, National Center for Environmental Information

At right is a repeat of Figure 4 from the previous post, showing the annual Palmer Hydrological Drought Index for California. You can see that the first two decades of the 20th Century were the wettest in the record. This was true for the entire western USA, not just California. That means that Colorado River allocation levels were based on abnormally wet years; they overestimated the amount of water reliably available from the river. Since then, the region has dried, and the water available from the river has decreased. Combined with the tremendous surge in demand, it means that demand exceeds supply.

To get a rough estimate of the current state of California’s surface water resources, one way is to look at reservoir levels. The major reservoirs on the Colorado River are Lake Powell and Lake Mead, which together account for the vast majority of water stored on the Colorado River System. Recall that the Colorado River provides water for much of Southern California, including the agricultural area of the Imperial Valley.

Figure 10. Lake Mead. Source: NASA.

Figure 10. Lake Mead. Source: NASA.

Much has been made of the “bathtub ring” at Lake Mead. The water level there has dropped 142 feet (see photo at right) and is at the lowest level since Lake Mead first filled. It is currently at 39% of full pool (5/12/2015). This represents 52% of Lake Mead’s average historical storage on this date. (Lake Mead Water Database, 5/12/2015) Lake Powell fluctuates with the snowmelt, being fullest in early summer, emptiest in late winter/early spring. Currently, it is at 44% of full pool, which represents 59% of average historical storage for this date. (Lake Power Water Database, 5/12/2015)

A system of reservoirs inside California impounds and stores water from California’s rivers. California’s largest “reservoir” is its mountain snowpack, which stores water during the winter months , and then releases it as the snow melts during the spring and summer. The snowpack accounts for fully 30% of California’s water, or 29 million acre-feet of California’s dedicated water supply. The snowpack is typically at its deepest around April 1 each year. As noted above, however, climate change is predicted to reduce the water content of the snowpack by 40%. The situation this year (2015) is much worse than predicted, however. On April 1, the water content was 5% of its historical average for that date. Thus, as the dry season begins, California’s largest “reservoir” is 95% empty. (California Department of Water Resources, 2015)

Table 2: Current Conditions on 14 Major Reservoirs (5/12/2015).

Reservoir (largest to smallest)
% of average historical storage on this date
% of average historical storage on this date
Lake Mead
San Luis
Lake Powel
Lake Shasta
Pine Flat
Lake Oroville
Folsom Lake
New Melones
Millerton Lake
Castaic Lake
Don Pedro
Lake Perris

Lake Mead data from the Lake Mead Water Database. Lake Powell data from the Lake Powell Database. Other reservoir data from the California Data Exchange Center – Reservoirs.


The largest man-made reservoir in California is Lake Shasta, about 1/6 the size of either Lake Mead or Lake Powell. California’s man-made reservoirs are also very low compared to historical averages for this date. Table 2 summarizes the current percent of average historical storage on May 12, 2015 for the two Colorado River reservoirs and the 12 largest California reservoirs.

Figure 11. When Will Lake Mead Go Dry? Source: Barnett and Pierce, 2008.

Figure 11. When Will Lake Mead Go Dry? Source: Barnett and Pierce, 2008.

Climate projections suggest that, while the current drought in California may be especially severe, the drought in California is likely to become a long-term normal pattern. If it does, how long before the reservoirs run dry? I don’t know. But a scientific paper from the Scripps Institution of Oceanography predicted a 50% chance that Lake Mead would be empty by 2021, and a 75% chance by 2040. (See Figure 8) If water deliveries were reduced by 10%, then the 50% probability of going dry would be postponed until about 2035. If deliveries were reduced by 25%, then the chance of going dry would be postponed until about 2060. Be sure to understand the implications here: water withdrawals from the Colorado system could be reduced by 25%, a huge reduction, and the reservoir would still have a 50% chance of going dry in the middle of the century.

The next post in the series will try to develop an estimate of the overall average annual water deficit that will be faced by California later this century.


California Department of Water Resources. 2015. Sierra Nevada Snowpack Is Virtually Gone; Water Content Now Is Only 5 Percent of Historic Average, Lowest Since 1950. News release published 4/1/2015. http://www.water.ca.gov/news/newsreleases/2015/040115snowsurvey.pdf.

California Data Exchange Center – Reservoirs. http://cdec.water.ca.gov/cdecapp/resapp/getResGraphsMain.action.

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

California Department of Water Resources. 2003. California’s Groundwater, Bulletin 118, Update 2003. http://www.water.ca.gov/groundwater/bulletin118/report2003.cfm.

Faunt, C.C., ed., 2009, Groundwater Availability of the Central Valley Aquifer, California: U.S. Geological Survey Professional Paper 1766.

Farmiglietti, Jay. 2014. “Epic California Drought and Groudwater: Where Do We Go From Here?” National Geographic News Watch: Water Currents. 2/4/14.

Howard, Brian. “California Drought Spurs Groundwater Drilling Boom in Central Valley. National Geographic. 8/16/2014. http://news.nationalgeographic.com/news/2014/08/140815-central-valley-california-drilling-boom-groundwater-drought-wells.

Klausmeyer, Kirk, and Katherine Fitzgerald. 2012. Where Does California’s Water Come From? San Francisco: The Nature Conservancy.

Lake Mead Water Database. Accessed online 5/12/2015 at http://lakemead.water-data.com.

Lake Powell Water Database. Accessed online 5/12/2015 at http://lakepowell.water-data.com.

NASA. 12/16/2014. “Needed: 11 Trillion Gallons to Replenish California Drought.” Nasa Science News. http://science.nasa.gov/science-news/science-at-nasa/2014/16dec_drought.

NASA. 2014. “Satellite Study Reveals Parched U.S. West Using Up Underground Water.” News and Features, Release 14-200. https://www.nasa.gov/press/2014/july/satellite-study-reveals-parched-us-west-using-up-underground-water/#.VVpNrOdpey0.

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.

USGS. 2009. California’s Central Valley Groundwater Study. Factsheet 2009-3057. http://pubs.usgs.gov/fs/2009/3057.

USGS California Water Science Center. CVHM Numerical Model. Web page accessed 5/20/2015 at http://ca.water.usgs.gov/projects/central-valley/cvhm-numerical-model.html.

USGS Water Science School. Land Subsidence in California. Web page accessed 5/20/2015 at https://water.usgs.gov/edu/earthgwlandsubside.html.


  1. […] noted in my original series, significant subsidence has already occurred over California’s aquifers. More seems to be occurring every year. A recent study attempted to […]

  2. […] were diverted to provide water to Los Angeles. (See my post on California’s water supply, here. For movie buffs, this is also the subject of the famous movie Chinatown.) The lake has been almost […]

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