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Climate Change Impacts on California Water Resources

Posted on 16 October 2010 by dana1981

California's water resources face significant strain on two fronts - a state population which is expected to grow from 35 to 55 million over the next 40 years, and a declining Sierra snowpack as a result of rising temperatures.

There is uncertainty regarding how total precipitation in California will change as the average planet and state temperatures continue to warm. Different climate models project anywhere from a modest decrease to a modest increase in net precipitation. Where the models agree is that less precipitation will fall as snow, and more as rain.

Temperature Projections

In 2003, the California Energy Commission’s Public Interest Energy Research program established the California Climate Change Center to conduct climate change research relevant to the state, a virtual organization with core research activities at Scripps Institution of Oceanography and the University of California, Berkeley. The Center performed a “Climate Scenarios” project (2005), which analyzed a range of impacts that projected rising temperatures would likely have on California. The project used three potential anthropogenic greenhouse gas emissions scenarios (Figure 1) and three climate models with a range of climate sensitivity parameters to conclude that by 2099, average temperatures in California will rise by 3 to 10°F (1.7 to 5.5°C; Figure 2).

Figure 1: Projected CO2 emissions for the three scenarios considered by the Climate Scenarios project

Figure 2: Projected California warming under various emissions scenarios by 2064 (left) and 2099 (right)

Rainfall Replacing Snow

The Center's report summarizes the state water resource problem succinctly:

"Most of California’s precipitation falls in the northern part of the state during the winter while the greatest demand for water comes from users in the southern part of the state during the spring and summer. A vast network of man-made reservoirs and aqueducts capture and transport water throughout the state from northern California rivers and the Colorado River. The current distribution system relies on Sierra Nevada mountain snowpack to supply water during the dry spring and summer months. Rising temperatures, potentially compounded by decreases in precipitation, could severely reduce spring snowpack, increasing the risk of summer water shortages."

The study finds that the spring Sierra Nevada snowpack could decrease by 70 to 90% under the higher warming scenario. However, if emissions are significantly curbed and temperature increases are kept in the lower warming range, snowpack losses are expected to be only half as large as in the higher warming range (Figure 3).

Figure 3: Decreasing Sierra snowpack in low and high warming scenarios

As winter precipitation falls increasingly as rain rather than snow, reservoirs may need to be filled to a higher capacity in order to meet Californian water demand in the summer months when the Sierra snowpack no longer can. However, reservoirs also act as winter flood control, and thus filling them higher in the winter poses a greater threat for floods in the winter months.

New reservoirs may need to be constructed, but this increased storage will not come cheap. A study by Tanaka et al. (2006) found,

"California’s water supply system appears physically capable of adapting to significant changes in climate and population, albeit at a significant cost. Such adaptation would entail large changes in the operation of California’s large groundwater storage capacity, significant transfers of water among water users, and some adoption of new technologies."

Sea Level Rise

Aside from the obvious impacts to the coast of California, sea level rise also poses a threat to state water resources.

"An influx of saltwater would degrade California’s estuaries, wetlands, and groundwater aquifers. In particular, saltwater intrusion would threaten the quality and reliability of the major state fresh water supply that is pumped from the southern edge of the Sacramento/San Joaquin River Delta."


California has a $30 billion agriculture industry that employs more than one million workers. It is the largest and most diverse agriculture industry in the USA, producing more than half the country’s fruits and vegetables. Tanaka et al. found that agricultural water users are the most vulnerable to water resource changes as a result of global warming.

Schlenker et al. (2007) investigated the impact of climate change on irrigated agriculture in California and concluded,

"changes in water availability due to climate change hence have the potential to severely impact the value of farmland."

The study found that due to decreased runoff in late spring and early summer (due to increased precipitation falling as rain rather than snow, and a decreased Sierra snowpack, as discussed above), there will be less water availability when it is needed most, during the growing season. This will increase the demand for irrigation, putting more pressure on river and groundwater systems.

In terms of economic impacts, the study concludes that a reduction of one or two acre-feet of water per acre of land would result in a decrease in the value of the affected farmland of as much as $1,700 per acre.

Overall Water Sustainability

A study by Tetra Tech (2010) evaluated a Water Supply Sustainability Index (WSSI) which takes into account natural available precipitation; the extent of water development already in place; dependence on groundwater; the region’s susceptibility to drought; projected increases in water use; and the difference between peak summer demand and available precipitation, a measure of storage requirements. The projected WSSI throughout the USA in 2050 with and without climate change effects taken into account is shown in Figure 4. As you can see, climate change exacerbates an existing water problem in California, and many regions within the state face high to extreme water sustainability issues mid-century when climate change is taken into account.

Figure 4: Water Supply Sustainability Index in 2050 including (above) and excluding (below) climate change effects

The study found that water withdrawals in California are estimated to be greater than 100% of the available precipitation in 2050.


Overall, while there may not be a significant change in overall precipitation falling in California as the average temperature continues to rise due to anthropogenic global warming, the challenge is in coping with the transition as the precipitation falls more as rain than snow.  Finding a way to store the rainfall as we lose nature's storage medium (the Sierra snowpack) will be difficult.  Winter sports enthusiasts will also be adversely impacted by the decline in snowfall, as will the state economy.  And the major agricultural industry of California is very vulnerable to these changes.  A decrease in California agricultural production will not just impact the state and country, but the entire world.

The more we can reduce anthropogenic greenhouse gas emissions, the less the planet and state will warm, and the easier it will be to cope with these changes.  In fact as shown in Figure 4, in order to minimize water sustainability challenges nationwide, we must minimize the warming of the planet.

This blog post was written as part of Blog Action Day 2010
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Comments 1 to 9:

  1. Canary in the mineshaft? This is an excellent article. One might say that as California goes so does most of the western half of the United States. And let us not forget how the dust bowl of the 1930s impacted drought and heat in much of the United States. Could global warming be heading us into such conditions again? In addition to snowpack runoff, much of the country depends heavily on groundwater for irrigation and domestic use. What effect will increased drought and heat associated with global warming have on groundwater usage and replenishment?
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  2. Thanks Roger. I don't think California is a canary, I think it's a miner. We can't afford to lose California's agricultural productivity, which is just massive. There is a very good chance that climate change could lead to another Dust Bowl in the SW USA. And groundwater resources will no doubt be depleted in areas facing significant droughts. It's hard to say how California will be impacted, since the state will still receive a similar amount of precipitation. It depends on whether we manage to capture it successfully.
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  3. Fascinating, thanks! There's a journalist by the name of John Fleck who makes somewhat of a specialty of water resources in the SW United States. His blog is worth checking in on from time to time.
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  4. This is not just a problem of the future: For several years I have driven on Interstate Highway 5 past Mount Shasta (14,179 ft / 4,322 m) in Northern California. Until recently the mountain was covered in snow year-round, and could be seen as a white cone from much of the Sacramento Valley north of the city of Sacramento. In the past few years this has not been the case. In August, 2009, the mountain was bare with only a couple of patches of snow near the summit (as seen from the south and west). Two weeks ago there was more snow, covering the top half of the mountain except for some large areas of bare rock. With the current La Niña event, we may hope for some precipitation this winter, but it is likely to be rain rather than snow in the Sierra. -- Bill, getting thirsty in the SF Bay Area.
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  5. Some things don't make sense. In a 1944 treaty between the United States and Mexico, Texas was to take water from Mexico out of the Rio Grande and Arizona was to give water back to Mexico from the Colorado River. Arizona dumps a tremendous amount of water from the Colorado River on the ground to raise crops year-round, and a lot of it percolates through the sand to flow without fanfare back into Mexico where they can reuse it. But that hidden water doesn't count toward satisfying the treaty requirements, so the United States has installed an elaborate system of pumps and canals along the border between Arizona and Mexico to extract the groundwater and deliver it to Mexico. If the actual flow of groundwater into Mexico is already known, why not just renegotiate the treaty to avoid the expense (including CO2) of running pumps and the water distribution system? In either case that doesn't help California. Since Mexico has stopped shipping water to Texas (see first link above), maybe Arizona should reuse their recovered water, saving more of the Colorado River water for California. That would probably be a temporary fix, as the Colorado River is not immune to Global Warming.
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  6. provides information on the findings of the GRACE satellites. Although the importance of California’s Central Valley to US agricultural output seems over-stated, the fact is that dwindling water supplies have already resulted in population/agriculture competition for the inadequate supply available. Outcome to date is that population needs are being met. Those of agriculture are not. Aquifers are already being pumped at unsustainable rates in a bid to overcome shortage of water for agriculture and yields are falling. This is happening in 2010. With predicted population growth and the effects of climate change by 2050, agriculture in its present form will no longer be possible. As in Australia, farmers will have to learn to use water in a more cost-efficient and effective manner, growing those crops which provide the best returns, given the price of water. Farmers do not have until 2050 to learn how to make best use of water. They must do so now. Those that do not will go out of business and crop yields will fall further as population increases.
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  7. There's already a lot of acrimony over trying to keep Californian River flows high and cool enough for salmon. Regardless of scenario, I guess it's good-bye to California's salmon.
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  8. Influx of seawater in aquifers is a problem in the Orange County wherabouts for a long time. Sea level rise would certainly worsen this. The problem and -mainly- the efforts in litigation and rule making to solve it have been subject of extensive research, including the Nobel Prize Elinor Ostrom. Her work about common-pool resource management should have more attention than it has had in these times of natural resource overuse. Two recommended books: - Rules, Games and common-pool resources (very academic and technical. Lots of math) - Governing the commons (a still academic but more readable text) Both have these Californa aquifers as one of their case studies.
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  9. I did a project on the Colorado basin recently. Major issues are due to demand. Some of my conclusions. • The initial apportionment of water under the Colorado River Compact was based on data collected over a short period, at a time of relatively high flow. Current water balance suggests that the sum of the entitlements is higher than current flows. • No consideration was taken for environmental needs within the Compact as a result discharge to the Gulf of California is close to zero. • Pollution is a major problem, partly due to very low river flow, with high salinity of river water especially in the southern reaches. Desalination plants at the Mexican border reduce the salinity of the river water before it enters Mexico. • There is no organisation which has overall control over the basin. • Agricultural use of water has grown rapidly over the last century and accounts for 80% of the water used. • Many of the crops grown in the basin are exported to other regions or countries for example exporting hay and rice to Japan. • Huge subsidies on water prices are allowing farmers to grow crops which would otherwise not be economically viable. • Rapid population growth has increased water demand substantially. The population grew by 10-35% between 2000 and 2010 in the 7 states within the Colorado basin. This growth is expected to continue. • Population growth is, in part, indirectly driven by cheap water prices. • Residential water usage is amongst the highest in the United States, however, water prices are some of the lowest. • Much of the Infrastructure in Colorado basin is funded partly by the federal government. The United States collectively is paying for infrastructure • Open water evaporation from reservoirs is a major problem, with losses estimated to be approximately 2MAFY. • Climate Change will increase temperatures in the Colorado basin by 2-4°C. Reduced run off will decrease flows in the Colorado River by 10-30%. By tackling some of the issues with demand and getting rid of subsidies on water prices many of the issues could be dealt with relatively easily. For instance increasing block tarrifs need to be employed. Also the actual cost of water needs to be evaluated including the payments for existing infrastructure.
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