In This Issue
Spring Bridge Issue on Engineering and Climate Change
March 15, 2020 Volume 50 Issue 1
The seven articles in this issue cannot cover all engineering-related aspects of climate change, but they highlight several areas of concern.

How Will Climate Change Affect California's Water Resources?

Monday, March 16, 2020

Author: Dennis P. Lettenmaier and Jay R. Lund

Water is the lifeblood of civilization. From the earliest times, civilizations managed water extremes—too much (flooding) or too little (drought). More economically advanced civilizations developed infrastructure and institutions to manage these extremes.


Anglo settlement of North America began in regions with relatively reliable natural water supplies that required modest engineering. Westward migrants in the 1800s encountered more variable climates (particularly precipitation and runoff) than in the eastern part of the country. Recognition of the challenges in “reclaiming” (providing water to) western lands resulted in formation of the US Geological Survey (USGS) in 1879, and a decade later construction of the first modern stream gauge (on the Rio Grande River at Embudo, NM), followed 13 years later by formation of the US Bureau of Reclamation, which was charged with “making the desert bloom.”

Figure 1 

Figure 1 shows the much greater variability of annual precipitation and, to a lesser extent, annual streamflow in the Western states than in the East. Generally, high streamflow variability follows high precipitation variability, and both increase from east to west. Coefficients of variation (standard deviation divided by the mean) are typically greater for streamflow than for precipitation.

Although managing the variability of water resources, particularly in the Western states, is challenging, water infrastructure has helped support reliable water supplies. Usually, reservoirs and use of groundwater help store water from wetter periods for use in drier periods.

Reservoir planning methods date at least to the work of Rippl (1883), Hazen (1914), and Sudler (1927) and typically use observations (usually of streamflow, for reservoir design) to represent the natural variability that reservoirs and aquifers dampen to provide a reliable supply. Early hydrologists recognized that a historical record (time series) is but one realization of many outcomes that could occur in a future planning period.

When the computer age arrived, hydrologists developed approaches termed synthetic hydrology or streamflow synthesis to incorporate uncertainties in hydrologic design. These methods are essentially variants of ensemble prediction now widely used in weather forecasting. Variations of these approaches are embedded in the design and operation of reservoir systems to provide reliable water supplies given interseasonal and inter-annual streamflow variability. Reservoir systems also serve other purposes, including flood protection, hydropower generation, and recreation.

The Changing Climate of the American West

Although the particulars of design and planning methods vary and their sophistication has evolved, essentially all assume statistical stationarity. Stationarity holds that the probability distribution (of, for instance, streamflow) is unchanging in time (both at the margin, say as pertains to streamflows in any given year, and jointly, as pertains to, e.g., covariances of streamflows among years). That assumption does not hold if the probability distributions change over time in ways more complex than, for instance, recurring seasonal variations.

This foundational assumption of stationarity has been challenged as climate change increasingly affects hydrologic processes (Milly et al. 2008). Climate nonstationarity poses new challenges for managing water, especially in the West. When large water works were being planned in the Western states (from the late 1800s to about 1970), a key limitation was short record lengths—few historic climate or hydrologic records exceeded about 30 years.

Now, a half-century past the era of large water infrastructure construction in the United States, much longer records are available, but how best to use them isn’t obvious because of nonstationarity. Nor, for that matter, is the question of how best to incorporate non-stationarity in water planning resolved. Most studies of the sensitivity of water resources to climate change have employed scenario analysis (e.g., Wang et al. 2018), which is useful for examining possibilities but less so for prescribing management responses.

The problem of nonstationary climate for water management is especially prevalent in California, which has a large water infrastructure, mostly designed and constructed decades ago. Well over half of the state’s population (and millions of acres of irrigated agricultural lands) depends on a system of dams and aqueducts that moves water long distances. Most notably, water is transported from Northern California and the Colorado River basin for agricultural use in the Central and Imperial Valleys and coastal cities, notwithstanding trends toward increased conservation and greater use of local water supplies. 

Figure 2 

While figure 1 indicates the high natural variability that water systems in California are intended to manage, the elephant in the room is nonstationarity in the hydrologic system due mostly to warming temperatures. Figure 2 shows winter temperatures in the Sierra Nevada (headwaters for most of California’s water supply) and in the most hydrologically productive sub-basins of the upper Colorado River basin (over 20 percent of the flow of the Colorado is diverted to California via the All-American Canal near Yuma, AZ). Both river basins have clear warming trends, somewhat higher in the Colorado basins than in the Sierra Nevada. Also, both head-water areas have higher trends (on an annual basis) since about 1970 than over the entire ~100-year period (this partly reflects the relative warmth early in the -period, which includes the Dust Bowl years of the 1930s). The larger post-1970 trends (prevalent across the Western states) also may reflect more rapid growth in global greenhouse gas emissions since about the 1970s. In any event, the warming in both headwater regions exceeds that for the United States as a whole, consistent with many studies that show greater warming in the West (and generally less in the Southeast) (Vose et al. 2017).

Less Snow, Earlier Runoff

A major consequence of generally warmer winters in the West has been reductions in snowpack (typically measured by snow water equivalent, SWE, the depth of a snow core multiplied by its density). Figure 3a shows, for about 50 snow courses over the Sierra Nevada where long-term observations have been collected since 1950, trends in April 1 SWE binned by the average December–February temperature. As expected with a warming climate, the largest trends are at the warmest sites (generally lower elevations), with smaller trends at colder, higher elevations.

Figure 3

An interesting aspect of the results for 1950–97 (an anomaly in Mote et al. 2005, from which the data were taken) is positive trends at the highest elevations, where increased precipitation more than compensated for warming. In the longer 1950–2019 dataset, the positive trends no longer appear, which arguably is due to the addition of 22 years of record (1998–2019), a period that has been quite warm (especially the 2007–09 and 2012–16 droughts).

An important hydrologic consequence of warming is less snow (seen in downward trends in figure 3a), which shifts seasonal peak runoff (from snowmelt) to earlier in the year, increasing winter flows and reducing summer flows. Stewart and colleagues (2005) showed such trends across the Western states.

Figure 3b shows trends in spring pulse onset (essentially the beginning of the snowmelt period, as defined by Stewart et al. 2005) for a set of USGS stream gauges in the Sierra Nevada with long records, and minimum upstream effects of dams and diversions. Most sites show spring pulse onset advancing over the last ~60 years by amounts ranging from a few days to 3–4 weeks. This change in streamflow timing effectively reflects a loss of natural seasonal storage, which augments manmade reservoir storage. We discuss below implications of this loss of natural storage for California water management.

Streamflow Sensitivity to Precipitation and Temperature Changes

Although the spring pulse onset in the Colorado basin has advanced similar to Sierra Nevada–-heading streams (Stewart et al. 2005), the consequences are small for reservoir system operation (particularly the two immense reservoirs, Lakes Powell and Mead, which are the source of water transfers to California). This is because the combined usable storage in Lakes Powell and Mead is about four times their natural average annual inflow, so the reservoirs greatly reduce the effects of interseason and interannual streamflow variability on water deliveries. Therefore, the Colorado River system is much more sensitive to changes in annual inflow volumes than to their seasonal timing. Annual inflow volumes are sensitive to precipitation and to factors that influence basin evapotranspiration (often linked to temperature, notwithstanding arguments that temperature sensitivities can be somewhat misleading; see, e.g., Milly and Dunne 2011).

Sensitivity of river discharge to precipitation can be quantified by the elasticity of (average annual) streamflow to (average annual) precipitation (where elasticity is defined as in economics: the fractional change in streamflow divided by the fractional change in precipitation). For the Upper Colorado, various elasticity estimates (see Vano et al. 2014) center around about 2.0, implying that a 5 percent reduction (increase) in annual precipitation reduces (increases) annual streamflow by about 10 percent.

In contrast to precipitation elasticities, which are relatively easily estimated from observations (e.g., Sankarasubramanian et al. 2001), direct estimation of temperature sensitivities (which are more convenient than elasticities; e.g., the change in annual average streamflow per degree change in average annual temperature) is more challenging. This is because effects of temperature variations tend to be obscured by larger effects (on streamflow) of interannual precipitation variability.

Seasonal effects also can be important, with substantial differences in the sensitivity of annual streamflows to winter versus summer warming (Das et al. 2011). Nonetheless, overall most recent work suggests temperature sensitivities of Colorado River annual streamflow to warming in the range of 5–10 percent per degree Celsius—notwithstanding that recent coupled model results (Hoerling et al. 2019) suggest somewhat smaller values.

An important point in interpreting likely future changes is that essentially all climate models predict continued warming across the West (particularly in the Colorado River basin), consistent with observed warming over the last century shown in figure 2. Climate models also tend to show drying over the Colorado River basin (Milly et al. 2005), although more recent results (e.g., Brekke et al. 2014) are less conclusive and tend to show, for both the Colorado Basin and California, small (albeit slightly negative) changes in precipitation. This suggests that the temperature signal may be the most important driver of future change.

For discussion, a conservative temperature increase estimate of about 2°C (by, say, the end of the century) and precipitation change from zero to a 5 percent decrease implies reduction of the mean annual flow of the Upper Colorado of 10–20 percent—the midpoint of which (15 percent) is comparable to the observed change in Colorado River runoff over the last century (Hoerling et al. 2019; Xiao et al. 2018). The possibility that changes of this magnitude could continue in the Colorado River basin and California (where water infrastructure is challenged by changes in both annual volumes and spring runoff timing) has given rise to considerations of how to strengthen existing water systems. We discuss below some possible adaptive responses.

Water Management Challenges

Changes in climate will bring operational challenges for water management in California. First, rising sea levels (Hinkel and Nicholls 2020) will reduce some coastal aquifer yields (due to sea water intrusion) and threaten the stability of lowlands and salinity control in the Sacramento–San Joaquin Delta. This delta is the major hub for California’s interregional water conveyance, and delta-related issues impact the management of flows from contributing river basins.

Figure 4 

Second, seasonal shifts in runoff from spring to winter from the loss of snowpack with warmer temperatures (figure 4) will disrupt reservoir operations based on historical reservoir inflow patterns, specifically refilling reservoirs with spring snowmelt after the winter flood season. Changes in both reservoir inflow timing and reservoir operations will challenge both water supply and flood management. Although runoff is shifting from spring to winter (figure 3b; Stewart et al. 2005), the implications of climate warming for flooding are less well understood (Wasko et al. 2019; Willis et al. 2011).

Third, higher temperatures may increase agricultural water demands, although the jury is still out on this effect (the effects of increased plant water use in a warmer climate, shorter growing season, and CO2 fertilization effects are not completely understood and in any event likely are crop- and site-specific; Cai et al. 2015). A shift toward increased crop water use would challenge reservoir operation (given that roughly 80 percent of California’s human water use is agricultural), which would be complicated by other challenges for maintaining cold water and flows for native fishes and other species.

Furthermore, higher temperatures could, without substantial precipitation increases, decrease California’s access to Colorado River water, which today supplies about 10 percent of California’s water use. Colorado River “surplus” diversions to California effectively ended in the last decade as the Upper Colorado River basin states take more of their allocations under the 1922 Colorado River Compact, even as total river flow has failed to reach total Compact allocation amounts (which were based on anomalously high pre-1922 flows). California historically took more than its -Compact allocation when the Upper Basin states took less, which is no longer the case.

Finally, greater interannual variability in precipitation (predicted by many climate models) may increase the severity of droughts, especially when accompanied by warmer temperatures that accelerate spring and summer soil moisture depletions. All these effects will bring new operating challenges and needs for water policy changes.

California’s water infrastructure (constructed mostly in the second half of the last century) is massive. However, total reservoir storage capacity (about 50 km3) is small compared with average annual inflows of about 90 km3 (the ratio of storage to mean inflow of about 0.44 compares with around 4.0 for the massive Colorado storage system). An additional 20–25 km3 of water (on average) is stored seasonally as snowpack (Mao et al. 2015). This means that most reservoir storage is seasonal (intended to move inflows from the high-runoff spring–early summer to the higher-demand, lower--runoff summer). Nonetheless, the largest reservoirs usually have storage capacity sufficient to supply water for one or two years of drought—but not longer droughts like the most recent (2012–16), which was mitigated mostly by groundwater pumping (and some water use reductions in agriculture and cities) (Lund et al. 2018).

Adaptive Responses

Two obvious responses to the additional stress on water operations from a changing climate are demand management and supply management (partly through increases in reservoir storage).

In recent decades, total US withdrawals of water (and likely consumptive use) have declined, with especially large declines in the most recent period for which data are available (2010–15; Dieter et al. 2018). In California declines in surface water withdrawals, especially for municipal use, were amplified by drought in 2012–16.

Additional reservoir storage would be useful in some cases but can only provide modest overall improvements in water reliability (partly because the most economical reservoir sites are already developed). California anticipates spending $2.7 billion to partially fund additional surface and groundwater storage capacity. However, even if all funded reservoirs were built, they would increase surface water storage capacity by only about 10 percent, with a smaller (percentage) effect on the ability of increased storage to provide reliable water deliveries.

Moving drought water storage from larger existing onstream reservoirs to aquifers or offstream reservoirs, combined with increases in some downstream flood flows and wetland capacities for groundwater recharge, along with better use of hydrologic forecasts, are options that can more flexibly, rapidly, and less expensively increase overall system abilities to manage floods and droughts. However, these options are limited by a combination of legal constraints (e.g., who “owns” flood flows directed to groundwater recharge) and (for flood flows) the limited volume of water available for groundwater recharge (Alam et al. 2020). Furthermore, such changes bring costs, impacts to summer reservoir recreation and hydropower, and higher water supply pumping and energy costs—although failure to act will likely have costs as well.

Groundwater as a Supplemental Water Supply

Recent droughts have highlighted the importance of groundwater as a supplemental water supply. The importance of such supplies, especially for agriculture during long droughts, has grown as California’s agriculture has shifted to more profitable permanent crops that cannot easily be fallowed in dry years. However, greater reliance on groundwater has increased depletions of aquifers—by some estimates, over 55 km3 in the recent 2007–09 and 2012–16 droughts alone (Lund et al. 2018; Xiao et al. 2017). (Some estimates show interdrought recovery, others do not; Xiao et al. 2017.)

Recent California groundwater legislation (the Sustainable Groundwater Management Act) is intended to stabilize groundwater levels by ending overdraft, which should better accommodate growing seasonal and interannual variability in water availability. This also will require reducing overall irrigated area substantially, with economic harm especially in the southernmost part of the Central Valley. In any event, the often decades-long drawdown-refill periods expected for California’s large aquifers and long droughts bring policy and opera-tional challenges for local groundwater management and statewide groundwater regulations—challenges likely to increase as the climate continues to warm (Alam et al. 2019; Dogan et al. 2019).

New Technologies

New technologies may help California’s water system adapt to climate change. In addition to increased aquifer recharge and capture of some seasonal floods, options include wastewater treatment and artificial recharge (already used by the Orange County Water District), advanced hydrologic flood forecasting for reservoir operation, and modeling to coordinate operation of multiple reservoirs.

Agronomic changes in crops and use of high-tech irrigation methods—both on the ground (e.g., drip irrigation) and through remote sensing (e.g., to better determine crop water use in real time)—also could help. However, improved crop irrigation efficiency often “saves” little water as it usually reduces aquifer recharge or return flows to streams—water already committed for droughts and instream or other uses.

Management of Ecosystems

One especially challenging area will be mitigation of water-related environmental and ecosystem management effects of climate change. Natural ecosystems are adapted to long historical hydroclimatic regimes, not the comparatively recent changes due to global (and local) human activities.

Sustaining ecosystem functions will require defining ecosystem objectives achievable under uncertainty and may lead to expensive actions with many challenges, given the extensive impacts of human activities on virtually all California’s ecosystems (Herman et al. 2018). One example is the mandate, under the Endangered Species Act, to restore native salmonid populations in the Sacramento and San Joaquin River systems. Environmental and ecosystem management is likely to be where climate change brings the greatest and most difficult impacts and challenges.

Balancing Management Actions with Climate Effects

California already deals with exceptional hydro-climatic variability (see figure 1). Responses to the challenges of operating the massive water infrastructure have included, among other actions, water conservation and water trading. Climate-related challenges will force changes in the state’s water management, many of which are desirable even without climate change (Connell-Buck et al. 2011).

Importance of a Portfolio Approach

In part because of climate-related and other -stresses, water management in California is increasingly -portfolio-based, an approach that balances the use and operation of a variety of water sources with management options and activities intended to better align the behaviors of water users, system managers, and regulators. Expansion of the portfolio approach must be central in California’s response to climate change; there is no silver bullet.

Arguably, the extreme variability of California’s historical climate might make the state better prepared for still greater variability as the climate continues to warm, as contrasted with other regions with less variable climates (Madani 2019; Pinter et al. 2019). California water management has changed significantly in the past as it has dealt with nonstationary demands, technologies, and legal issues. Now water managers face non-stationary supplies as well.

Adaptation Strategies

Climate change gives California more incentive to accelerate and hone adaptation strategies, which will likely include the following:

  • major changes to reservoir and aquifer operations, to respond to seasonal streamflow shifts, greater interannual variability, and higher water temperatures (Connell-Buck et al. 2011; Dogan et al. 2019);
  • additional wastewater reuse and targeted desalination (e.g., of brackish waters) to help some urban areas, as well as continued urban water conservation efforts and more effective use of groundwater supplies by coastal cities;
  • reductions in irrigated areas in the Central -Valley to meet state requirements to end groundwater overdraft—it is estimated, for example, that, to the -roughly 3 km3/yr Central Valley groundwater overdraft of the recent past, climate change could add about 2 km3/yr by the year 2100 (Alam et al. 2019);
  • water markets to greatly reduce the costs of these transitions; and
  • more effective and flexible regulations and environmental management.


If well managed, climate change effects to -California’s water management systems will not be catastrophic statewide for humans, but they may be catastrophic for many ecosystems and for people in some local areas (e.g., where currently irrigated land is retired). The associated changes and need for adaptation will also bring sizable statewide costs.

The greatest impacts of climate change on water uses in California are likely to be environmental and ecosystem losses, exacerbation of already large agricultural losses to end groundwater overdraft in the southern Central Valley, and an increase in overall costs of water. If state and local water managers adopt effective measures, the state economy seems likely to suffer more from other climate change effects. And the costs of not adapting to change may be much greater.


Thanks to Mike Dettinger (USGS, ret.) for providing figure 1; to UCLA graduate students Qian Cao, Mu Xiao, and Kim Wang for preparing figures 2 and 3; and to Iris Stewart (Santa Clara University) for providing the list of stations used in her 2005 paper, data from which are included in figure 3.


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About the Author:Dennis Lettenmaier (NAE) is a Distinguished Professor in the Department of Geography at the University of California, Los Angeles. Jay Lund (NAE) is a Distinguished Professor in the Department of Civil and Environmental Engineering at the University of California, Davis.