Hydrologic Effects and Water Resources

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Higher temperatures mean changes in precipitation patterns that will have a dramatic impact on California's water resources and hydrology. Most researchers agree that global warming will change the form, timing, intensity, and distribution of precipitation in very significant ways, whether or not there is any change in the overall amount of precipitation.22-26 This has profound implications for California's surface water supply.

Available surface water depends largely on how much snow falls and how long it is stored in the Sierra Nevada and Cascade mountain ranges each winter. Higher temperatures mean that more precipitation will fall as rain and less as snow.27 Growing evidence suggests that the rain-to-snow ratio is already increasing as a consequence of global warming. Several studies indicate that annual stream flow from the Sierras in the fall and winter has increased during the second half of the twentieth century, and spring flow has decreased.28,29 In general, snow lines in California's mountains will rise 500 vertical feet for each degree centigrade increase in atmospheric temperature.30 Therefore, a future 3°C increase will raise the snowline 1,500 feet. Higher temperatures also mean earlier snowpack melt.31-33 Assuming a 3°C temperature increase, the combined effect of less snow and earlier snowpack melt will reduce the amount of water stored in the snowpack by an estimated 33 percent in an average year.34 Such changes in the snowpack will cause a shift in the timing of water runoff from the mountains toward the winter and early spring.35 Runoff will increase in winter and early spring and decline in late spring and early summer.36,37 This probably will be the case even if there is no change in the amount of precipitation.38

FIGURE 3.2. Hypothetical Changes in Runoff for a Western Snowmelt Basin.

Should winter precipitation double by the end of the century (as some models predict), runoff will increase three-fifths by the 2030s and double by the 2090s.39 Figure 3.2 illustrates the shift in timing of the spring runoff.

Currently, winter runoff from Sierra Nevada and Cascade rivers is captured and stored in reservoirs. These reservoirs, together with a lengthy canal system, make water available throughout the year to urban population centers and agricultural areas statewide. This elaborate system moves water from northern California, where two-thirds of the state's surface water originates, to southern California, which has 70 percent of the population and 80 percent of the water demand.40 The reservoirs are also used to prevent flooding of agricultural lands and urban areas that would otherwise occur as a result of natural variations in the amount of runoff.

Changes in the timing and amount of runoff will alter the frequency, timing, and severity of floods. Today, more than 75 percent of California's communities are built on floodplains or Special Flood Hazard Areas.41 Continued population growth probably will mean more development in floodplains. Increased spring runoff will expand flood-prone acreage to include many communities that are not yet at risk. Further urbanization, which increases surface runoff during storms, will exacerbate the problem. An increase in wildfires caused by higher summer temperatures and intensified winds is another likely consequence of global warming in California.42 Wildfires denude watersheds and increase soil erosion. This increases the amount of sediment deposited in California's streams and rivers, thereby reducing their flood-carrying capacity and placing an even heavier burden on the reservoir system.

The water delivery system was constructed under the federally funded Central Valley Project and the California State Water Project in the mid-twentieth century. It was engineered based on runoff patterns that had been observed over the preceding hundred years. Changes in the spring runoff patterns are likely to reduce summer water supplies unless major changes are made to California's water delivery system or demand is reduced.43 This is true even if annual precipitation increases because the increase will occur in the winter. Existing reservoirs will not have enough capacity to store the increased winter volume for use during the summer and, at the same time, prevent flooding unless current operating rules change, and even that may be insufficient.44

Construction of additional infrastructure may be necessary to capture the increased winter and spring runoff. However, there are significant environmental and cost impediments to such construction.45,46 In the absence of new infrastructures, existing reservoirs would need to be maintained at lower levels during the winter. This could reduce statewide water supply in the summer by an estimated 7—20 percent.47 Alternative flood control measures could be implemented so that more of the winter runoff could be stored in reservoirs. Development could be restricted in floodplains, for example. Rivers could be restored to their natural state so that floods would spread out along their length and concentrated flooding downstream would be avoided. Californians could also adopt conservation technologies (e.g., low-flush toilets and better landscaping practices) to improve the efficiency with which municipalities and industry use the reduced summer water supply.48

Without infrastructure or water management policy changes, the surface water supply will also be adversely affected by another major consequence of global warming: sea level rise. The Sacramento and San Joaquin Delta is the major source of California's water, providing 65 percent of the state's total water supply and 45 percent of its drinking water.49 Today, water is released from upstream reservoirs to maintain delta outflow at the level needed to prevent saltwater intrusion into pumping stations that supply freshwater to the rest of California. Increased seawater intrusion caused by higher sea levels, together with lower dry season runoff, would significantly degrade water quality in the delta.

FIGURE 3.3. Sea-Level Rise and Delta Flooding.

Twice as much water would have to be released from reservoirs to maintain water quality if sea level rises by 1 meter.50

Today, a significant portion of land in the delta lies as much as 25 feet below sea level.51 It is protected from inundation by a system of levees, sumps, and pumps. If sea level rises 1 foot, today's 100-year high tide mark will become the 10-year high mark (Fig. 3.3).52 A 1-meter sea level rise, together with increased winter runoff from the Sacramento and San Joaquin rivers, probably would overtax the current levee system.53 A large inland lake with fresh to brackish water would then replace the existing delta.

Furthermore, sea level will rise in the San Francisco Bay during the twenty-first century due to global warming and other factors such as land subsidence caused by tectonic movements and depletion/compaction of subterranean aquifers. Intertidal areas that provide winter homes and migration stopovers to many species of shorebirds will be inundated. Sea level in the Bay has been rising historically. A rise of 1.5 meters is expected in the southern Bay where the change is particularly pronounced over the next hundred years if the historic rate of change continues. More than half of the current tidal flats may be lost as a result. If global temperature increases another 2°C to 4.7°C as expected, the rise will be even higher, between 2 and 2.5 meters by 2100. As illustrated in figure 3.4, this will result in a 70 percent to 83 percent loss of intertidal habitat. Similar effects can be expected for salt marsh and upland habitat as well.54

Today, surface water supplies approximately 60 percent and groundwater

2120

FIGURE 3.4. Projected future percent changes in intertidal and upland habitat in northern and southern San Francisco Bay under three sea level rise scenarios.

supplies 40 percent of the water used for all purposes in California.55 In many areas, the use of groundwater could be increased to compensate for a reduction in surface water during the summer. However, groundwater quality will be compromised in areas along the heavily populated coastal strip if aquifers are inundated by seawater. Hazardous waste sites and landfills along the coast could be flooded and contaminate aquifers, further reducing the groundwater supply.

More reservoirs and levees probably would be needed to ensure an adequate surface water supply in the summer.56 The entire water delivery system will have to be reengineered to maintain water supply and flood control at current levels. However, major construction projects of this sort take 30—50 years to plan and build. Voters must approve them. Serious disruptions in the water supply system may have to occur to create the political will to undertake a major new construction project.

FIGURE 3.4. Projected future percent changes in intertidal and upland habitat in northern and southern San Francisco Bay under three sea level rise scenarios.

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