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The Arid West—Where Water Is Scarce - Sources Of Western Water Supplies

groundwater aquifer land surface

Surface Water and Runoff

Precipitation (rain, snow, and sleet) is the main source of essentially all freshwater supplies in the West. The amount of precipitation largely controls the availability of surface water and groundwater. In the arid regions of the West, much of the available precipitation evaporates shortly after rains. Tucson, Arizona, for example, receives most of its annual rainfall from heavy thunderstorms during the hottest months of the year—between July and September—when much of the rainfall is lost through evaporation.

Runoff refers to water that is not immediately absorbed into the ground during a rain and runs off into lower-lying areas or surrounding lakes and streams. Runoff is the primary measure of a region's renewable water supply. In addition to rain, a large share of the West's runoff comes from the melting of mountain snowpacks, which are essentially huge reservoirs of frozen water that slowly release their supplies during the spring and summer. Much of western agriculture depends on this meltwater becoming available during the growing season.

FIGURE 8.1
Changes in Western snowpack, 1999–2095
SOURCE: "Figure 6-9. Projected Reductions in Western Snowpack Resulting from Potential Changes in Climate," in Climate Action Report 2002: The United States of America's Third National Communication under the United Nations Framework Convention on Climate Change, U.S. Global Change Research Program, May 2002, http://www.usgcrp.gov/usgcrp/Library/thirdnatcom/default.htm (accessed April 13, 2005)

Changes in climate can adversely affect this important water source. The U.S. Global Change Research Program (USGCRP) projected that the snowpack of the West's mountain ranges was very likely to decrease as the climate continued to warm, despite a projected increase in precipitation. (See Figure 8.1.) Their research suggested that in the coming years more precipitation would fall as rain (rather than as snow) and that snow-pack would develop later and melt earlier. As a result, peak streamflows would very likely come earlier in the spring, and summer flows would be reduced. The change in the timing of runoff from snowmelt is likely to have implications for water management, flood protection, irrigation, and planning.

Groundwater

The other source of water in the West is groundwater, or subterranean supplies. Most groundwater is found in aquifers, underground saturated zones full of water (all the spaces between soil and rocks, and the rocks themselves, contain water). These saturated zones are recharged (replenished) primarily from rainfall percolating through the soils. Water from streams, lakes, wetlands, and other water bodies may also seep into the saturated zones. In the saturated zone, water is under pressure that is higher than atmospheric pressure. When a well is dug into the saturated zone, water flows from the area of higher pressure (in the ground) to the area of lower pressure (in the hollow well), and the well fills with water to the level of the existing groundwater. If the pressure is strong enough, the water will flow freely to the surface; otherwise the water must be pumped.

The Ogallala or High Plains Aquifer is one of the world's largest. Located in the United States, it covers 156,000 square miles stretching from southern South Dakota to the Texas panhandle and is the largest single source of underground water in the United States. The Environmental Protection Agency (EPA) has designated the Ogallala Aquifer a sole source aquifer, meaning that at least 50% of the population in the area depend on it for its water supply. The Ogallala Aquifer provides water to portions of eight western states—Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. Like many aquifers in the West, this once plentiful source of underground water is rapidly being depleted because its water supply is being extracted by thousands of wells at a faster rate than can be replenished through annual rainfall. Falling water tables invariably signal that withdrawal of groundwater is exceeding the rate of replacement and that, eventually, the source of water could disappear.

In July 2002 concerns about excessive pumping of the Ogallala Aquifer prompted Senators Sam Brownback (Republican of Kansas) and Jeff Bingaman (Democrat of New Mexico) to introduce to Congress the Ogallala Aquifer Bill. Officially named the High Plains Aquifer Hydrogeologic Characterization, Mapping, and Modeling Act, the bill proposed establishing a program within the Department of the Interior to map and study the aquifer. The United States Geological Survey (USGS) would coordinate the program and would provide grants and technical assistance to those eight states (or local agencies and educational institutions) for projects that address groundwater issues. Reintroduced in 2003, the bill was passed by the Senate but stalled in the House of Representatives.

Because of the arid and semiarid climate found in much of the West, the natural replenishment of aquifer water is slow, and the consequences of the large-scale removal of groundwater are becoming more evident. These consequences include land subsidence; loss of springs, streams, wetlands, and their associated habitat; and degradation of water quality. Land subsidence is the sinking of the land surface, and can be caused by the slow drainage of water from the clay and silt sediments in and next to aquifers. As water levels in aquifers decline and the water is drained from the silt and clay soils, they compact, causing the land surface to drop. Uneven land subsidence can cause large cracks and holes in the ground, causing damage to roads, pipelines, buildings, canals and drainage ditches, railroads, and other structures. Significant land subsidence ranging from six to twenty-nine feet in depth has already occurred near Mendota and Lancaster, California; Las Vegas, Nevada; and Eloy and Phoenix, Arizona.

California has a vast network of underground water reserves. This groundwater resource is especially valuable when surface water is in short supply, as in a drought. In 1991 alone (during a seven-year drought), farmers withdrew a record 4.9 trillion gallons more than was put back in—an amount that could support every Californian at that time for almost three years. As long as the amount of groundwater extraction continues to increase, as is expected with the growing population, and recharge (the process by which water is added back to the reserves) remains below normal, the levels of available groundwater will continue to decline.

As water levels go down, a water supplier can deepen a well. The amount of energy, however, necessary to lift the remaining water to the surface increases, adding to the farmer's or water utility's cost of producing a crop or providing water supplies for cities. These additional energy costs are ultimately passed along to the water's users. Alternatively, a farmer may switch to a higher-value crop or let the land lie fallow (unplanted). Ultimately, if the groundwater is depleted, the well will go dry.

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