Precipitation Trends and Water Consumption in the Southwestern United States

by
Henry F. Diaz
National Oceanic and Atmospheric Administration
Environmental Research Laboratory/Climate Diagnostics Center
and
Craig A. Anderson
Cooperative Institute for Research in the Environmental Sciences
University of Colorado, Boulder

This presentation is abstracted from Diaz, H.F., and Anderson, C. A., 1995, "Precipitation trends and water consumption related to population in the southwestern United States: A reassessment": Water Resources Research, v. 31, p. 713-720, March 1995. The reader is urged to refer to the full paper and(or) contact the authors for more information.

Introduction

This study compares trends in water consumption, regional precipitation, and population growth in the southwestern states representing the major users of Colorado River Basin water (Arizona, California, Colorado, Nevada, New Mexico, and Utah). Although much of the western United States was characterized by wet weather during the early 1980s, the years since then have been predominantly dry. A decline in water use has also taken place in many of the western states since the mid-1980s, notably in California, which accounts for the majority of water depletions from the Colorado River Basin. However, population in the six-state area has increased at approximately the same rate as in previous decades. A continuation of these trends may result in heightened competition for available water supplies, exert increased pressure on water pricing policies, and force users to increase water conservation efforts.

Climate Variability

The Southwest receives precipitation from several different sources. In California, most of the precipitation results from Pacific storms originating in the Gulf of Alaska during the winter. In contrast, Arizona and New Mexico receive the bulk of their precipitation from the "summer monsoons", whereas Nevada, Utah, and Colorado receive their moisture from a variety of sources.

One of the most important characteristics of precipitation in the Southwest is the high degree of seasonal, interannual, and decadal variability. FIGURE 1 shows the precipitation record for California since 1895; the data have been smoothed slightly to suppress the high interannual variability. As can be seen, the high rainfalls of the early 1980's (due to a strong El Niño event) and the subsequent severe drought of the late 1980's were not unique events. Similar very wet and very dry conditions have occurred in the Southwest throughout this century. The level of Great Salt Lake (Utah) and the streamflow of the Colorado River show similar decadal-scale variations (FIGURE 2).

The recurrence of low precipitation and reduced streamflow is a fundamental characteristic of the Southwest. When severe droughts or severe wet conditions occur, they can affect large portions (up to 50%) of the western U.S. (Southwest + Northwest). FIGURE 3 shows the fraction of the West affected by moderate-to-severe drought or moderate-to-severe wet conditions since 1895. Again, the decadal-scale variability is clearly visible.

Water Supply

Most of the water supply for the Southwest comes from melting snow during the spring and early summer. However, changes in storm tracks, in the proportion of precipitation that falls as snow, and in seasonality, can result in earlier snow melt, diminished snowpack, increased evaporative losses, and lower runoff. This translates to less water being available for storage in the network of western reservoirs. The largest watershed in the Southwest is the Colorado River Basin (FIGURE 4). When filled to capacity, the reservoirs of this watershed can store 60 million acre feet of water. This amounts to about a 4-year supply at current consumption rates.

Water Use
FIGURE 5 shows the water-use rates in the six southwestern states since 1950. The usage is divided into four components: 1) civil use, 2) irrigation, 3) consumptive and irrigation conveyance losses, and 4) hydroelectric power generation. Category 3 refers to water lost to the system through various processes (e.g. evapotranspiration and irrigation conveyance losses) and hence is unavailable for further use. Between 1950 and 1990, the population in the Southwest tripled (FIGURE 6) and the overall demand for water has increased accordingly. This is particularly true of civil use, which is the smallest of the four categories. The next largest category, irrigation use, increased between 1950 and 1980 and then showed a small decrease. The consumptive use and hydroelectric categories display similar trends. At this time, it is unclear whether the observed decline in the total water use figures between 1980 and 1990 represent a systematic change (due to increased efficiency) or a temporary slowdown that will reverse itself in the next decade. Ultimately, the demand for water is expected to increase as the population of the Southwest continues to grow.

Summary

The climate record clearly shows the occurrence of significant precipitation and streamflow fluctuations on interannual to decadal time scales. These fluctuations can change the amount of water available in Southwestern watersheds by 50% or more for several years. A severe drought lasting more than 5 years will have a major impact on water deliveries and hydroelectric production in the Southwest. Given the rapidly growing population of the Southwest, water scarcities may become increasingly evident, even in the absence of major dry episodes.

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U.S. Department of the Interior | U.S. Geological Survey
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