Drought is a nebulous concept for which there is no universal definition. All definitions—whether based on precipitation, soil moisture, or availability of water in rivers or reservoirs—are ultimately driven by conditions of abnormally low precipitation or high evaporative demand. Those conditions are particularly chronic in the western United States where water is scarce. The settlers of the 1800s found,for instance, that although land was in ample supply, the success of settlements depended heavily on ample rainfall. Post-Civil War settlers flourished during a period when precipitation generally was ample, but immense hardship followed in the generally dry decade of the 1880s. In modern history, the Dust Bowl years of the 1930s made an indelible impression on a generation of Americans. Although the 1930s drought was not restricted to the West (see Figure 11.1.1), its implications were most serious there (few of the major water systems now in place existed then). The drought of the 1950s was also widespread, but its effects were felt more in the Great Plains region than in the far West. The most recent western U.S. drought began in the late 1990s and persisted for at least 5 years over parts of the region. It has resulted in damages estimated at tens of billions of dollars. Reservoirs in the Colorado River system in particular have declined to near record low levels (see Figure 11.1.2).

An important property of droughts in arid and semiarid regions is that small decreases in precipitation can produce large decreases in runoff. Figure 11.1.3 shows stream flow in the Rio Conchos River of northern Mexico, a major tributary of the Rio Grande. During the 1990s, precipitation fell short of its long-term average by only about 10 percent. Runoff, however, fell by about 50 percent. In contrast, in humid basins, a 10 percent dropoff in precipitation would produce only about the same decrease in runoff, which helps to explain why the severity and duration of droughts tend to be greater in the western than in the eastern United States.

FIGURE 11.1.1 Drought extent in August 1934. Soil-moisture percentiles expressed relative to 1960–2003 climatology. SOURCE: See www.hydro.washington.edu/forecast/monitor.shtml. Courtesy of Land Surface Hydrology Research Group, University of Washington.