WSP04710

• File Number: 8056
• Description: Drought Preparedness
• State: CO
• Basin: Statewide
• Date: 6/1/1997
• Author: Donald Wilhite
• Title: Improving Drought Management in the West
• Water Supply Pro - Doc Type: Report/Study

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The Climatology of Drought in the West: 1895 to 1996

Standardized Precipitation Index (SPIJ (McKee et aI., 1993; 1995) at 1-, 3-, 6-,
9-, and 12-month time intervals, These maps are available on the NDMC's
home page. They can be used in conjunction with the PDSI maps to assess
the status of moisture conditions nationwide.

Figure 1 reveals two features of drought in the United States: (1) its variable
but recurrent nature and (2) the magnitude and duration of the droughts of
the 1930s and 1950s in comparison to other episodes during the time series.
Drought frequently affects more than 10 percent of the Nation, and it is not
uncommon for more than 30 percent ofthe Nation to be affected. The most
benign climatic periods occurred around the mid-1940s, between the late
1960s and mid-1970s, and from 1978 to 1985. In contrast, the 1930s drought
continued for nearly a decade; PDSI values of ~-3,0 (severe and extreme
drought) were recorded over approximately 65 percent of the country and
more than 95 percent of the Great Plains at the peak of the drought in 1934,
The 1950s drought began in the Southwest and Southern Great Plains States
in the late 1940s and persisted through 1957. The geographical area affected
during the 1950s was quite similar to the area affected by the 1996 drought.
At its peaks in 1954 and 1956, severe to extreme drought affected nearly
50 percent of the Nation.

Figure 2 illustrates the percent area in severe and extreme drought for three
western river basins (Pacific Northwest, Missouri River, and Upper Colorado)
during the period 1895-1995. Each of these time series illustrates the
frequency of drought in the region. Particularly revealing is the number of
times that drought affected more than 80 percent of each basin. Figure 2 also
reveals the intensity and duration of the 1930s drought in the Pacific
Northwest and Missouri River basins, the 1950s drought in the Missouri and
Upper Colorado basins, the late 1890s and early 1900s drought in the Upper
Colorado, and the 1987 to 1995 droughts in all three basins. These drought
time series for the other river basins in the West are shown in figures 3-5.

Clearly, no portion of the West is immune to the ravages of drought, An
analysis ofPDSI values for the period from 1895 to 1995 indicates that
drought occurs with much greater frequency in the West than it does in other
regions of the country. Most of the West experiences severe to extreme
drought more than 10 percent of the time, and a significant portion ofthe
region experiences severe to extreme drought more than 15 percent of the
time. For the period from 1985 to 1995, large portions of Washington,
Oregon, Wyoming, and Idaho and smaller portions of North Dakota, Nevada,
Utah, and California experienced severe to extreme drought more than

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