The scarcity of water in the southwestern United States is exacerbated by the occurrence of frequent and prolonged droughts. Assessments of the historical and prehistorical occurrences of drought in the Colorado River basin were made using streamflow measurements and estimates for the period 1906-83 and treering reconstructions of flows from the 1500s to 1960s. If a drought is simply a continuous period with flows below the long-term average, then severity of droughts can be described in several ways depending on the resource being affected. For example, from the Colorado River flow series, severe sustained droughts could be identified as the periods with the maximum accumulations of flow deficits (relative to the long-term average flows) or as periods in which a hypothetical level of reservoir development, management, and demand would suffer the greatest reservoir- storage depletions. In the present analysis, results were relatively similar using either approach, and the resulting "worst-case" drought scenarios were used in other studies of the likely impacts on hydrologic, ecological, and social systems of the Southwest.
Figure 3. Cross section of tree rings (Photo courtesy the NOAA Paleoclimatology Program)
Figure 4. Annual streamflow totals and (heavy) 20-year moving averages of flows at Lees Ferry (Compact Point), as estimated from tree-ring series.[MAF = millions of acre-feet per year.] (Modified from Meko and others, 1995)
Figure 5A. Annual flows, 1570-1610, illustrating three hydrologic droughts in quick succession between 1579 and 1600. (Modified from Tarboton,1995) [MAF = millions of acre-feet per year.
Figure 5B. Definition of the artificially severe sustained drought for the Colorado River basin. [MAF = millions of acre-feet per year. (Modified from Tarboton,1995)
Using the tree-ring series and a variety of statistical models of the variability of Colorado River flows, droughts like the worst-historical drought (1943-1964) are expected about once every 50-100 years, on average. The realistic worst-case severe sustained drought (1579- 1600) might recur once in 400-700 years while the artificial (sorted) drought scenario might be expected about once in 2,000 to 10,000 years, on average.
It has been conjectured that greenhouse warming might
lead to an enhancement of the global hydrologic cycle
(e.g., Graham, 1995) and that enhancement, in turn, might result in
greater hydroclimatic variability. If so, then the
probabilities of droughts such as those identified by
Meko and others (1995) and by Tarboton (1995) could change and the
expected time between such severe sustained droughts
might be much shorter or much longer. Notably,
Lamb (1995) describes quite varied long-term changes in precipitation
over the western United States during the 1100s to 1300s
when general cooling prevailed (leading up to the
Little Ice Age); some areas experienced more drought
during this cooler period and others were wetter. Thus
there is no certainty as to whether climate change would
bring more or less drought to the Southwest.
Other References Cited
Graham, N.E., 1995. Simulation of recent global temperature
trends. Science, v. 267, p. 666-671.
Lamb, H.H., 1995. Climate, history, and the modern world (2nd
ed.). New York, Routledge, 433 p.
Back to Motivation section
Graham, N.E., 1995. Simulation of recent global temperature trends. Science, v. 267, p. 666-671.
Lamb, H.H., 1995. Climate, history, and the modern world (2nd ed.). New York, Routledge, 433 p.
What kind of hydrologic, environmental, and economic impacts would result?
What management alternatives might help?