Historical perspective of Statewide streamflows during the 1977 and 2002 droughts in Colorado

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Since the late 1800's, Colorado has experienced several periods of drought that have varied from about 2 to 12 years in duration. The Statewide drought from 1999 extended into 2002, a year characterized by lower than normal precipitation, snowpack accumulation, and streamflow.  Statewide, mountain snowpack was about 53 percent of normal on April 1, 2002, the lowest since 1977 Accumulated precipitation during September, 2001-August, 2002, was the lowest on record for 9 of 15 long-term precipitation stations analyzed.

The low snowpack and overall lack of precipitation resulted in very low streamflows throughout Colorado during 2002.

An analysis of streamflow and water-quality data obtained at 146 stations during 1978-2002 (after 1977, the last year of greatly diminished streamflows throughout Colorado) indicated that mean summer streamflows (July-September) during 2002 were the lowest on record during that 25-year period at 116 of the stations. 

Because most streamflows in Colorado are derived from melting of mountain snowpacks during April through July, streamflows primarily were analyzed for the snowmelt (high-flow) period, but streamflows also were analyzed for the winter (low-flow) period. The snowmelt period is defined as April 1 through September 30 and the winter period is defined as October 1 through March 31. Historical daily average streamflows were analyzed on the basis of 7, 30, 90, and 180 consecutive-day periods (N-day) for 154 selected stations in Colorado. Methods used for analysis of the N-day snowmelt and winter streamflows include evaluation of trends in the historical streamflow records, computation of the rank of each annual N-day streamflow value for each station, analysis for years other than 2002 and 1977 with drought-diminished streamflows, and frequency analysis (on the basis of nonexceedance probability) of the 180-day streamflows.

Ranking analyses for the N-day snowmelt streamflows indicated that streamflows during 2002 were ranked as the lowest or second lowest historical values at 114–123 stations, or about 74–80 percent of the stations; by comparison, the N-day snowmelt streamflows during 1977 were ranked as the lowest or second lowest historical values at 69–87 stations, or about 47–59 percent of the stations. Many of the stations in the mountainous headwaters where snowmelt streamflows were ranked lowest during 2002 were ranked second lowest during 1977. These results indicate that snowmelt streamflows during 2002 were considerably more diminished than those during 1977.

The 180-day snowmelt streamflows were ranked among the five lowest historical values at about 90 percent of the stations during 2002 and were ranked among the five lowest historical values at about 77 percent of the stations during 1977. Other years during which the 180-day snowmelt streamflows were ranked among the five lowest values at a substantial percentage of stations include 1934, 1954, 1963, and 1981, but the percentages of stations with 180-day snowmelt streamflows ranked among the five lowest values were smaller during those years than during 2002 and 1977.