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

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PROJECTS

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

Study Area: Statewide
Period of Project: 2004-2005
Project Number: BCV00
Project Chief: Gerhard Kuhn
Cooperator: Colorado Water Conservation Board

BACKGROUND:

Since the late 1800's, Colorado has experienced several periods of drought that have varied from about 2 to 12 years in duration. The most recent Statewide drought began during 1999 and extended into 2002, a year characterized by precipitation, snowpack accumulation, and streamflow much lower than normal. Statewide, mountain snowpack was about 53 percent of normal on April 1, 2002, the lowest since 1977, and 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. Although streamflows throughout much of Colorado during 2002 were the lowest on record since at least 1977, a longer-term perspective of streamflows during 2002 in relation to recorded historical streamflows throughout Colorado is needed. This perspective will provide water-resource managers and planners an indication of the likelihood of similarily low streamflows and enable better planning for droughts.

OBJECTIVES:

To provide a perspective analysis of the non-exceedence probabilities and non-recurrence intervals of streamflows recorded throughout Colorado during the drought of 2002, as well as other drought years, such as 1977. The proposed study will analyze historical discharge records for streamflow-gaging stations throughout Colorado that (1) were operating during both 2002 and 1977, (2) have at least 25 years of record, and (3) are not immediately downstream from reservoirs. About 150 stations in Colorado meet these criteria.

Because most streamflow in Colorado is derived from melting of mountain snowpacks during April-July, the proposed study will not be a typical low-flow analysis, focusing only on the base-flow period that usually extends from October-March. Rather, the study will analyze streamflow records during both the snowmelt period and the base-flow period. The snowmelt period will be defined as April 1-September 30, and the base-flow period will be defined as October 1-March 31; for the 2002 base-flow period, discharge records through March 31, 2003, will be included.

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