By Gerhard Kuhn
USGS Scientific Investigations Report 2005-5174, 84 p., 22 figs.
The citation for this report, in USGS format, is as follows:
Since 1890, Colorado has experienced a number of widespread drought periods; the most recent statewide drought began during 1999 and includes 2002, a year characterized by precipitation, snowpack accumulation, and streamflows that were much lower than normal. Because the drought of 2002 had a substantial effect on streamflows in Colorado, the U.S. Geological Survey, in cooperation with the Colorado Water Conservation Board, began a study in 2004 to analyze statewide streamflows during 2002 and develop a historical perspective of those streamflows. The purpose of this report is to describe an analysis of streamflows recorded throughout Colorado during the drought of 2002, as well as other drought years such as 1977, and to provide some historical perspective of drought-diminished streamflows in Colorado.
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 114123 stations, or about 7480 percent of the stations; by comparison, the N-day snowmelt streamflows during 1977 were ranked as the lowest or second lowest historical values at 6987 stations, or about 4759 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.
Frequency analysis of snowmelt streamflows indicated that recurrence intervals for the 180-day snowmelt streamflows during 2002 were greater than 50 years for about 57 percent of the stations and were more than 100 years for about 14 percent of the stations. By comparison, recurrence intervals for the 180-day snowmelt streamflows during 1977 were greater than 50 years only for about 15 percent of the stations and were more than 100 years only for about 1 percent of the stations. Generally, snowmelt streamflows during 2002 were more diminished and have higher recurrence intervals than snowmelt streamflows during 1977.
The N-day winter streamflows during 2002 and 1977 were not ranked among the five lowest historical values at about 86103 stations, or about 5870 percent of the stations, compared to about 1027 percent of the stations for the N-day snowmelt streamflows. These results indicate that winter streamflows during the 2002 and 1977 droughts were diminished to a lesser extent than the snowmelt streamflows. The reasons for this partly are the result of streamflow augmentation from reservoir releases, winter ground-water discharge resulting from irrigation during summer, and return flows from municipal water use; the effects of these factors likely have resulted in increasing winter streamflows during the past 100 years.
The 180-day winter streamflows were ranked among the five lowest historical values only at about 39 percent of the stations during 2002 and only at about 37 percent of the stations during 1977, compared to about 90 percent of the stations during 2002 and about 77 percent of the stations during 1977 for the 180-day snowmelt streamflows. The 180-day winter streamflows were ranked among the five lowest historical values at a larger percentage of the stations during 1934, 1956, and 1963, which had the largest percentage, about 63 percent.
Frequency analysis of winter streamflows indicated that
recurrence intervals for the 180-day winter streamflows during 2002 were
greater than 50 years only for about 20 percent of the stations, were
greater than 100 years only for about 6 percent of the stations, and were
less than or equal to 20 years for about 66 percent of the stations. Thus,
the 180-day winter streamflows recorded during 2002 were not exceptionally
low compared to the 180-day snowmelt streamflows recorded during 2002;
frequency analysis results for winter streamflows during 1977 were similar
(less than 10 percent difference) to those for 2002. Winter streamflows
in Colorado during 2002 and 1977 were not affected by lack of snowpack
accumulation and melt to the same extent as snowmelt streamflows. Human
activities probably have a greater effect on winter streamflows, and these
effects probably have changed with the course of changes in water-resource
management and infrastructure.
Purpose and Scope
Methods of Study
Computation of 7-, 30-, 90-, and 180-Day Streamflows
Streamflow Trends Analysis
Historical Perspective of Statewide Streamflows
Analysis of 7-, 30-, 90-, and 180-Day Snowmelt Streamflows
Historical Ranking of Snowmelt Streamflows During 2002 and 1977
Other Years with Drought-Diminished Snowmelt Streamflows
Frequency Analysis of Snowmelt Streamflows
Analysis of 7-, 30-, 90-, and 180-Day Winter Streamflows
Historical Ranking of Winter Streamflows During 2002 and 1977
Other Years with Drought-Diminished Winter Streamflows
Frequency Analysis of Winter Streamflows
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