Changes in Streamflow Timing in the Western United States in Recent Decades Fact Sheet March 2005

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Changes in Streamflow Timing in the Western United States in Hecent Decades ... from the National Streamflow Information Program This Fact Sheet is one in a series that highlights information or recent research findings from the USGS National Streamflow Information Program (NSIP). The investigations and scientific results reported in this series require a nationally consistent streamgaging network with stable long -term monitoring sites and a rigorous program of data quality assurance, management, archiving, and synthesis. NSIP produces multi- purpose, unbiased surface water information that is readily accessible to all. Introduction Mountain snow fields act as natural reservoirs for many western water -sup- ply systems, storing precipitation from the cool season, when most precipita- tion falls and forms snowpacks, until the warm season when most or all snow - packs melt and release water into rivers. As much as 75 percent of water supplies in the western United States are derived from snowmelt. Thus, water - resource management of western rivers com- monly is planned around the knowledge that much of the runoff to reservoirs and lowlands occurs during the early parts of the warm season, when water demands for irrigation and other uses are at their greatest. During the cool season, water demands are low and, in West Coast states, the potential is high for winter storms to cause disastrous floods, Sepa- ration in time between the cool- season risks of flooding and the warm - season benefits of snowmelt runoff is a funda- mental assumption of water - resource management strategies in the West. Trends toward diminished snow - pack and earlier snowmelt in western states may be related to global warming (Knowles and Cayan, 2002; Mote, 2003; Stewart and others, 2004) or to naturally occurring variability in winter and spring temperatures and in precipitation form or timing (Dettinger and Cayan, 1995; Cayan and others, 2001). These changes also affect streamflow timing (Dettinger and Cayan, 1995; Cayan and others, 2001). Trends toward earlier snowmelt and streamflow, whatever the causes, threaten finely tuned water- resource and flood - management systems and proce- U.S. Department of the Interior U.S. Geological Survey 1a� \ 11. ia. I Tumultuous springtime snowmelt -fed streamflow in the eastern Sierra Nevada, East Fork of the Carson River above Grovers Hot Springs, Markleeville, Califor- nia, May 2000. dures in many western settings. Therefore recently observed trends toward earlier snowmelt and streamflow in western rivers measured by the U.S. Geological Survey (USGS) are a source of consid- erable interest and concern to resource managers. I® Printed on recycled paper Streamflow Data Benjamin Franklin observed that "When the well is dry, we learn the worth of water." A similar sentiment applies to the availability of information on water, particularly information on streamflow Fact Sheet 2005 -3018 March 2005