U.S. Department of the InteriorU.S. Geological SurveyFact Sheet 2005-3018March 2005Changes in Streamflow Timing in the Western United States in Recent Decades... from the National Streamflow Information ProgramPrinted on recycled paperThis 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.IntroductionMountain 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-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.Streamflow DataBenjamin 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 streamflow1900 1920 1940 1960 1980 20000.20.30.40.50.60.7Spring FractionApril-July Flows as Fraction of Water-Year Total180 150 120 90 60 70 60 50 40 30 20 10 March - MayTemperature ( C) Trends1950-19976420-2-4-622222222221111111100000000000000-1-1-1-1-2-2-2-2oo o o ooooooooomagnitude and timing. Streamflow information is required for numerous water-facility design and operational needs, as well as for research bearing on the long-term implications of climate variability and human activities. The USGS has been measuring and recording streamflows in the western United States since the late 19th century. Because USGS streamflow data (http://waterdata.usgs.gov/usa/nwis/sw) are collected to consistent national standards, span long periods of time, cover the full range of streamflow from floods to droughts, and include observations for unregulated natural streams, they can be analyzed to determine when and where snowmelt and streamflow-timing vary. Streamflow timing can be described by many different measures, depending on data availability and on the aspects of streamflow timing that are of most concern. Roos (1991) and Dettinger and Cayan (1995) analyzed the frac-tions of annual streamflow that occur in spring and summer seasons, because this fraction of total flow is, in many water-resource systems, the most readily stored and distributed for warm-season uses. Cayan and others (2001) characterized streamflow timing by the day of year when wintertime low-flow conditions rapidly transition to springtime high-flow conditions with the onset of warm-season snowmelt. These “spring-pulse dates” are important because they indicate the tim-ing of snowmelt and the divide between winter and spring conditions. Stewart and others (2004) characterized streamflow timing by the “center of volume” of each year’s hydrograph. The center-of-volume date is the date by which roughly half of the streamflow for a year has passed. These center-of-volume dates provide direct measures of overall streamflow timing based on runoff conditions throughout the year.Changes in Streamflow TimingAnnual streamflow in most western rivers has come progressively earlier dur-ing the past several decades. The long-term tendency of springtime streamflow (that fraction of overall flow that occurs from April to July) to decline during the 20th century in the central and northern Sierra Nevada is shown as a fraction of overall flow in figure 1. Regressions of data prior to the 1945 indicate no statistically significant trend, whereas regression of data after 1945 show a clear statistically significant trend toward ear-lier streamflow. As the springtime frac-tions of yearly flows have declined, the fraction in winter, and especially flows in March, have increased. These results broadly reflect a regional trend toward warmer winters and springs during the same period (fig. 2). Changes in daily streamflow variations and center-of-volume dates of western rivers can be illustrated by comparing Figure 1. April - July streamflow in eight major rivers of the western Sierra Nevada, Cali-fornia, as a fraction of water-year (October through September) total streamflow. Dots indicate yearly values, blue curve is 9-year moving averages, and dashed line is linear trend prior to 1945 and solid line is trend after 1945.Figure 2. Springtime temperature trends in North America, 1950-1997, using gridded temperature anomalies from Peterson and Vose (1997).mean measured flows in the Clark Fork Yellowstone River, Wyoming, during the 1950s with those during the 1990s (fig. 3a). The