USGS to Develp Indicators to Better Measure Nation's H2O Supply: Greenwire

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BOX A. -- LONG -TERM STREAMFLOw TIMING TRENDS IN NEW ENGLAND AND WESTERN STATES An important objective of the proposed water availability assessment is the ability to document long -term changes in water availability. An example of this ability has been demonstrated recently in studies of changes in streamflow timing in New England and the western United States. In both regions, changes have been documented in winter and spring streamflow. These changes reflect recent warmer winter and spring temperatures and the affiliated increase in the fraction of precipitation falling as rain rather than as snow (in the West) and to earlier snowmelt (in both regions). Such changes have considerable practical importance because, if they continue, water - resources managers may need to modify long- standing rules for water supply and flood control operations. Snow pack is an important natural component of the water storage system in these regions. The loss of snowpack storage will either lead to do a decrease in the size of reliable water supply or the construction of new man -made storage to keep reliable supplies up to historical levels. In New England, changes in the timing of streamflow have been observed in many unregulated, rural rivers during recent winter - spring seasons. Center -of- volume dates, that is, the date when half of the total volume of streamflow for the period January 1 to May 31 has passed the streamgaging station, were determined annually for 27 rivers (Hodgkins and others, 2003). Despite considerable year -to -year variability, a clear shift toward earlier center -of- volume dates by one to two weeks has occurred during the past 30 years (Fig. Al). In the West, a similar shift in streamflow timing of one to two weeks sooner has been observed since the 1940s ( Cayan and others, 2001). As in New England, western streamflow- timing trends are largely restricted to rivers that receive large snowmelt contributions each spring. They are also most evident in rivers draining middle - altitude basins. The trends are of concern because, historically, nearly 75 percent of western water is derived from spring snowmelt. Were they to continue, these trends threaten to reduce the amount of "useful" streamflow into some western reservoirs by reducing the fraction of each year's streamflow that can be diverted or captured in spring and summer (Fig. A2; Dettinger and Cayan, 1995). 10 Apr Age so cs Mar 31 _� IM N [t. 21 11 1900 _...... 1920 19W 19@0 2000 Year 0$ 0.7 06 O.ta O.A 6 1 Year Fig. Al. Smoothed winter- spring center -of- volume Fig. A2. Smoothed spring fractions of annual dates dates for the 13 longest record rivers in New England. for streamflow volumes for eight western rivers 503 1920 1980 1060 lt5 "it Year Fig. Al. Smoothed winter- spring center -of- volume Fig. A2. Smoothed spring fractions of annual dates dates for the 13 longest record rivers in New England. for streamflow volumes for eight western rivers