CLIMATIC FLUCTUATIONS AND THE TIMING OF WEST-COAST STREAMFLOW

Since about 1950 there has been a trend in the California Sierra Nevad a toward a decreasing portion of the total annual streamflow occurring during April through July, while the streamflow during autumn and win ter has increased. This trend not only has important ramifications wit h regard to water management, it also brings up the question of whethe r this represents a shift toward earlier release of the snowpack resul ting from greenhouse warming. Therefore, the observed record has been examined in terms of relative influences of temperature and precipitat ion anomalies on the timing of streamflow in this region. To carry out this study, the fraction of annual streamflow (called the fractional streamflow) occurring in November-January (NDJ), February-April (FMA), and May-July (MJJ) at low, medium, and high elevation basins in Calif ornia and Oregon was examined. Linear regression models were used to r elate precipitation and temperature to the fractional streamflow at th e three elevations for each season. Composites of monthly temperature and precipitation were employed to further examine the fractional stre amflow in its high and low tercile extremes. Long time series of clima tic and hydrologic data were also looked at to infer the causes in the trend toward earlier runoff. For the low-elevation basins, there is a dominant influence of precipitation on seasonal fractional streamflow . Middle-elevation basins exhibit a longer memory of precipitation and temperature in relation to their fractional streamflow. In-season pre cipitation is still the most important influence upon NDJ and FMA frac tional streamflow; however, the influence of temperature in melting th e snowpack is seen on MJJ fractional streamflow, whose strongest influ ence is FMA temperature. At higher elevations, prior-season precipitat ion exerts a greater influence than at low and middle elevations, and seasonal temperature anomalies have an effect on all seasonal streamfl ow fractions. There are several causes for the trend toward decreasing fractional streamflow in the spring and summer. Concomitant with the trend in the timing of streamflow was an increase in NDJ (most notably November) precipitation. There also has been a trend toward higher sp ring temperatures over most of the western United States, but since th ere has also been a trend toward decreasing temperatures in the southe ast, we do not interpret this as a signal of anthropogenic warming. Ot her factors in the trend toward earlier streamflow may include a decre ase in MJJ precipitation and an increase in August-October precipitati on.