Climatic Fluctuations and the Timing of West Coast Streamflow

Since about 1950 there has been a trend in the California Sierra Nevada toward a decreasing portion of the total annual streamflow occurring during April through July, while the streamflow during autumn and winter has increase. This trend not only has important ramifications with regard to water management, it also brings up the question of whether this represents a shift toward earlier release of the snowpack resulting from greenhouse warming. Therefore, the observed record has been examined in terms of relative influences of temperature and precipitation 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 California and 0regon was examined. Linear regression models were used to relate precipitation and temperature to the fractional streamflow at the three elevations for each season. Composites of monthly temperature and precipitation were employed to further examine the fractional streanflow in its high and low tercile extremes. Long time series of climatic 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 stream-flow. In-season precipitation is still the most important influence upon NDJ and FMA fractional streamflow; however, the influence of temperature in melting the snowpack is seen on MJJ fractional streamflow, whose strongest influence is FMA temperature. At higher elevation prior-season precipitation exerts a greater influence than at low and middle elevations, and seasonal temperature anomalies have an effect on all seasonal streamflow 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 spring temperatures over most of the western United States, but since them has also been a trend toward decreasing temperatures in the southeast, we do not interpret this as a signal of anthropogenic warming. Other factors in the trend toward earlier streamflow may include a decrease in MJJ precipitation and an increase in August–October precipitation.