SEASONAL HYDROCLIMATE CHANGE IN THE SACRAMENTO RIVER BASIN, CALIFORNIA

Abstract

Hydroclimate change that alters runoff seasonality is an increasing concern in snowmelt-influenced watersheds throughout the western United States. Detecting seasonal runoff change is complicated by the interaction of atmospheric and topographic influences and by human alteration of streamflow. Reconstructed or unimpaired discharges for 1921 to 1994 for the Sacramento River are employed as an indicator of natural discharge and are analyzed for evidence of seasonal change. Rescaled adjusted partial sums (RAPS) improve visualization of trends and fluctuations in annual discharge and the seasonal fractions of annual discharge. The statistical significance of trends indicated by the RAPS is evaluated using nonparametric tests. Small increasing trends beginning in the late 1940s in annual unimpaired discharges and the seasonal fractions for fall, winter, and summer are not statistically significant. In contrast, the spring fraction of annual discharges shows a decreasing trend beginning in the late 1940s that is statistically significant. The decline in spring fraction discharges when annual discharges are increasing is consistent with a greater proportion of annual precipitation in the watershed occurring as rain rather than snow. Reduced spring discharges will impose new limits on water-management alternatives for maximizing use of the water supply. [Key words: Sacramento River Basin, hydroclimatology, precipitation, stream discharge, seasonal changes.]