Seasonal hydroclimate change in the Sacramento River Basin, California

Hydroclimate change that alters runoff seasonality is an increasing concern in snowmelt-influenced watersheds throughout the western United States. De tecting seasonal runoff change is complicated by the interaction of atmosph eric and topographic influences and by human alteration of streamflow. Reco nstructed or unimpaired discharges for 1921 to 1994 for the Sacramento Rive r are employed as an indicator of natural discharge and are analyzed for ev idence of seasonal change. Rescaled adjusted partial sums (RAPS) improve vi sualization of trends and fluctuations in annual discharge and the seasonal fractions of annual discharge. The statistical significance of trends indi cated 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 signi ficant. In contrast, the spring fraction of annual discharges shows a decre asing trend beginning in the late 1940s that is statistically significant. The decline in spring fraction discharges when annual discharges are increa sing is consistent with a greater proportion of annual precipitation in the watershed occurring as rain rather than snow. Reduced spring discharges wi ll impose new limits on water-management alternatives for maximizing use of the water supply.