Effects of Multidecadal Changes in Temperature, Snowpack and Precipitation on Streamflow in Southwestern Rivers

We diagnose the effects of multidecadal changes in temperature, snowpack, and precipitation on the interannual variability of streamflow in southwestern snow-fed rivers. Consistent with many previous studies, we find that snowmelt runoff is shifting toward earlier peak flows as spring season temperatures rise. Many studies conclude that continued decreases in snowpack (primarily driven by long-term temperature change) should generate reduced melt season streamflow, but this effect is confounded in observations by variations in precipitation that are consistent with natural fluctuations. Hence the rivers under study here show no significant secular trends in annual runoff. Total melt season streamflow depends on runoff from cold season snowpack, and on decadally varying precipitation. We use a combination of dynamical model simulations and simple statistical models to attempt to disentangle the effects of temperature and precipitation changes on streamflows.

The results allow us to interpret observed changes in the skill of operational regression-based seasonal forecast models for streamflow in southwestern rivers. Clearer interpretation of these forecasts is important for stakeholders who make water allocation decisions early in the water year based on expected streamflows.