Observed Sacramento Basin streamflow response to precipitation and temperature changes and its relevance to climate impact studies

Observational studies of Sacramento Basin annual mean streamflow response to precipitation and temperature indicate that streamflow amounts in the basin are strongly sensitive to precipitation, but virtually insensitive to mean seasonal temperature. This result is in accord with studies of the climate mean sensitivity of streamflow amount to changes in climatological mean precipitation and temperature as simulated by conceptual hydrological models of the basin. Simpler regression models of the Sacramento Basin show a strong dependence of streamflow amount on temperature, which is not evident in the observation-based annual mean streamflow sensitivity. The interannual variability in Sacramento Basin streamflow response to precipitation exhibits substantial nonlinearity in that the partitioning of precipitation to runoff strongly depends on the precipitation volume. During very wet years in the basin, streamflow response to precipitation exhibits a proportionally greater change (increase) than during normal or dry years. For most dry years in the basin the streamflow response to precipitation is fairly linear. However, on the tails of droughts the streamflow response to precipitation is diminished relative to other dry years. This implies that a shift to a much drier or more drought-prone climate would result in a more substantial reduction in streamflow amounts than might at present be expected from observations of streamflow responses in isolated dry years. This conclusion assumes no changes in the features controlling water retention in the basin, which is an important issue to be addressed in basin climate change studies.