Tracking the Impact of Climate Change on Central and Northern California's Spring Snowmelt Subbasin Runoff

A simplified index system was set up at Pacific Gas & Electric Company (PG&E) to evaluate the current impact of climate change and produce a simplified trend forecast of spring snowmelt runoff for relatively small hydroelectric operational subbasins in central and northern California. The need for PG&E to classify and index these relatively small subbasin drainages in terms of runoff impact from climate change became apparent with the need to increase overall awareness of the relatively rapid change in water availability timing since about 1970 and to focus on identifying potential associated business risks. Spring runoff from snowmelt has historically been depended upon for filling the nearly one hundred seasonal reservoirs that PG&E manages. Within any given large watershed, there are considerable differences in the characteristics of contributing subbasin reaches, and those characteristics influence how the subbasin runoff will respond to climate change. Isohyetal maps can sometimes be utilized to identify these areas from averaged precipitation amounts that are often related to topography and elevation differences. During this review, it was found that orographically influenced subbasins were least impacted from the effects of climate change, while those areas that were either in a rain shadow or were behind topographic barriers revealed larger climate change impact in the form of reduced snowpack, spring runoff, and sometimes runoff for the water year.