The relationship between snowpack and seasonal low flows in the Sierra Nevada: climate change and water availability in California

Seasonal low flows are important for sustaining aquatic ecosystems, and for supplying human needs during mid-summer. When the timing of water supply and demand do not coincide, humans rely on both natural and artificial storage. In California, the gap in timing between supply and demand is bridged primarily by the Sierra Nevada snowpack, which slowly melts throughout the spring and summer. However, most future climate scenarios suggest a decreased snowpack in the Sierra. Previous studies have investigated changes in snowmelt timing and spring snowmelt flood events. Here, by contrast, we explore how changes in the Sierra Nevada snowpack will affect annual low flows. We have identified all of the gauged catchments in the Sierra Nevada with unimpaired streamflow records and with at least ten years of overlapping snowpack and streamflow data. In each of these catchments, we have analyzed up to 40 years of historical snow and streamflow records. We find that annual minimum, mean, and maximum flows in these catchments all increase and decrease proportionally, or more-than-proportionally, as the annual peak snowpack water content changes from year to year. For every 10% decrease in snowpack, there is a 9-17% decrease in annual minimum flow. Minimum flows also occur earlier in years with smaller snowpacks; for every 10% decrease in snowpack, minimum flows occur 3-7 days earlier in the year. Finally, we find that in some catchments, annual low flows are significantly correlated not only with that year's snowpack, but with the previous year's snowpack as well. That is, seasonal low flows in some Sierra Nevada catchments exhibit a multi-year "memory" of snowmelt water inputs. We evaluate possible mechanisms that might underlie this observed memory effect. If these observed relationships between snow and flow hold in the future climate regime, the projected decrease in snowpack is likely to have a severe effect on seasonal low flows.