Characterizing changes in streamflow under historical and current climates for the Russian River, California

Abstract:

Precipitation in California is naturally more variable than elsewhere in the United States, and climate change is expected to increase the frequency and severity of precipitation and streamflow anomalies. As part of a larger effort to assess flow conditions under historical, current, and future climates, we characterized the change in the Russian River’s mainstem flows between two 30-year periods that represent historical (1951 to 1980) and current (1981 to 2010) climate conditions. Analyses included measured data from one mainstem gage (Ukiah) that represents natural flow conditions, and three mainstem gages (Hopland, Healdsburg, and Guerneville) regulated by diversion into the Russian River from the adjacent Eel River and by reservoir storage. Analysis of natural flows at the Ukiah gage under the current climate indicates statistically significant increases in low flow metrics that include: median monthly flows from July to October; number of zero flow days; and 1-, 3-, 5-, 7-, 30- and 90-day minimum flows. In contrast to the Ukiah gage, decreases in low flows under the current climate at the three regulated-streamflow gages varied with distance downstream. Statistically significant declines in median monthly flows occurred during the second period (1981-2010) from August to November at Hopland, September to November at Healdsburg and in October at Guerneville. Although mean annual flow declined at all four gages during the second period and median monthly low flows declined at the downstream gages, median monthly low flows and minimum flows at the Ukiah gage which represents natural flows increased during the driest months (July to October). Results from this study will be used to support ecological studies and water resource planning within the Russian River watershed. The relative importance of climate and watershed response on the quality and quantity of streamflow under historical and current climates will be assessed and results compared to analyses of unimpaired flows estimated using a basin-scale water balance model calibrated to low flow conditions.