Russian River Watershed Hydrograph Separation Using Stable Water Isotopes to Inform Watershed Models and Reservoir Operations

Within the US, reservoirs are most often operated according to a rule curve that specifies target water levels based on the time of year. In California, the rule curve specifies releases of water prior to winter (the rainy season) to maximize flood control, but it may not result in optimal reservoir operations for other authorized uses, such as water supply. Lake Mendocino lies within the Russian River watershed of California’s Coast Ranges and is the first test watershed for Forecast Informed Reservoir Operations (FIRO). This multi-agency initiative is meant to explore the outcomes of a proposed management strategy whereby reservoir managers would incorporate state of the art watershed monitoring, hydrologic modeling, and weather prediction to optimize both water supply operations and flood control of Lake Mendocino and the Russian River. One objective of FIRO is to better understand how tributaries to Lake Mendocino react to high-impact precipitation events, like atmospheric rivers (ARs).

Hydrograph separation using stable water isotopes and natural geochemical tracers distinguishes between the volume of surface runoff (new water from precipitation) and groundwater (old water or baseflow) contributing to streamflow. Correctly representing these distinct stream water sources in the Gridded Surface Subsurface Hydrologic Analysis (GSSHA), a US Army Corps of Engineers (USACE) distributed watershed model, will improve streamflow forecasts and anticipated inputs to Lake Mendocino. In an effort to include these parameters in GSSHA, the Center for Western Weather and Water Extremes (CW3E) measured stream level from October 2017 to May 2018 at six recently installed gaging stations located on tributaries in the headwaters of the Russian River. Rating curves were created to produce continuous discharge hydrographs. Precipitation, stream water, and groundwater samples were collected across several AR events and analyzed for stable water isotopes and anions/cations. Results from these analyses and hydrograph separation generally show a high groundwater contribution to streamflow, even after the system has been primed. Further analyses of these results will focus on source water differences between the AR events and between sample sites, as well as the anticipated impacts to the reservoir system.