The Role of Forecasts in Managing Endangered Salmon in an Engineered River Basin

The recent intensification of drought in California's Sacramento River basin has highlighted the importance of seasonal hydroclimatic forecasts for management of the region's interconnected water resources infrastructure to meet irrigation demands and protect spawning conditions for endangered Winter Run Chinook salmon. Cold river temperatures, regulated through selective withdrawal from the seasonally-stratified Shasta Reservoir upstream of current spawning habitat, are required to maintain salmon egg viability throughout their summer incubation period. Advance planning to prevent exceeding certain levels of temperature-dependent egg mortality (TDM) requires a suite of forecasts: reservoir inflow, agricultural demand, meteorology, reservoir and conveyance facility operations, and salmon spawning distribution. Each of these has considerable uncertainty, especially at the 6-month lead time required for early decision-making. Yet the relative importance of errors in each forecast in affecting projected TDM outcomes is not well-characterized. To address this, we present a retrospective analysis of the 2021 Sacramento River temperature management season to evaluate the sensitivity of seasonal TDM to each forecast component. The analysis considers original forecasts of reservoir inflow, meteorology, spawning distribution, and reservoir operation at monthly increments throughout the spring planning period along with a hindcast of actual realized conditions. Outcomes are evaluated through the use of a reservoir-river-egg incubation simulation framework for the upper Sacramento River system. Results will help inform and direct efforts to improve forecasting methods for better advanced decision-making in drought years.