Synthetic Forecast Ensembles for Evaluating Forecast Informed Reservoir Operations

Forecast informed reservoir operations (FIRO) is an important advance in water management, but the design and testing of FIRO policies is limited by relatively short (10-35 year) hydro-meteorological hindcasts. We present a novel, multisite model for synthetic forecast ensembles to overcome this limitation. This model utilizes parametric and non-parametric procedures to capture complex forecast errors and maintain correlation between variables, lead times, locations, and ensemble members. After being fit to data from the hindcast period, this model can generate synthetic forecast ensembles in any period with observations. We demonstrate the approach in a case study of the FIRO-based Ensemble Forecast Operations (EFO) control policy for the Lake Mendocino-Russian River basin, which conditions release decisions on ensemble forecasts from the Hydrologic Ensemble Forecast System (HEFS). We explore two generation strategies: (a) simulation of synthetic forecasts of meteorology to force HEFS; and (b) simulation of synthetic HEFS streamflow forecasts directly. We evaluate the synthetic forecasts using ensemble verification techniques and event-based validation, finding good agreement with the actual ensemble forecasts. We then evaluate EFO policy performance using synthetic and actual forecasts over the hindcast period (1985-2010) and synthetic forecasts only over the pre-hindcast period (1948-1984). Results show that the synthetic forecasts highlight important failure modes of the EFO policy under plausible forecast ensembles, but improvements are still needed to fully capture FIRO policy behavior under the actual forecast ensembles. Overall, the methodology advances a novel way to test FIRO policy robustness, which is key to building institutional support for FIRO. We develop a method to produce synthetic hydrologic ensemble forecasts based on hindcast ensembles Synthetic and actual forecast ensembles exhibit good agreement based on ensemble forecast verification statistics Synthetic forecasts can extend robustness testing of forecast informed reservoir operations to better estimate out-of-sample performance