Using GEFS ensemble forecasts for decision making in reservoir management in California

Abstract:

Reservoirs such as Lake Mendocino in California's Russian River Basin provide flood control, water supply, recreation, and environmental stream flow regulation. Many of these reservoirs are operated by the U.S. Army Corps of Engineers (Corps) according to water control manuals that specify elevations for an upper volume of reservoir storage that must be kept available for capturing storm runoff and reducing flood risk, and a lower volume of storage that may be used for water supply. During extreme rainfall events, runoff is captured by these reservoirs and released as quickly as possible to create flood storage space for another potential storm. These flood control manuals are based on typical historical weather patterns - wet during the winter, dry otherwise - but are not informed directly by weather prediction. Alternative reservoir management approaches such as Forecast-Informed Reservoir Operations (FIRO), which seek to incorporate advances in weather prediction, are currently being explored as means to improve water supply availability while maintaining flood risk reduction and providing additional ecosystem benefits.

We present results from a FIRO proof-of-concept study investigating the reliability of post-processed GEFS ensemble forecasts to predict the probability that day 6-to-10 precipitation accumulations in certain areas in California exceed a high threshold. Our results suggest that reliable forecast guidance can be provided, and the resulting probabilities could be used to inform decisions to release or hold water in the reservoirs. We illustrate the potential of these forecasts in a case study of extreme event probabilities for the Russian River Basin in California.