Assessment of Folsom lake response to historical and potential future climate scenarios: 1. Forecasting

In collaboration with operational forecast and management Agencies, an integrated forecast-control system is designed and applied to a major reservoir in California to evaluate the potential benefits of climate information for flood control, hydroelectric energy production, and low flow augmentation. In addition to retrospective studies involving the historical period 1964–1993, system simulations were performed for the future period 2001–2030, under a control and a 1% greenhouse-gas-increase scenario. This paper presents the forecast component formulation and validates ensemble 30-day reservoir-inflow forecasts under a variety of situations. The control component formulation and corresponding reservoir management results are presented in Yao and Georgakakos, this issue. The forecast component is based on ensemble flow forecasting. Quantiles of the distribution of climate-model precipitation simulations are used to select catchment-scale historical daily precipitation time series for the generation of an ensemble of daily reservoir-inflow by hydrologic models. Ensemble generation takes into consideration both atmospheric-forcing and hydrologic-model uncertainties. Principal conclusions of this paper are that the integrated system provides reliable ensemble inflow volume forecasts for the majority of the deciles of forecast frequency, and that the use of climate model simulations is beneficial mainly during high flow periods. It is also found that to maintain reliability for future climate periods, generation of ensemble inflow forecasts should use input time series that reflect potential sharp changes in precipitation amount.