U.S. DEPARTMENT OF THE INTERIOR U.S GEOLOGICAL SURVEY Assessment of Folsom Lake Watershed Response to Historical and Potential Future Climate Scenarios

An integrated forecast-control system was designed to allow the profitable use of ensemble forecasts for the operational management of multi-purpose reservoirs. The system ingests large-scale climate model monthly precipitation through the adjustment of the marginal distribution of reservoir-catchment precipitation to reflect occurrence of monthly climate precipitation amounts in the extreme terciles of their distribution. Generation of ensemble reservoir inflow forecasts is then accomplished with due account for atmosphericforcing and hydrologic-model uncertainties. These ensemble forecasts are ingested by the decision component of the integrated system, which generates noninferior trade-off surfaces and, given management preferences, estimates of reservoir-management benefits over given periods. In collaboration with the Bureau of Reclamation and the California Nevada River Forecast Center, the integrated system is applied to Folsom Lake in California to evaluate the benefits 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 (constant future greenhouse-gas concentrations assumed at the present levels) and a greenhouse-gasincrease (1-% per annum increase assumed) scenario. The present paper presents and validates ensemble 30-day reservoirinflow forecasts under a variety of situations. Corresponding reservoir management results are presented in Yao and Georgakakos, A., this issue. Principle conclusions of this paper are that the integrated system provides reliable ensemble inflow volume forecasts at the 5-% confidence level 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, for future periods with potential sharp climatic increases of precipitation amount and to maintain good reliability levels, operational ensemble inflow forecasting should involve atmospheric forcing from appropriate climatic periods. _________________________ Hydrologic Research Center, 12780 High Bluff Dr., Suite 250, San Diego, CA 92130 Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0224