Pii: S1366-7017(98)00023-3

A case study of the Sacramento Basin is undertaken to examine problems of planning for the impacts of climate change on water resources utilizing output from general circulation climate models. This work provides a sensitivity analysis linking climate, stream ̄ow, and planning decisions. Water resource planners want reliable time series of stream ̄ow in the basin. Sacramento basin hydrological models do well at simulating monthly stream ̄ow time series from the observed station precipitation and temperature data. However, climate models poorly simulate the large scale processes and features that are ultimately important in determining precipitation and stream ̄ow in the Sacramento Basin region. Observational data for the Sacramento Basin indicates that basin stream ̄ow is sensitive to precipitation changes of the size indicated by GCM 2 CO2/1 CO2 studies. We devised a broad range of plausible scenarios for basin stream ̄ow changes and carried out a qualitative examination of the consequences of planning on any one of the scenarios should any of the di€erent scenarios actually come to pass. For the problem of maintaining basin water deliveries, some of the possible long range planning decisions in the Sacramento Basin depend on choice of climate scenario. This dependance will be in ̄uenced by demand changes and normative decision criteria. Thus, basin stream ̄ow is sensitive to uncertainty in the range of GCM simulated basin precipitation, and long range supply planning decisions for the basin are potentially sensitive to the choice of stream ̄ow scenario; contingent upon the range of scenarios assessed and on normative decisions related to risk aversion and the evaluation of impacts. This contingent result runs counter to recent quantitative climate impact studies that have adopted the convenient ®ction that current GCM regional climate scenarios provide comprehensive estimates of potential regional changes. The challenge is to ®nd robust planning options that smooth out the sensitivity to a wide variety of di€erent climate realizations. # 1998 Elsevier Science Ltd. All rights reserved. Water Policy 1 (1998) 321±34