On the use of climate models to assess the impacts of regional climate change on water resources

We critically analyse some of the major assumptions typically employed in climate model-based impacts studies of water resources. In particular, climate impacts studies have not considered whether current climate models are capable of simulating the factors important for determining regional hydrology. We set up a case study of the Sacramento Basin to test these assumptions. The Sacramento Basin has been the subject of a number of climate impacts studies. By comparing climate model simulations of the current climate with observational data, we show that there are critical shortcomings in the climate models' simulations of the synoptic-climatological processes important for Sacramento Basin precipitation. The stationary waves, jet streams, and storm tracks simulated in the climate models show significant differences from their observational counterparts in the climate mean, and respond differently in wet and dry winters and for individual storms than the observations. On the scale of the Sacramento Basin, the climate model precipitation is too weak and too frequent relative to observations, and the models smear out the sharp spatial precipitation gradients observed along the west coast. The smearing effect is substantially reduced for a climate model run at high horizontal resolution. However, the model defiencies in the temporal distribution of precipitation, and in the simulation of large scale circulation features persist at high resolution. The use of time series of observed ocean boundary conditions also does little by itself to improve model deficiencies. Deficiencies in the model large scale circulation features are related to the model subgrid parameterizations. Historical data indicates that streamflow amount in the Sacramento Basin is sensitive to precipitation, while the timing of streamflow is sensitive to temperature. During particularly wet years in the basin, streamflow increases proportionally more than precipitation. During particularly dry years the streamflow is more linearly related to precipitation. However, after runs of dry years the streamflow response to precipitation is diminished relative to other dry years. Since streamflow is nonlinearly related to precipitation, the amount of basin streamflow implied for a model's doubled CO 2 basin precipitation simulation depends on how much basin precipitation is simulated for the current climate. For water resource planning, the GCM basin scenarios are probably no more uncertain than the use of historical data to plan for the future. For decisions involving long range investment in infrastructure to maintain existing water supplies in the basin, the choice of climate scenario is critical in determining the array of costs and benefits associated with any climate change. For decisions involving crop management policy in the basin, the time scales for implementation and operation are short relative to the likely rate of climate change, and the choice of climate scenario is less critical. Thesis Supervisor: Peter H. Stone Professor of Meteorology This thesis is dedicated to the memory of my grandfather