KEYNOTE: LINKING GCMS TO REGIONAL HYDROLOGIC MODELS TO ASSESS POTENTIAL FUTURE CONJUNCTIVE USE

Climate change and climate variability play an important role in successful conjunctive management of surface-water and groundwater resources. Analysis of conjunctive use and related adaption require an ensemble of linked models that represent the variations in climate, and related movement and use of the water throughout the landscape, and the availability of surface-water and groundwater components. The linkage uses output from Global Climate Models (GCMs) to drive inflows and outflows of regional hydrologic models of the surrounding watersheds and regional-aquifer systems. This framework provides the foundation to simulate and analyze a supply-constrained and demand-driven hydrologic cycle at the watershed scale. The conjunctive use of natural and anthropogenic components are simulated in a holistic context that allows for the assessment of climate change and ancillary demands, such as urbanization, as well as adaptation. This physically-based process approach yields additional insight that may compliment more traditional ensemble, perturbation, or probabilistic approaches to climate change analysis.

This approach is demonstrated with the application and analysis of the Central Valley, California using the long-term climate change with the GFDL-A2 "business as usual" scenario and 1.2 percent of urban growth. The potential of intermittent to severe persistent droughts simulated with climate change and increased urbanization do not trigger a regional "operational drought" where demand exceeds the physical engineered supply capacity. However these changes result in a transition of conjunctive use to a groundwater dominated agricultural water supply. These changes also include increased streamflow infiltration, captured recharge, and reduced surface waters used for maintaining riparian habitat. The modeling results suggest that potential limiting factors for conjunctive use in the Central Valley may be secondary effects, such as increased land subsidence, reduced water for riparian habitat, or changes in flows at the Delta. This example shows that global-scale climate variations can be translated into realistic local hydrologic responses that can provide a foundation for a Decision Support System for operational, project, and policy planning of conjunctive use.