Climate change is likely to have important influences on management strategies for conjunctive use of surface water and groundwater. In all watersheds, there is a single resource, composed of inextricably connected precipitation, surface water and groundwater. For groundwater, climate-related effects may include significant changes in recharge, discharge, and groundwater withdrawals.
Historically, operational decision making and longer term planning of water resources have not considered climatic influences on seasonal to interannual to multi-decadal time scales. Insights into effects of climate variability and climate change on water-resource management can be provided by observationally informed modeling and conjunctive-use analyses. In the Central Valley of California, the demand for water resources for people and agriculture competes with environmental needs, such as maintaining minimum streamflows, preventing seawater intrusion into the Sacramento-San Joaquin Delta, and preserving habitats for fish and birds. Sustainable development of this complex system requires an integrated water-management approach; a suite of linked models is being developed to support that approach.
A method is needed to assess how climate change could affect surface water and groundwater use in highly developed agrourban watersheds. An emerging approach to providing this method is holistic modeling with conjunctive use analysis using linked and physically based hydrologic models that combine the natural and human components of use and movement of water.
A scenario based on a projection of 21st Century climate from the GFDL CM2.1 (Geophysical Fluid Dynamics Lab Climate Model 2.1) global climate model (GCM) responding to assumptions of rapidly increasing greenhouse-gas emissions (SRES A2), has been used to drive mountain-watershed, agricultural, native vegetation and urban water-use, and groundwater flow models of the integrated Central Valley, This scenario, which has been selected from the range of plausible possibilities for initial analysis, characterizes California's climate as becoming quite warm (+2 to+4°C) and substantially drier (10-15%) during the mid- to late-21st Century, relative to historical conditions. This climate scenario is one of the warmest and driest over California, from among the wider range of possibilities projected by currently available climate models. The suite of hydrologic models used to quantify the hydrologic effects of this climate scenario includes a model of runoff and recharge from the watersheds of the Sierra Nevada Mountains, and a model of agricultural water-deliveries and use in the Central Valley. These models have been shown to perform well in replicating historical hydrologic observations in the Sierra/Central Valley region.
This supply and demand modeling framework provides a method to evaluate a suite of linked models as part of the sort of decision support system that will be required for the analysis of conjunctive use in regional flow systems throughout the world.
The following figures provide more detail about the Global Climate Model's architecture.
Model simulations indicate declines in streamflow (up to 40%) that lead to reduced surface-water deliveries for agriculture and riparian habitat.
Model results also indicate that, under this scenario, increased demands for irrigation water would be met by increased groundwater pumpage. In turn, increased pumpage contributes to increased streamflow infiltration, reduced base flow, reduced groundwater outflows to the Delta, increased depths to groundwater, and land subsidence. Overall model results for this scenario indicate a likely change of the Central Valley from a surface-water to groundwater-dominated system.
The following figures compare historical data with future projections for a variety of environmental factors, such as precipiation, temperature, streamflow, hydrologic response, and groundwater levels.
This project demonstrates the importance of USGS integrated observations and models for evaluating vulnerabililties in the Central Valley system and for exploring potential trends and management responses. An expanded set of simulations is being designed that will include other climate and growth scenarios. This broader investigation should provide a basis for decision support for planning and testing of adaptation strategies in the supply-constrained and demand-driven Central Valley.