GROUND-WATER AVAILABILITY IN CALIFORNIA'S CENTRAL VALLEY

For more than 50 years, California's Central Valley has been one of the most productive agricultural regions in the world. Large increases in population have increased the competition for water within the Central Valley and statewide. As a result, an assessment and quantification of the ground-water resources within California's Central Valley is being undertaken. The first step in this analysis is developing a numerical model that can be used to quantitatively address ground-water issues in the Central Valley. The modeling effort consists of three major tasks. The first task, “texture modeling,” develops a better understanding of the internal architecture of the deposits in the Central Valley. The second task, the “Farm Process”, develops a systematic approach for estimating water-budget components, which in this study are based upon the consumptive use of water by plants and available surface-water deliveries. The third task, “ground-water modeling,” develops a model at a scale relevant to management decisions, including water availability issues. A coupled farm-process and ground-water model is being used for water-balance regions with in the Central Valley, to estimate un-metered historical pumpage and to simulate the delivery of surface water since the 1960s. The simulated deliveries and ground-water pumpage reflect differences among the water-balance regions. Overall, surface-water deliveries supply most of the consumption in the initial part of the growing season, whereas increased ground-water pumpage augments these supplies later in the season. Throughout the last 50 years, the simulated relative proportions of surface water and ground water used for irrigation also vary from year to year in response to climate. Hence, the model is a useful tool for assessing ground-water availability and sustainable management of both ground water and surface water.