Conjunctive Groundwater and Surface Water Use in Southern Tulare County
Nels Ruud, Alec Naugle
United States Bureau of Reclamation
University of California Division of Agriculture and Natural Resources (UC ANR)
Changes in the management of the Central Valley Project and the effects of future droughts could significantly impact groundwater levels and availability in California's San Joaquin Valley. Under the Central Valley Project Improvement Act and California state regulations, local groundwater management agencies must assess long-term impacts of groundwater use and prepare water conservation plans. An improved understanding of the connection between groundwater resources and surface water supply in irrigated agricultural areas located in the San Joaquin Valley is critical for the development of effective long-term water conservation programs.
As a pilot project, we developed a conjunctive use groundwater-surface water model of the Tule River basin in the eastern-central San Joaquin Valley. The basin area is approximately 1500 square-miles and encompasses 24 federal irrigation districts of differing size and hydrologic conditions. Interactions between surface water, groundwater, and the vadose zone are being modeled to evaluate the relationship between surface water supply and groundwater levels. Areal recharge and groundwater pumpage are computed for individual crop-field (see figure above) to account for spatial variability in landuse and water demand. MODFLOW is used to model stream seepage and groundwater flow. A comprehensive, user-friendly basin-scale hydrogeologic database for the Tule River basin, including the input and output data relevant to the model, is developed using a Geographic Information System (GIS). The integrated conjunctive use model consists of the GIS database, surface water model, vadose zone model, and groundwater flow model. The information from the GIS database was synthesized for MODFLOW using Argus ONE, a dynamic GIS for modeling, and a plug-in extension developed by the USGS (MODFLOW GUI PIE) for Argus ONE. Conjunctive use management options were tested to assess impacts on irrigation districts based on their size, hydrologic conditions, and water rights situation.
Potential users of the conjunctive use model will include federal and state agencies, county planning agencies, and irrigation districts. Due to the wide variety of users and the broad spectrum of potential applications, a central tenet of the the project is to provide data management in this GIS format. Building the database in a standard GIS format (e.g., ArcView) allows users to easily access and manipulate data, and to incorporate data into a broad spectrum of applications. Also, the visualization and manipulation capabilities of a GIS allow us to interact directly with county and district personnel, thereby improving the database and the calibration of the conjunctive use model by the inclusion of their anecdotal expertise. This anecdotal expertise is considered crucial because little documented information exists on the distribution of surface water within irrigation districts or on the areal distribution of agricultural groundwater pumpage, which comprises up to 95% of the total pumping in the model area and is often estimated based on crop consumptive use, irrigation practices, irrigation efficiencies, and surface water availability.
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