A Ground Water Adaptive Management Tool (GAMT) to Facilitate Conjunctive Use Planning in Yuba County, California

To adapt to its large natural range in climate variability, the State of California relies in part on flexible annual water transfers from local water agencies to meet the demands of users across the State as well as fulfill environmental in-stream flow requirements.  Since 1991, Yuba County Water Agency (YCWA) of northern California, in cooperation with its Member Units, has participated five times in large (>276 TAF total) transfers to help meet out-of-agency demands.  To meet local needs, transferred surface water has then been replaced by pumped groundwater as part of YCWA’s conjunctive use program.  Because no groundwater model exists, effects of such substitution transfer pumping on the Yuba groundwater basin have only been quantified after the fact, despite potential local third-party impacts.  Additionally, as YCWA increases the frequency of groundwater substitution transfers to meet Yuba River instream flow obligations under a 2008 water right settlement, the need to ensure the long-term sustainability of the groundwater basin becomes crucial. In order to quantify the groundwater basin response to and recovery from future substitution transfer pumping, YCWA has developed a Groundwater Adaptive Management Tool (GAMT).  Based on historical groundwater level data across the Yuba basin, the GAMT is a regression-based spreadsheet tool that can be used as a predictor of basin behavior to plan for future groundwater transfers.  Furthermore, with its easy-to-use graphical interface, the tool transparently communicates complex data and interpretations, facilitating a decision-making process that involves users of varying technical skills.  This new forward-looking analysis allows YCWA to improve its ability to deliver highly reliable supplies to its Member Units, help meet statewide needs, and expand its conjunctive use program to address future supply challenges due to additional regulatory constraints and climate change effects.