Preface

While conjunctive use of surface and groundwater supplies is occurring in Yolo County, the coordinated and integrated management of these two supply sources within the County has not yet happened. This report is an investigation of conjunctive use management opportunities to improve the overall yield and sustainability of Yolo County's water supply system. Groundwater under Yolo County is the single most important supply source and provides 45 percent of water demands under average supply conditions. In severe droughts, groundwater can be expected to supply over 75 percent of the County's demands. By conjunctively managing the County's surface supplies with the management of the groundwater basin, specific groundwater overdraft-related problems could be addressed, the continued availability of groundwater as the primary urban water supply could be assured, and the reliability and role of groundwater supplies during droughts for agricultural and urban users could be enhanced. This study describes ways that these benefits to the County might be realized and suggests various concrete conjunctive use schemes. Furthermore, the potential roles of water transfers, both within the County and with agents outside the County, have been explored in the context of a County-wide conjunctive use planning and management framework.

Chapter 1 provides an introduction to this study. The County's existing water supply situation is described in Chapter 2 to establish the setting for the consideration of conjunctive use management. The present balance in the County between agricultural and urban/domestic water demands, and the matching of surface and groundwater supplies to the uses, are reported from estimates for 1990 water demands and average and drought year water supply conditions. Chapter 3 provides details of the spatial and time distributions of groundwater pumping, with attention to the impacts of droughts and water transfer. Implications for the sustained yield and the occurrence of overdraft conditions are made. Chapter 4 describes conjunctive use management concepts as they apply to Yolo County's water situation and presents various alternative projects. Constraints to implementing such types of County-wide conjunctively integrated water use strategies are reviewed in Chapter 5 and their implications for planning are presented. Conclusions are presented in Chapter 6 along with some recommendations for initiating a planning process to realize potential benefits from conjunctive use management of the County's water supply. Data, assumptions and calculations for this investigation are reported in the Appendices. This study was conducted by the author in 1991 under the direction of Jay R. Lund and William K. Johnson of the Department of Civil and Environmental Engineering at University of California, Davis.