Conjunctive use planning of surface water and groundwater in Southern California, USA

摘要

Southern California is one of the major metropolitan centers of the United States of America, over 18 million people. The population and economy were founded upon extensive development of both local groundwater resources and supplemental imported water supplies. Over the years, several groundwater basins in southern California have experienced over-pumping of groundwater. This has caused the decline of water table, land subsidence, and other negative environmental impacts. To recover the water table, surplus imported surface water is being used to recharge the groundwater basin by means of surface ponds and injection wells. By controlling the total water resources of a region, conjunctive use planning of surface water and groundwater increases the efficiency, reliability, and cost-effectiveness of water use, particularly in river basins with spatial or temporal imbalances in water supply and natural supplies.This paper presents the results of a conjunctive use study in southern California. Historically, the town of Yucca Valley located in the southwest part of the Mojave Desert in southern California relies on groundwater pumping from the Warren groundwater basin as its sole source of water supply. This significant dependency has resulted in a large imbalance between groundwater pumping and natural recharge, causing groundwater levels in the basin to decline substantially from the late 1940s to 1994. Consequently, in 1995 an artificial recharge program was proposed and implemented for the purpose of recovering the groundwater levels. However, the rise in groundwater levels due to artificial recharge has entrained septage stored in the unsaturated zone and caused nitrate concentration in the groundwater to increase simultaneously. This papeT develops an optimal pump and recharge strategy for a planned conjunctive use project to remove the high nitrate concentration, while maintaining groundwater levels at desired elevations at specified locations as well as meeting water demand. A simulation model for flow and mass transport is coupled with a management model to determine the optimal pumping and recharge pattern.