Evaluation of artificial recharge in the Mojave River Ground-Water Basin, California

Abstract

The Mojave River Basin relies almost entirely on ground water to meet the needs of its growing population and agriculture, which has resulted in overdraft conditions. Some of the ground-water management alternatives being proposed to mitigate the effects of overdraft include artificial recharge using water from the California State Water Project (SWP) and using SWP water in lieu of ground-water pumpage. A calibrated ground-water flow model was used to evaluate six proposed water-management alternatives using SWP water during a 20-year simulation period, 2000-2019, using constant rates from 1999 for recharge and pumpage (with the exception of recharge derived from Mojave River streamflows which were variable). The measured streamflow for the period of 1970-1989 was used to simulate the Mojave River streamflow. Water-management alternative 1 assumed that none of the Mojave Water Agency allocation of SWP water was available for mitigation measures and resulted in increases in hydraulic head in the floodplain aquifer in years of above-average streamflow (2008-2010, 2013) and decreases in years of below average streamflow. In general, simulated hydraulic heads in the regional aquifer declined with the exception of the El Mirage and Harper Lake areas. Also, average storage depletion for the entire ground-water basin over the 20-year simulation was 40,940 acre-feet per year. Water-management alternative 2 assumed that 30,000 acre-feet per year of SWP water was artificially recharged at Rock Springs Road Outlet (RSO). By 2019, the simulated hydraulic heads were as much as 75 feet higher in the Alto at the recharge site, 24 feet higher in the Transition zone, 15 feet higher in the Centro, and 17 feet higher in the Baja model subareas than the hydraulic heads resulting from water-management alternative 1. Water-management alternative 2 affected simulated hydraulic heads by as much as 5 feet in an area totalling 290 square miles; most of the change occurred in the Alto and Baja model subareas. Average storage depletion for water-management alternative 2 for the entire ground-water basin for the 20-year simulation period was 15,880 acre-feet per year, 25,060 acre-feet per year less than water-management alternative 1. Also, water-management alternative 2 indicated that the artificial recharge at RSO resulted in less simulated ground-water recharge from stream leakage in the Alto model subarea, which led to greater streamflow at the Lower Narrows, Barstow, and Afton Canyon streamflow gages. This increased streamflow resulted in an increase in simulated ground-water recharge from stream leakage, primarily in the Centro and Baja model subareas. Water-management alternative 3 assumed that 4,000 acre-feet per year of SWP water was artificially recharged at Manzanita and Oro Grande Washes (a total of 8,000 acre-feet per year) in the Alto model subarea. By 2019, the simulated hydraulic heads beneath the recharge sites were as much as 278 feet higher than heads resulting from water-management alternative 1. Changes in simulated hydraulic head greater than 5 feet covered almost 138 square miles in the Alto model subarea. Water-management alternative 3 had little effect on simulated hydraulic heads in the other model subareas. Model results indicated that the average storage depletion for the entire ground-water basin during the 20-year simulation was 32,940 acre-feet per year, about 8,000 acre-feet per year less than water-management alternative 1. Water-management alternative 3 had essentially no effect on simulated streamflows during the 20-year simulation period and, therefore, little effect on simulated net stream leakage. Water-management alternative 4 assumed 10,000 acre-feet per year of SWP water was artificially recharged near Newberry Springs in the Baja subarea. By 2019, the simulated hydraulic heads beneath the recharge site were as much as 193 ft higher in the Baja model subarea than the hydraulic heads resulting from water-management alternative 1. Increases in simulated hydraulic heads greater than 5 feet extended about 5 miles east of the Calico-Newberry Fault and affected about 71 square miles east of the Calico-Newberry Fault. Water-management alternative 4 had no effect on simulated hydraulic heads in the other model subareas. Model results indicated that the average storage depletion for the entire ground-water basin over the 20-year simulation period was 30,860 acre-feet per year, about 10,000 acre-feet per year less than water-management 1. Water-management alternative 4 had little effect on simulated streamflows during the 20-year simulation period and, therefore, essentially no effect on simulated net stream leakage. Water-management alternative 5 assumed 23,800 acre-feet per year of SWP water was delivered directly to municipal water districts in lieu of pumpage in the Alto model subarea. By 2019, the simulated hydraulic heads were as much as 98 feet higher in the Alto and 7 feet higher in the Centro and Baja model subareas than hydraulic heads resulting from water-management alternative 1. Changes in simulated hydraulic head were greater than 5 feet in an area of almost 245 square miles. Most of the change in simulated hydraulic heads occurred in the regional aquifer west of the Mojave River in the Alto model subarea. Average storage depletion for the entire ground-water basin over the 20-year simulation period was 19,170 acre-feet per year, 21,770 acre-feet per year less than water-management alternative 1. Results from water-management alternative 5 were similar to those of water-management alternative 2 in that they indicated that the in lieu replacement of ground water resulted in less simulated ground-water recharge from stream leakage in the Alto model subarea, and thereby streamflow at the Lower Narrows and Barstow gages. This increased streamflow resulted in an increase in simulated ground-water recharge from stream leakage primarily in the Baja model subarea. Water-management alternative 6 assumed 3,800 acre-feet per year of SWP water was delivered directly to municipal water districts in lieu of pumpage in the Transition zone model subarea. By 2019, the simulated hydraulic heads were as much as 30 feet higher in the Transition zone model subarea than the hydraulic heads resulting from water-management alternative 1. Changes in simulated hydraulic head were greater than 5 feet in an area of almost 16 square miles in the Transition zone model subarea. Water-management alternative 6 had little effect on simulated hydraulic heads in the other model subareas. Average storage depletion for the entire ground-water basin over the 20-year simulation period was 38,090 acre-feet per year, 2,850 acre-feet per year less than water-management alternative 1. Also, simulation of water-management alternative 6 indicated in-lieu replacement of ground water resulted in less ground-water recharge from stream leakage in the Alto, Transition zone, and Centro model subareas, and thereby greater streamflow at the Barstow gage. This increased streamflow resulted in an increase in simulated ground-water recharge from stream leakage, primarily in the Baja model subarea.