OVEREXPLOITATION OF WATER RESOURCES FOR IRRIGATED AGRICULTURE: CASE STUDIES IN THE US HIGH PLAINS AND CALIFORNIA CENTRAL VALLEY

Irrigated agriculture is the primary consumer of water resources accounting for ~ 70% of total fresh water consumption in the US. Using satellite and ground-based data and modeling analyses from the High Plains and Central Valley, we evaluate the impacts of irrigation on water resources and assess strategies towards more sustainable management. In the High Plains, high correlations between GRACE (Gravity Recovery and Climate Experiment) total water storage changes and ground-based measurements of soil moisture and groundwater levels (r=0.88) show the strong potential for GRACE to track changes in water storage in these systems. In the Central Valley, recent declines in GRACE water storage are attributed to reductions in groundwater storage in response to drought. Effects of irrigation on groundwater resources are greatest in the southern part of the High Plains and Central Valley, reflecting the spatial disconnect between water supply in the north and water demand in the south. Groundwater declines ≥30 m have been recorded over 10,000 km² in the southern High Plains. Low groundwater recharge (≤ 10 mm/yr) in much of the central and southern High Plains means that irrigation cannot be practiced sustainably and abstractions represent mining of an essentially nonrenewable resource. In the Central Valley, groundwater declines up to 120 m are generally restricted to the Tulare Basin in the south. Approaches towards more sustainable water resources require increasing supplies through managed aquifer recharge or reducing demands through more efficient irrigation, deficit irrigation, or conversion to rainfed agriculture. Conjunctive use of surface water and groundwater and managed aquifer recharge have been practiced in the Central Valley for decades and are increasing in recent times. Deficit irrigation is practiced in the southern High Plains but is leading to incipient soil salinization because of insufficient water to flush accumulating salts. Traditional irrigation practices are coming under increasing pressure from increased variability in supplies related to climate change and competing demands for environmental flows and urban water; therefore, approaches towards more sustainable management practices are essential.