Nonsustainable groundwater sustaining irrigation: A global assessment

Water used by irrigated crops is obtained from three sources: local precipitation contributing to soil moisture available for root water uptake (i.e., green water), irrigation water taken from rivers, lakes, reservoirs, wetlands, and renewable groundwater (i.e., blue water), and irrigation water abstracted from nonrenewable groundwater and nonlocal water resources. Here we quantify globally the amount of nonrenewable or nonsustainable groundwater abstraction to sustain current irrigation practice. We use the global hydrological model PCR-GLOBWB to simulate gross crop water demand for irrigated crops and available blue and green water to meet this demand. We downscale country statistics of groundwater abstraction by considering the part of net total water demand that cannot be met by surface freshwater. We subsequently confront these with simulated groundwater recharge, including return flow from irrigation to estimate nonrenewable groundwater abstraction. Results show that nonrenewable groundwater abstraction contributes approximately 20% to the global gross irrigation water demand for the year 2000. The contribution of nonrenewable groundwater abstraction to irrigation is largest in India (68 km(3) yr(-1)) followed by Pakistan (35 km(3) yr(-1)) the United States (30 km(3) yr(-1)), Iran (20 km(3) yr(-1)), China (20 km(3) yr(-1)), Mexico (10 km(3) yr(-1)), and Saudi Arabia (10 km(3) yr(-1)). Results also show that globally, this contribution more than tripled from 75 to 234 km(3) yr(-1) over the period 1960-2000.