by Bill Schlesinger
Agriculture is the world’s top user of surface water, and irrigation is increasingly used to extend agriculture to marginal lands and seasonal climates to maintain crop yield. In the U.S. 15% of agricultural land is irrigated, but these areas produce 40% of the nation’s food supply. Increasingly, irrigation waters are derived from groundwater, which now accounts for 45 to 60% of the total irrigation water. The fruits and vegetables of California’s Central Valley are nearly all grown with irrigation, much of it from pumped groundwater.
Of course, some of this groundwater is not easily renewable, having been accumulated over thousands of years. Removal of ground waters that were recharged during the wetter Pleistocene epoch is a classic example of non-sustainable human behavior. Irrigation in the southern High Plains of Texas and New Mexico has dropped groundwater levels by as much as two feet per year and threatens the future of irrigated agriculture, mostly for cotton, in that region.
A recent paper in Water Resources Research—the leading journal of this topic—indicates that growing the feed for animals tops the consumption of irrigation waters (46 km2/year in the U.S.) of which about 30% comes from groundwater. To produce a 16-ounce steak takes more than the water for the cow; it includes all the water used to grow the fodder that feeds the cow—something on the order of 1800 gallons. Other crops with large irrigation demand in the United States include wheat (13 km3/yr), corn (11 km3/yr), and cotton (4 km2/yr). About 20% of global irrigation is derived from ground waters that are regarded as non-renewable, with India (68 km3/yr), the U.S. (30 km3/yr) and China (20 km3/yr) topping the list.
Globally, irrigation waters amount to about 874 km3/yr, of which 113 to 283 km3/year is derived from groundwater. The delivery of groundwater to Earth’s surface increases the amount of water that can evaporate to the atmosphere or flow to the oceans in rivers. Regions with much irrigation, such as the Central Valley of California, are more humid than 100 years ago, and at least one-third of the rise in sea level over the past few decades is attributed to an increased flow of groundwaters to the world’s oceans.
Reflecting the linkage between Earth’s biogeochemical cycles, profligate use of non-renewable groundwater and large fossil fuel consumption to pump irrigation waters suggest that irrigation of arid and semi-arid lands is not likely to provide an important sink for atmospheric carbon dioxide by enhancing carbon sequestration in marginal agricultural soils.
References:
Halpern, B.S. et al. 2022. The environmental footprint of global food production. Nature Sustainability 5: doi: 10.1038/S41893-022-00965-X
Ruess, P.J., M. Konar, N. Wanders, and M. Bierkens. 2022. Irrigation by crop in the continental United States from 2008 to 2020. Water Resources Research doi: 10.1029/2022WR032804
Sahagian, D.L., F.W. Schwartz, and D.K. Jacobs. 1994. Direct anthropogenic contributions to sea-level rise in the twentieth century. Nature 367: 54-57.
Wada, Y., L.P.H. van Beek and M.F.P. Bierkens. 2012. Non-sustainable ground-water sustaining irrigation: a global assessment. Water Resources Research 48 (6) WOOL06
