Agricultural irrigation is so widespread that it accounts for about 4% of the total evapotranspiration of water from Earth’s surface. Scientists have known for some time that water vapor from irrigation affects regional and global climates. Now, for the first time, researchers have shown that irrigation in one region can directly affect the climate of another region thousands of kilometers away.
De Vrese et al. used the Max Planck Institute for Meteorology’s Earth System Model to simulate the fate and impact of water used for irrigation in South Asia from 1979 to 1999. In the simulations, early spring winds carried water vapor from irrigation in South Asia across the Arabian Sea and into East Africa, increasing humidity there. By late spring, when irrigation in the Middle East, Turkmenistan, and Afghanistan is in full swing, wind transported water vapor into Africa, increasing humidity as far west as Nigeria.
In the summertime, wind patterns shifted, and less water vapor was blown into Africa, although the Arabian Peninsula and the Horn of Africa still experienced increased humidity. At this point, winds swept water vapor from South Asia into Southeast Asia and China, increasing humidity there. In addition, irrigation in South Asia cools the land surface, reducing the land-ocean temperature gradient and, through this, weakening the monsoons in Asia.
The simulations show that water vapor transport from South Asian irrigation increases springtime rainfall in Africa by up to 1 millimeter per day. Increased rainfall and cloud cover may cool the surface by up to 0.5 kelvin. In the arid parts of East Africa, as much as 40% of the total yearly rainfall may be attributed to irrigation in Asia.
That level of influence means that changes in South Asian agriculture could have profound effects on African rainfall and temperatures. For example, if South Asian irrigation declines, perhaps as a result of a depleted water supply, reduced water vapor transport to East Africa could worsen drought conditions in a region where millions of people already face the effects of water scarcity.
Worldwide irrigation is expected to increase 50% by 2050 to support the food needs of a booming population. Improved understanding of the remote effects of irrigation could help predict how increased irrigation in certain areas will affect distant weather patterns. (Geophysical Research Letters, doi:10.1002/2016GL068146, 2016)
—Sarah Stanley, Freelance Writer