All early agriculture depended upon adequate rainfall. Man’s inevitable next step was to free himself from the vagaries of the weather by developing systems of irrigation.
By 5500 bce, Sumerian farmers were diverting floodwaters from the Euphrates and Tigris rivers onto their fields. Some irrigation canals extended 5 kilometers from the river.
By 4300 bce, the rice first found growing in the swamps of southern China was planted on leveled banks with mud borders to keep water from draining away. Such paddies became a staple regime for growing rice throughout East Asia.
Around the world, every civilization developed, and has since depended upon, irrigation systems to sustain their agriculture. Farmers at least have always appreciated water as a precious resource, however inefficiently they may have used the water they secured.
The intensification of food production in the last 2 centuries was achieved by a massive increase in irrigation. In 1800, there were 8 million hectares under irrigation across the world. In the 19th century, irrigated land quintupled to 40 million hectares. A 7-fold increase occurred during the 20th century, to 275 million hectares. Overall, growth in irrigated land from 1800 to 2000 was 34-fold.
70% of the world’s available freshwater is diverted to irrigation-intensive agriculture (with nearly all the rest consumed by industry). Over 15% of the world’s arable land is now irrigated, with the highest proportion in Asia. Irrigation allows crops to be grown in places where they could otherwise not be supported, and commonly in places where the water withdrawal is unsustainable.
Nearly 40% of the world’s food supply is produced using highly wasteful irrigation systems that are depleting nonrenewable groundwater. ~ American ecologist Mark Briscoe
In south Asia, only 1/3rd of the rice-growing region is not irrigated, yet it yields nearly 2/3rds of the harvest. The lands irrigated are marginal.
Irrigation can badly degrade soil and lead to waterlogging and salinization, as the ancient societies in Sumer discovered over 4,000 years ago. These effects despoil half the irrigated land in the Middle East, 25% of the irrigated fields in the US, and 80% of the irrigated area in northern India (Punjab). Punjab had normal rainfall in the mid-2010s, but groundwater extraction for irrigation resulted in a rapid decline in water availability.
Throughout history, most extracted water has been for irrigation. Wasteful inefficiency has been a constant problem. In India and China, 2/3rds of irrigation water evaporates or seeps from irrigation canals; in the US, half the water is lost.