Short answers, pulled from the story.
Irrigation has been a key aspect of agriculture for over 5,000 years. Some of the earliest known use dates to the 6th millennium BCE in Khuzistan in south-west Iran, and the site of Choga Mami in present-day Iraq is believed to show the first canal irrigation in operation at about 6000 BCE.
The main methods are surface irrigation, sprinkler irrigation, micro-irrigation including drip irrigation, and subirrigation. Surface irrigation, also called gravity irrigation, is the oldest form, while drip irrigation delivers water to the root zone one drop at a time with field water efficiency typically between 80 and 90 percent.
By 2023-23 percent of all croplands were equipped for irrigation, and the global land area equipped for irrigation reached 355 million hectares. In 2025, irrigated croplands produced 48 percent of all crops in value terms, making irrigated land 3.2 times more productive than rainfed land in value terms.
India and China have the largest equipped areas for irrigation, at 76 million hectares and 75 million hectares respectively, far ahead of the United States at 25 million hectares. Asia held 71 percent of the world total in 2023, where irrigation was a key component of the green revolution.
Irrigation can deplete underground aquifers through overdrafting, cause ground subsidence and salinization, and dry up surface water sources, leading to more extreme regional climates. Still water in canals and ponds has spread diseases like malaria and schistosomiasis, and in Australia over-abstraction for intensive irrigation has put 33 percent of the land area at risk of salination.
In the 1950s, Soviet officials began diverting the Syr Darya and the Amu Darya, the rivers that fed the Aral Sea, mainly to grow cotton. Before diversion the rivers delivered 55 cubic kilometers of water to the sea each year, but afterward only 6, and the Aral Sea shrank to less than half its original seabed.
Global irrigated area grew from 8 million hectares in 1800 to 94 million in 1950 and 235 million in 1990, when 30 percent of global food production came from irrigated land. In the United States, irrigated land rose from 300,000 acres in 1880 to 7.3 million in 1900, aided later by the center-pivot sprinkler that arrived after World War II.