How Does Irrigation Work? Moving Water to Where the Crops Are
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Around a fifth of the world's cropland is irrigated and it produces about forty percent of the food, which means the watered fifth is roughly twice as productive as the rest. Getting water onto a field is easy. Getting it there without wasting most of it, without ruining the soil within a generation, and without emptying the source, is the part that has defeated civilisations.
The methods and what they cost
Irrigation systems differ chiefly in how much water reaches the plant rather than the air, the subsoil or the drainage ditch:
- •Surface irrigation, flooding or running water down furrows, which needs no pumping or equipment and typically delivers only forty to sixty percent of the water to the crop; it remains the most widely used method worldwide because it is cheap
- •Sprinkler systems, including the centre pivot that draws the circular green fields visible from the air, reaching perhaps seventy to eighty percent, with losses to evaporation and wind drift
- •Drip or micro irrigation, delivering water slowly through emitters at the base of each plant, reaching ninety percent or more and allowing fertiliser to be dissolved in the supply, at a high installation cost and with emitters that clog
- •Subsurface drip, with the lines buried, which removes evaporation almost entirely
- •Traditional systems that exploit terrain, including the qanat, a gently sloping underground tunnel carrying groundwater from a mountain aquifer to a settlement without pumping and with little evaporation, some of which have run for two thousand years
- •Rainwater harvesting and flood spreading, which capture seasonal flows rather than drawing continuously from a river or aquifer
The salt problem
All water carries dissolved salts, and irrigation water carries more than rain. When the water evaporates or is taken up by the plant, the salt stays behind, so every irrigation season adds to what is in the soil. If drainage removes some of it, the system is stable. If not, salt accumulates until the soil becomes too saline for crops, a process that has ended agriculture in irrigated regions repeatedly. Southern Mesopotamia is the textbook case: cuneiform records document a shift from wheat to more salt-tolerant barley over centuries and then declining yields, and salinisation is a leading explanation for the decline of the region's agricultural base. The same process affects modern schemes, with substantial areas in Pakistan, Central Asia, Australia and the western United States degraded or lost. It is made worse by a rising water table, because irrigation without drainage raises groundwater until it reaches the root zone and evaporates from the surface, bringing salt up with it. The remedy is drainage, applying more water than the crop needs so that the excess flushes salt downward and away, which requires both extra water and somewhere for the drainage to go.
Where the water comes from
Surface water from rivers is the traditional source and is limited, shared and increasingly contested, since a river that is fully allocated leaves nothing for a downstream user or for the river itself. The Colorado has not reliably reached the sea in decades, and the Aral Sea lost most of its volume after the rivers feeding it were diverted for cotton, an environmental collapse visible from orbit. Groundwater became the dominant new source once cheap pumps and rural electrification spread, and it is being withdrawn faster than it recharges across major agricultural regions including northern India, northern China, the Arabian peninsula and the High Plains of the United States, where the Ogallala aquifer is being depleted in places that took thousands of years to fill. Satellite gravity measurements have made the losses measurable directly. Because groundwater is typically owned by whoever pumps it, the incentive structure encourages each farmer to pump faster, which is a textbook case of a shared resource being exhausted by individually rational decisions.
Efficiency and its paradox
The obvious response to water scarcity is more efficient irrigation, and the result is frequently that water use rises rather than falls. When drip equipment makes each litre more productive, farmers commonly expand the irrigated area, switch to thirstier and more valuable crops, or irrigate more intensively, so total consumption increases even though efficiency per hectare improved. This is the rebound effect, and it has been documented in enough places that water economists treat subsidised efficiency upgrades with suspicion unless they are paired with a hard cap on withdrawals. A related accounting error is that water lost from a flood-irrigated field is not necessarily wasted, since much of it percolates to groundwater or returns to the river and is used downstream, so eliminating that loss can reduce the supply someone else depends on. What matters is consumptive use, the water genuinely evaporated or transpired away, and measuring that rather than delivery efficiency is the change that makes water policy work.
The takeaway
Irrigated land is about a fifth of cropland and grows around forty percent of the food. Surface flooding is cheap and delivers barely half the water to the crop, sprinklers do better, and drip systems exceed ninety percent at higher cost. Every method concentrates salt in the soil unless drainage flushes it away, which has destroyed irrigated regions from ancient Mesopotamia onward. Groundwater is being withdrawn faster than it recharges across major farming regions, and efficiency gains frequently increase total water use rather than reducing it.