Irrigation

Irrigation brings water to crops that rain alone cannot feed, either by letting it run down gently sloping canals or by lifting it from a river or a well. The earliest known canals were dug on the dry plains of Mesopotamia about eight thousand years ago, and the same plains showed early on that a field watered without drainage can slowly turn salty.

What it is

Irrigation gives crops water when they need it instead of waiting for rain. In southern Iraq or in Egypt’s Nile valley there is no farming without it; elsewhere it raises yields and allows a second crop in the dry months.

If the source lies higher than the field, gravity does the work: a canal leaves the river upstream and follows a gentle slope down to the land. If the water lies lower, between high banks or at the bottom of a well, someone has to lift it. The plate shows a shaduf feeding a canal and its furrows, and an Archimedes screw.

How it works

On a bed that is too flat, water slows down, drops its silt and lets reeds grow; on one too steep, it scours the bed and breaks the banks. A small earth canal often falls little more than a metre per kilometre, and FAO recommends a furrow slope between 0.05 % and 0.5 %. Canals therefore follow the contours and lose height slowly.

In the field, water either floods a level basin closed by low earth banks and soaks in, or runs along furrows between the rows and seeps sideways into the ridges.

The shaduf: once the farmer lets go, the counterweight lifts the full bucket.

Lifting a cubic metre of water by one metre takes at least 9.8 kilojoules, and friction adds to that; a device only makes the work easier to supply. The shaduf is a lever with a bucket at the long end and a stone or a lump of dried mud at the short end. The farmer pulls the empty bucket down into the water, which raises the weight, and the weight brings the full bucket up. In field measurements in Chad, one person lifted between 39 and 130 litres a minute over heights of 1.8 to 6.2 m. For the same effort, a higher lift means less water.

The Archimedes screw is a helix turning inside a tube tilted at about 30°. Water stays at the low point of each turn, and as the screw rotates that low point moves up, carrying a pocket of water. It moves large volumes a few metres up; the steeper the screw, the smaller each pocket.

All irrigation water carries some dissolved salt. Plants and the sun take the water and leave the salt behind. Leaks from canals and fields also raise the water table, and salty groundwater then rises into the root zone. A field stays healthy only if extra water washes the salt down from time to time and drainage, natural or dug, carries it away. Otherwise salt builds up, sometimes as a white crust, and yields fall.

How to build it

The A-frame: when the plumb line hangs on the mark, both feet are at the same height.

Make an A-frame level. Lash two poles about 2.8 m long together at the top, with their feet 4 m apart, and fix a crossbar about 1 m above the ground. Hang a plumb line from the apex. Mark where it crosses the bar with the frame on two pegs, turn it round and mark again: halfway between is level.

Lay out the canal. Walk the frame from the intake, driving a stake at each foot. To set a fall, put a block under the downhill foot and move that foot until the line hangs on the level mark, then drive the stake: a 1 cm block on a 4 m frame gives 0.25 %. Dig along the stakes and tamp the spoil into a bank on the downhill side.

Cut the furrows. Lead them off the canal on the same gentle fall, 30 to 60 cm apart on sandy soil and 75 cm to 1.5 m on clay, each opened by a notch in the bank. Let water run until the soil at the far end is moist to root depth, then close the notch, and lead the furrow ends into a drain ditch so that surplus water leaves the field.

Build a shaduf. Set two posts firmly at the water’s edge, with a crossbar on which a pole three to four metres long pivots a quarter of its length from one end. Hang a bucket from the long end and a stone or bag of earth from the short end, adjusted until a full bucket rises with a light push.

Or wind a small screw. Wind a flexible tube round a pole, each turn advancing about as far as the pole is thick. Mount the pole at about 30° on two bearings, low enough that the mouth of the tube dips under water for part of each turn, and fit a crank at the top. Turn it so the mouth scoops water as it goes in. If nothing comes up, try the other direction first, then lower the angle or tighten the turns.

Safety. People drown even in narrow canals with steep, slippery banks: never work alone at the water and keep children away. Keep channels shallow, no deeper than the knee, and never enter a deeper trench: its walls can collapse without warning. Keep spoil back from the edge. Keep clear of the swinging shaduf weight. Drain standing water within a few days, as it breeds mosquitoes. Where bilharzia occurs, its parasites live in fresh water, still or flowing, and enter through the skin: avoid any skin contact with canal water. Irrigation water is not drinking water.

History and evolution

At Choga Mami, in eastern Iraq near the Iranian border, excavators found channels of about 6000 BCE that they read as irrigation ditches; some put them nearer 5500 BCE. The cities of Sumer later lived on canals from the Tigris and Euphrates; texts from Lagash describe full networks by about 2400 BCE.

In Egypt, earth banks divided the floodplain into basins. During the flood, sluices let the water in; it stood about a month, then drained away before sowing. The shaduf, shown on a Mesopotamian seal of about 2200 BCE, reached Egypt in the second millennium BCE.

A qanat taps groundwater under the hills and brings it to the surface through a tunnel.

Greek writers credit Archimedes, in the third century BCE, with the water screw used in Egypt; the Assyriologist Stephanie Dalley reads an inscription of Sennacherib, around 700 BCE, as describing bronze screws at Nineveh, a reading still debated. Qanats, tunnels that carry groundwater for kilometres, are usually traced to Iran in the first millennium BCE, though they are hard to date; about 37,000 were still in use there in 2015.

In 1958 Thorkild Jacobsen and Robert Adams argued that rising salt pushed Sumerian farmers from wheat to hardier barley; Marvin Powell challenged that theory in 1985. Drip irrigation, developed in Israel by the engineer Simcha Blass and made by Netafim from 1965, now feeds water to the foot of each plant.