Irrigation and water control

In a hot, dry growing season a crop uses roughly 4–8 mm of water a day. 1 mm over 1 hectare is 10 m³, so a hectare needs about 40–80 m³ a day at peak — 40,000–80,000 litres.

That single number decides everything: whether the source can supply it in the driest month, how big the channel must be, whether lifting by hand is even feasible, and how many hectares you can irrigate. Measure the source's flow in the dry season — see water power for how — before planning a single field.

Water runs downhill and will not be argued with. A water level — a long clear tube filled with water, each end tied to a marked stick — gives accurate levels over tens of metres for nothing: the water stands at the same height at both ends.

Lay out the channel route with pegs, measuring the fall from peg to peg, and record the levels. A channel built before the survey is a channel that discovers it runs uphill.

Gravity first. A contour channel taking water off a stream upstream of the fields is cheaper and more reliable than any lifting device.

Give earth channels a fall of roughly 1 in 1,000 to 1 in 3,000 and keep the water moving at roughly 0.3–0.6 m/s: slower and silt settles and weeds choke it; faster and it scours its own bed. Put a sluice at the intake and division boxes where water splits between fields, so shares can be set and closed.

Unlined channels in sandy ground can lose most of the water before it arrives. Measure it: the flow at the intake against the flow at the field.

Puddle with clay — a layer 15–30 cm thick, wetted and trampled or rammed into the bed and banks — or line with stone, brick or lime mortar where loss is high. Keep banks free of burrowing animals and tree roots, and clear silt and weeds on a schedule.

Where gravity cannot reach:

• Shaduf — a counterweighted lever and bucket. Lifts of about 1–3 m, slow, one person.
• Archimedes screw — a helix in a tube set at about 30°, turned by hand or animal. Low lifts, under about 1–2 m, large volumes.
• Noria — a wheel with pots driven by the current itself. Lifts up to nearly its own diameter, and needs no labour.
• Chain or rope pump — washers on a loop through a pipe. Deeper lifts.

No single device is best at every height. Compare litres per hour per person before choosing.

Irrigation without drainage concentrates salt in the topsoil until the field is sterile. It destroyed some of the earliest agricultural civilisations on earth, and it is entirely avoidable.

• Dig drains so the water table stays at least 1.5–2 m below the surface — closer than that, water rises by capillarity and leaves its salt at the top.
• Apply extra water — roughly 10–20% more than the crop uses — to flush salt below the roots, and let it drain away.
• Watch for white crusts, patchy stunted growth, and dark greasy-looking soil — the first signs.

Irrigation canals and still pools are where malaria mosquitoes breed and where the snails that carry schistosomiasis live — a parasite that enters through the skin of anyone wading, washing or swimming.

• Keep water moving; drain puddles and low spots.
• Clear vegetation from channel edges on a schedule.
• Line channels where you can.
• Keep latrines well away from irrigation water — see sanitation.
• Discourage wading and bathing in channels where these diseases occur.

Irrigation disputes are among the most reliable sources of lethal conflict in agricultural societies. Agree, in writing, while there is enough:

• Who gets water, in what order, for how long — by turn, by time or by share.
• What happens when there is not enough — proportional cuts, or a priority order.
• Who maintains which section of channel, and what happens to someone who does not.
• Who settles disputes, and how.

Store water as well: a tank, cistern or small dam buffers a dry spell and is worth more than a larger channel. See the first public good.