Water Harvesting in the Drylands

In a dryland the problem is not rainfall, it is where the rainfall goes. A hard storm on bare ground sheds most of its water in minutes, and the job of design is to catch that water and put it where a crop can drink it slowly. Every technique below did that, with no pump.

The qanat: water moved underground

The qanat was developed in ancient Iran about three thousand years ago, and the same function appears across Asia and North Africa under other names: kariz in Iran, karez in Afghanistan and Pakistan, foggara in Algeria, khettara in the Atlas, and falaj in Oman and the Emirates. It is a gently sloping tunnel that taps an aquifer in the hills and carries water downhill by gravity to a lower outlet, with vertical shafts at intervals for access and maintenance.

The engineering logic is evaporation. Water in an open channel under a desert sun is lost as it travels; water in a buried tunnel is not. The flow also changes only gradually from wet years to dry years, which makes the system reliable enough to build a settlement on.

The Nabataean runoff farm

Two thousand years ago, farmers in the Negev desert grew productive crops on four or five inches of rain a year. The system had three parts. A terraced wadi placed stone walls across a small dry riverbed, creating fields behind each wall that trapped part of the flood and cascaded the excess down. The diversion system caught the flood behind a low dam in a larger wadi and sent it down a channel to terraced fields on the floodplain. The conduit channel system, the one most widely used, ran terraced fields in the beds of small tributary wadis with a catchment on the surrounding slopes, linked by channels running diagonally across the hillsides.

Israeli scientists rebuilt farms at Shivta and Avdat in the late 1950s on the original layouts. The Avdat farm, about 20 acres, was planted with 3,000 orchard trees plus cereals, vegetables, legumes and pasture. Shivta was left unmanaged from the late 1970s and the crops kept yielding for decades without staff.

The Sahel: zai pits and half-moons

In the western Sahel, farmers restored degraded dryland with two cheap shapes. The zai pit is a hole about 15 to 50 centimetres across and 5 to 15 centimetres deep, spaced roughly 80 centimetres apart, dug into hard, crusted soil to catch runoff and hold compost in place. Farmers across Burkina Faso, Niger and Mali have used it for generations. From the 1980s the Burkinabe farmer Yacouba Sawadogo revived and enlarged it and filled the pits with manure and compost. The manure draws termites, whose tunnels open the soil, and reported yield gains for sorghum, millet and trees have been very large, though the biggest multipliers are reported rather than settled.

The semi-circular bund, also called the demi-lune or half-moon, is a crescent dug with its opening facing the flow of water, so it catches the runoff and holds it around a plant. They run 2 to 8 metres across, with the excavated earth piled along the rim to about 30 to 50 centimetres high, on slopes up to about 15 percent, staggered so runoff that passes one row is caught by the next. The catchment-to-cultivated area ratio runs between one to one and three to one. Large, widely spaced crescents rehabilitate grazing land; small, closely spaced ones establish trees and shrubs.

Check dams and the old reservoir

Where a channel carries a seasonal flood, a check dam is a low barrier across it. The water pools, percolates, and recharges the groundwater the wells draw on. India has built more than 200,000 check dams on ephemeral streams since the 1960s for this purpose. The johad is the community version, a percolation pond owned in common, and the NGO Tarun Bharat Sangh revived over 4,500 johads in Rajasthan's Alwar district.

What the swale borrows, and will not say

The contour swale is a level ditch on the contour, with the spoil thrown downhill to make a bank, built to hold water and let it sink rather than carry it away. Varahamihira's sixth-century rain chapters existed so farmers could predict the rains and be ready to catch them. The Sahelian contour bund, the stone line, the fanya juu ditch and the Nabataean catchment channel are the same shape doing the same job. The modern course teaches the swale and credits a twentieth-century designer. It is a renaming.