Irrigation

Posted: 26 March 2008

On a global basis, agriculture accounts for 70 per cent of all annual water withdrawals. (In developing countries water consumption for agriculture is typically 70-80 per cent of total water consumption). The overwhelming bulk of this water is used for irrigated agriculture; only a tiny amount is accounted for by livestock. Some 40 per cent of the global harvest comes from the 17 per cent of cropland that is under irrigation.

Since 1970 global water withdrawals have mirrored the rise in irrigated area. Click here for a graph showing this pattern (Source: GEO-3/FAO).

Most irrigation systems waste water. Typically, only between 35 per cent and 50 per cent of water withdrawn for irrigated agriculture ever reaches the crops. Most soaks into unlined canals, leaks out of pipes, or evaporates before reaching the fields. Although some water "lost" in inefficient irrigation systems returns to streams or aquifers, where it can be tapped again, water quality is invariably degraded by pesticides, fertilizers and salts that run off the land. Poorly planned and built irrigation systems have limited the yields on close to half of all irrigated land.

Salt build-up in soil has severely damaged around 30 million hectares of the world's 255 million hectares of irrigated land, according to the UN Food and Agriculture Organization (FAO). A combination of salinisation and waterlogging affects another 80 million hectares. At the same time, land under irrigation is expanding, and is expected to reach about 290 million hectares by 2010, and about 330 million hectares by 2025. But the area of land affected by salinisation is also eating into the total figure.

Striving to improve efficiency

Solutions are available. Two, in particular, show promise:

  1. Drip irrigation. This irrigation technique delivers nearly 95 per cent of all water withdrawn to the root zones of the crops via a network of porous or perforated piping, usually installed in the ground. This keeps evaporation rates very low and wastes very little water. Drip irrigation systems cut water use by up to 60 per cent compared to simple gravity systems. In the 1970s drip irrigation was used on only 56,000 hectares. Today, it is used on over 1.6 million hectares. Still, this accounts for only 1 per cent of all irrigated land.

  2. Low energy precision application (LEPA). This system offers substantial improvements over conventional sprinkler systems that spray water into the air. The LEPA method delivers water to the crops from drop tubes that extend from the sprinkler's arm. When used together with appropriate water-saving farming methods, LEPA can achieve efficiencies as high as 95 per cent. Also, since this system operates at low pressure, energy costs (in terms of pumping) drop by as much as 50 per cent.

Click here for a map comparing water use by the agricultural, industrial and domestic sectors between countries.