Irrigated soils are a much higher priority for conservation. In most large irrigation schemes, water from rivers, wells or qanats is fed by gravity to low-lying sites. The soils in these sites are vulnerable to salinization, because the added water raises the water-table within them. When the soil water nears the surface, some is drawn up further by capillarity and the salts it carries are concentrated by evaporation on the surface, where they reduce yields, ultimately to very low levels (Figure 5.10). Waterlogging also damages crops. In large schemes, where settlers are unaccustomed to irrigation, or where planners have ignored warnings about salinization, large areas have gone out of production.

In low-intensity irrigation, as in Iraq in the 3rd millennium BCE, salinity was kept at bay by fallowing the land. During the fallow, weeds and natural drainage drew down the water-table. In second millennium BCE Iraq, a centralized state replaced the ancient system with irrigation from a large canal. Salinity accumulated and steadily reduced yields (Jacobsen and Adams 1958). In intensive modern systems, where water is added to three or more crops a year, the best strategy to manage salinization and waterlogging is to maintain drainage through the soil, and to add enough water to carry away the salts. The bigger the scheme, the more expensive are the necessary drainage pipes, ditches, or tube-wells for lowering the water-table, and the canals to take salty water to the sea or to reservoirs where it can evaporate. Complementary strategies include using brackish water to irrigate salt-tolerant crops, and blending saline with fresh water, but there are dangers in both.