3.4.6 Soil salinization
The change from annual flooding by a river to perennial irrigation that can be provided from a reservoir can cause soil salinization, if salts normally present in the river water accumulate in the soil as the water evaporates. These salts were previously washed away by the flooding, but the reduced supply of water by irrigation leaves them in the soil. The water is taken in by plants, or evaporated by the sun, leaving the salts behind. This causes a decline in crop yields until eventually the soil becomes useless for agriculture. It can be prevented by using enough irrigation water to wash the salts through the soil, and draining this water from the fields.