Long low earthen banks to impound the monsoon run-off and allow it to soak into the ground are known as khadins in the desert state of Rajasthan. A khadin system is comprised of a rocky catchment slope and the cultivated area behind the embankment - which over time builds up a deep and fertile soil. The embankments are typically 300 to 500 metres long and one to three metres high. Although they are not included in official Indian irrigation statistics, researchers believe that around 300,000 hectares are watered by khadins and similar indigenous methods built to increase soil moisture and trap silt. The most widespread vernacular irrigation system in India and Sri Lanka is the tank, a small reservoir impounded by a dam across a seasonally flooded depression or gully.
Individual tanks can irrigate anything from a few hectares to several hundred.
Although tanks have been proven by centuries of experience to be socially, economically, and ecologically appropriate techniques of water and land management, over the last few decades their use has steadily declined and many are now abandoned. In some traditional systems, water is taken from behind a small wood, rock or earth dam. The Sonjo, who irrigate the slopes of Mount Kilimanjaro in Tanzania, for example, divert water with brushwood dams up to three metres high. Small dams of this type are easily destroyed by floods, a feature which can enhance the sustainability of the overall system as most of the sediments behind the dams will be washed away with the dam. Because the dams are built with local materials and labor, rebuilding them is not usually a major expense.