An irrigation tank or tank is an artificial reservoir of any size. They are mainly found in India. It can also have a natural or man-made spring included as part of a structure. Tanks are part of an ancient tradition of harvesting and preserving the local rainfall and water from streams and rivers for later use, primarily for agriculture and drinking water, but also for sacred bathing and ritual. Often a tank was constructed across a slope so to collect and store water by taking advantage of local mounds and depressions. Tank irrigation is practised by constructing mud banks across small streams to make a small reservoir which collect excess water during the rainy season.Tank use is especially critical in parts of South India without perennial rainfall where water supply replenishment is dependent on a cycle of dry seasons alternating with monsoon seasons.
Water is considered a purifying and regenerative element in India, and is an essential element of prayer and ritual. Water is also revered because of its scarcity in western India where dry and monsoon seasons alternate and failure of the monsoon season means famine and death while plentiful water replacing irrigations sources is a time of rejoicing. This resulted in building water storage tanks that combined the practical and sacred. Since ancient times, the design of water storage has been important in Indian architecture. As early as 3000 BC, sophisticated systems of drains, wells and tanks were built to conserve and utilise water. Tank building as an art form began with the Hindus and developed under Muslim rule.
An example of the art of tank design is the large, geometrically spectacular Stepped Tank at the Royal Center at the ruins of Vijayanagara, the capital of the Vijayanagara Empire, surrounding the modern town of Hampi. It is lined with green diorite and has no drain. The tank was filled by aqueduct.
Ralegaon Siddhi is an example of a village that revitalised its ancient tank system. In 1975 the village was drought-stricken. The village tank could not hold water as the earthen embankment dam wall leaked. Work began with the percolation tank construction by the villagers who donated their labor to repair the embankment. Once this was fixed, the village's seven wells below the tank filled with water in the summer for the first time in memory. Now the village has a supply of water throughout the year.
Tanks known as "Pushkarni" or "Kalyani" also known as "Kund" in Hindi are reservoirs with steps leading down to the water, generally found in South India, and sometimes constructed within the walls of a temple complex.
Stepwells were often used for leisure, providing relief from daytime heat. This led to the building of some significant ornamental and architectural features, often associated with dwellings and in urban areas. It also ensured their survival as monuments.
The development of large-scale water management methods and hydroelectric generation have replaced much of the local efforts and community management of water. For example, the state of Karnataka has about 44,000 artificial wetlands locally constructed over many centuries. At least 328 are threatened today.
However, recently a tank regeneration movement initiated by communities and non-governmental organisations (NGO) has arisen. Today, there are approximately 120,000 small-scale tanks, irrigating about 41,200 km² in semi-arid areas of India. This constitutes about one third of the total irrigated land in South India.