As per a 1996 97 estimate, there are 39,202 tanks in Tamilnadu. In the past, Chennai alone had about 150 tanks. In Tamilnadu, tank irrigation has not increased much since 1883, when 50 percent of the cropped area was under tank irrigation.
Today, less than 10 percent of the land under crops is irrigated by tanks, built mostly by the Chola and Pandya kings.
Inscriptions in temples tell us of the largesse provided by the kings for establishing drinking and irrigation water sources.
The Grand Anaicut as it stands now
Anicuts were small or medium dams built across rivers to divert water into irrigation channels.
The Grand Anicut or Kallanai was built in the second century A.D. by Karikala Chola. It was made of stone and situated on the river Cauvery where the River Kollidam branches off.
Anicuts have also been built across the Tambaraparani, Chittar and many others.
The anicuts in the Kanyakumari district are said to have been built 1000 years ago.
The construction and maintenance was however done entirely by the people. The Rajasmighamangalam tank of Ramnad was built more than 1000 years ago by the Raja of Ramnad.
Waterbodies in Tamilnadu fall under three categories: lakes or yeris, tanks or kulams and ponds or kuttais.
A yeri was a large earthenware tank dug out of the ground with the dug out mud making the side walls or bunds. A kulam was built with bricks (and occasionally granite) and was attached to a temple, giving it the name kovil kulam or temple tank.
A kuttai was a small pond. In Tamilnadu there are a few tanks that irrigate more than 1000 ha and even one which serves 6000 ha. Uranis are natural springs. Rectangular tanks are constructed above them to collect the water coming out from the ground. A winding staircase is provided for people to go down and collect the water for their domestic use.
The village community decided what crops to grow and where to grow them, how long the sluice gates of the yeris should be kept open, how much water should flow to each field and so on.
Tanks can be classified into system and non-system tanks. System tanks receive plenty of water as they catch the overflow from a reservoir, nearby stream and the runoff from around their catchment.
They help farmers to raise more than one crop. Non-system tanks depend entirely on rainfall and can support only one crop. The tanks used a system of canals and sluice gates to control and transport the water.
Several tanks linked by canals were also built in the watershed areas. The surplus of water in the higher tank thus flowed into the lower one, following the natural slope of the land.
Very Nice to Read ur message. One interseting information is that KALLANAI was the Oldest Civil Engineering Marvals of Human as per the Hydrology Wing of World BanK andThe Government of The Netherlands's Report.
Interesting info about the irrigation system. But the link is not working.
Unfortunately, Kallanai was not built by Karikalan. Sangam literatures mention that Karikal valavan built banks for kaveri on both sides. It is not present day kallanai. One research student of Delhi Jawaharlal Nehru University did his Ph.D on kallanai and proved that it is around 400 years old. Might have been built during late nayak or early british period.
The earliest evidence of dams built by tamil people is by first empire pandyas. Lalgudi inscriptions talk about this.