The densely irrigated zones are found roughly speaking along the principal rivers of Tamil Nadu, that is to say from North to South : Palar, Ponniyar, Cauvery and its affluent the Bhavani, Vaigai and Tambraparni. On each of them, dams have been built and explain the green spots along these rivers.

The delta of Cauvery is very famous for its rice production and for the Grand Anicut (a water diversion structure) built in the 2nd century, but the name of the river itself is now associated with political problems related to water. Indeed, the river takes its source in Karnataka, where several dams control its flow and its affluents (the Mettur dam located in Tamil Nadu also regulates the flow). Tamil Nadu has then to regularly request that Karnataka releases more water downstream to satisfy the needs of the farmers in the delta region.

Besides these very irrigated zones located along rivers and corresponding to intensive zones of rice growing [see the comment of the map of Andhra Pradesh ], it is difficult - as for Andhra Pradesh - to explain the geographical distribution of irrigation in Tamil Nadu simply by topography or hydrography. If the tanks for example are located in all the eastern part of the state, their density can be different as the green spot in the South of Tambraparni is only due to tanks. In the same way, some zones with 40 to 60% of irrigation can be the result of the use of various techniques, for example tanks and tubewells. The diversity of irrigation types with their own characteristics (dependance on rainfall and runoff for rainfed tank, type of soil and depth of the watertable for access to ground water, etc.) makes it difficult to characterize the variable percentage of irrigated lands.

O.A.