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Tamil Nadu, a coastal state in south India, is prone to droughts. The climate of the state ranges from dry subhumid to semiarid. The state has three distinct rainfall climates: (1) advancing monsoon period (from June to September), with strong southwest winds; (2) northeast monsoon (from October to December), with dominant northeast winds; and (3) dry season (from January to May). The normal annual rainfall of the state is about 945 mm.
Tamil Nadu is classified into seven agroclimatic zones: northeast, northwest, west, southern, high rainfall, high altitude hilly, and Cauvery Delta (the most fertile agricultural zone). The Cauvery Delta zone, located in the humid tropics, has a mean annual rainfall of 1,273 mm, with more than 60% of that coming from the northeast monsoon. An analysis of summer monsoon rainfall (June–September) for 1871–1991 shows that the state experienced below-normal rainfall in 30% of these years. During the summer, the average rainfall is 266 mm, but water demand is 663 mm. Hence it is a deficit season. The northeast monsoon brings an average of 529 mm rainfall and demand is relatively low—361 mm.