Importance of water consumption by perennial vegetation in irrigated areas of the humid tropics: evidence from Sri Lanka

In tropical, monsoon climates of South-East Asia, irrigation facilities supplement rain in the wet season and enable crops to be cultivated during the dry season. In the Dry Zone of Sri Lanka, 70% of the average annual rainfall of 1000 mm falls in a 3 month period. During the dry season, reference evapotranspiration has less rainfall — about 700 mm, indicating that much additional supply is meant to support crops, mainly paddy. In this climatic context, irrigation has dramatically changed the local environment, creating ecosystems quite similar to that of the wet zone to flourish. In these systems, recharge of shallow groundwater by percolation from irrigated fields, canals, and tanks, has provided a continuous supply of water for natural vegetation and homestead gardens. Much of the water used by this non-crop vegetation is beneficial. Growth of fruit and coconut trees can be quite profitable, while other trees enhance the environment.