Impacts of climate change on irrigation water demand of paddy: a case study from <i>Hakwatuna Oya</i> irrigation scheme in Sri Lanka

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Assessment of irrigation water requirement (IWR) is a prerequisite for planning and management of an irrigation scheme, particularly for a water short scheme. In this context, this study was conducted to estimate the current and future IWR under A2 (very heterogeneous world) and B2 (world in which emphasis is on local solutions to economic, social, and environmental sustainability) scenarios of IPCC emission for Hakwatuna Oya irrigation scheme using SDSM and CROPWAT models. It was found that the reference evapotranspiration (ETo) during Maha and Yala seasons at present are 407 mm and 506 mm, respectively. Water requirement of 3½ months paddy in Maha season is 1,282 mm whereas it is 1,381 mm in Yala season. The crop water requirement, which depends primarily on temperature, remains relatively stable over the simulation period up to 2070. When compared to the mean annual rainfall during 1972 to 2001, the rainfall is expected to increase by 32% and 27% from 2041 to 2070 under A2 and B2 scenarios, respectively. As a result, the total water availability from rainfall and irrigation water issues from the Hakwatuna Oya reservoir in Maha season would increase in the future with an overall water deficit to reduce from 28% to 18% and from 28% to 20% under A2 and B2 scenarios, respectively. For Yala season, this reduction is found to be 30% to 24% and 30% to 26% under A2 and B2 scenarios, respectively. The benefits from climate change could be further enhanced by adjusting the planting time to coincide with months of high rainfall.