Recent changes of rainfall regime of Hakwatuna Oya watershed of Sri Lanka

This study was conducted to verify farmers' claim, which was also verified by a field survey, that the availability of water in the Hakwatuna Oya irrigation scheme in Sri Lanka has decreased due to variability of rainfall in the watershed area as a result of climate change. Daily rainfall data of 10 selected stations representing “Hakwatuna Oya” watershed over a 50 year period (1961-2010) were used in this study. The linear regression analysis was used to study the annual trend of rainfall during the study period from 1961-2010. The variability of rainfall during recent decades (1991-2000, 2001- 2010) and the most recent 20-year period (1991-2010) was compared with the base period of 1961-1990, recommended by the World Meteorological Organization (WMO) for climate studies, using Coefficient of Variation (CV %), student t- test and confidence interval at 95 percent. The results showed that there was no significant trend of annual rainfall over the last 50 years. In relation to the four rainfall seasons, there was no significant variability of rainfall during the last two decades compared to the base period, except for few stations. However, the rainfall variability of South West Monsoon (SWM), Yala and Maha seasons displayed significant differences in mean rainfall for Siyabalangamuwa rainfall station, which is the closest to the Hakwatuna Oya scheme in the Intermediate zone. The mean rainfall during post 1990 period was lower than the base period during SWM and Yala season whereas it has increased during Maha season.