Abstract:Water is a basic need of all living beings and the management of water in an optimum manner has therefore now received worldwide attention. This optimum management of water is achieved through the construction of reservoir systems and their optimal operation. This paper presents a simulation study carried out using Water Evaluation and Planning System (WEAP) software at the proposed Uma Oya Downstream Development Project in the Kirindi Oya basin in Sri Lanka. To investigate the impact of the proposed system, one model was developed for the existing system and another for the proposed system. Crop water requirements and irrigation schedules for three climatic conditions were determined using CROPWAT software. Climatic conditions were those of the years which received rainfalls with 20%, 50% and 80% probability of exceedance representing wet, normal and dry years respectively. Daily rainfall, runoff and other meteorological data were the information collected for the study. Availability of water for the irrigation of existing and newly proposed areas was ascertained from the model. Optimal operation patterns were developed for the newly built Alikota Ara and Kuda Oya reservoirs and the enlarged Handapanagala reservoir for different climatic conditions. Results indicated that even when the system is operating in accordance with optimum operating rules, there can be supply deficits. A study was made on the system performance with reduced irrigable areas and different crop types which were observed to bring in reduced deficits.