Estimation of a Safe Reservoir Yield Using Weap: a Case Study of Thwake Multipurpose Reservoir

Abstract

Reservoir operation in this case multi-purpose reservoir involves several competing uses like irrigation, hydropower, flood control and water supply. This study aimed at using a simulation model, WEAP model to estimate the maximum, reliable continuous and dependable reservoir yield at a particular time. WEAP model schematic was set to develop current and reference scenarios. Parameters used were a GIS map of the sub-catchment, hydrological and water demand data from WRA. To achieve the objective, 1000 years’ synthetic flows were generated using SWAT Model, and 60 years (1952-2012) historical flows were used and the projected flows input into the WEAP model. The monthly reservoir balance for the base scenario and upstream dams’ development scenario were simulated. The performance of the Thwake Reservoir under different scenarios was assessed for base case and upstream development dams. The sets were tuned to the operational rules of the Thwake reservoir. The safe yield of the Reservoir is at 30% of the effective storage (488Mm3), when setting the demands on high priority and low priority. The zone totally available for Galana Kulalu flow requirement at 80% of the remaining effective storage for the high priority demands. For low priority demands, the zone totally available for Galana Kulalu is the storage between the 897M.a.s. l and 900.7 M.a.s.l. There are also different priorities for filling/storing water in the reservoir when setting the reservoir at base scenario and at upstream development.