The increase industrial and agricultural development, caused by the population growth on the one hand and the limited water resources available on the other hand has caused the groundwaters to be considered as a valuable water supply more than ever. Optimal operating policies are very much essential for sustainable management of reservoir as poor operating policies may lead to over or underuse of water. The purpose of this study was to investigate the water availability of Timah Tasoh Reservoir, water demand for minor scheme paddy, and hence balance the water inflow and outflow from Timah Tasoh Reservoir. The study area for agriculture is the minor scheme for whole Perlis state, and inflow areas are Lubok Sireh, Padang Besar at Titi Keretapi, Wang Kelian and Kaki Bukit. The result shows that the water inflow to Timah Tasoh Reservoirs is sufficient to supply for the needs of minor scheme agriculture. The highest water balance among the first planting season is in May. Total inflows calculated are much more significant than the total outflows; therefore, it concludes that the water available in Timah Tasoh Reservoir is enough to supply for minor scheme paddy in Perlis for a whole year.
