EVALUATION OF THE EXISTING WATER SUPPLY SYSTEM IN THIMPHU VALLEY, BHUTAN MUKAND SINGH BABEL Water Engineering and Management School of Civil Engineering, Asian Institute of Technology PO Box 4, Klongluang, Pathumthani 12120, Thailand ASHIM DAS GUPTA Water Engineering and Management School of Civil Engineering, Asian Institute of Technology PO Box 4, Klongluang, Pathumthani 12120, Thailand MAHESH PRADHAN Water Engineering and Management School of Civil Engineering, Asian Institute of Technology PO Box 4, Klongluang, Pathumthani 12120, Thailand This study evaluates the existing water supply system in Thimphu valley, Bhutan from two perspectives, namely technical and financial. The technical aspects include water demand and supply situation and uniformity in water distribution and consumption in the network. The financial aspects considered are present cost recovery status, present level of payment and affordability for water. Based on the findings the study recommends measures to enhance the performance of the water supply system. INTRODUCTION The present water supply system in Thimphu was built some four decades ago and since then has undergone several augmentation, expansion and rehabilitation programs as the city grew and the need arose for increased water supplies. The system was initially designed to cater the demand until 2000 and is presently stressed beyond its hydraulic capacity due to increased water demand. The population within the municipal boundary is 60,000 and about 60% of which is supplied with piped water from the system. The treatment plant has an optimal design capacity of 6,500 m 3 /d with retention time of 2 h in the up-flow clarifiers and filtration velocity of 5 m/h in the rapid sand filters. The treatment process consists of flocculation, sedimentation, rapid sand filtration and chlorination in a sequential order. Raw water is abstracted from three nearby streams. The plant is presently running at its maximum capacity of about 10,000 m 3 /d, which is 50% more than the optimal design capacity thereby causing water quality problems with high turbidity, especially during the rainy season. The whole service area is fed by gravity with 70% of it getting intermittent water supply and only 30% receiving continuous supply. The difference in hydraulic head from the sources to the lowest parts 1