The purpose of this study was the development of a model to estimate the effects of various operations on water storage in reservoirs. Investigation area was the Chao Phraya river basin in Thailand. Reservoirs in the water use system are multi-purpose reservoirs. Water distribution for various purposes must be considered. Furthermore the water balance of reservoirs cannot be determined during 1 year. At first a model to estimate future effects of each purpose was developed in this water use system. Next, present operations of reservoirs were evaluated by using the estimation model. Recently water shortage has occurred occasionally in this basin presumably due to operation rules. Evaluation of operation rules showed that the current rules enable to achieve stable water use. The probability of water shortage was estimated at 0.059. Finally relations among effects of purposes were investigated.