Supplementary water should be estimated even in areas where data is scarce because of the importance of proper water management. The Mekong River, particularly the lower part (Lower Mekong River Basin), is one area where supplementary water has not been well researched despite the fact that this international river runs through four countries: Laos, Thailand, Cambodia and Vietnam. Studies to grasp the amount of supplementary water are restricted due to a lack of hydrological data, thus methods for obtaining these amounts are needed. One solution is model simulations. In this study, supplementary water was estimated by calculating back from rice production targeted at a macro scale. As a result, the supplementary water in Cambodia, Laos and Thailand was estimated from 1991 to 1995 through model calibration with rice production.