We developed a water circulation model that incorporates paddy water management, including reservoir operation and water allocation schemes, which is applicable to extensive and well-developed irrigation areas. First, reservoir operation and water allocation and management models were introduced into the water circulation model previously developed by the NIRE. Subsequently, we applied the developed model to assess the impact on irrigation reservoirs of climate change in the Mun River basin of northeast Thailand. As a result, taking into consideration the fluctuation in rainfall, the developed model reproduced the seasonal changes in storage water and water release at two large reservoirs, and monthly diverted water at a major diversion weir. Moreover, introducing the operation of reservoirs and diversion weirs improved the simulation accuracy of river discharges, particularly in dry seasons. According to climate change projections, the impacts of climate change on reservoir operations differed in each reservoir, although the reservoirs are in the same basin. This means that the impacts of climate change on the operation of water-use facilities should be individually assessed. Since the developed model requires only basic information and assumes simple operation rules, this model is useful for assessing impacts on the operation of numerous water-use facilities simultaneously at the macro level.