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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. Discipline: Watershed and regional resources management Additional key words: anthropogenic flow regulation, atmospheric general circulation model, reservoir operation model, water allocation and management model, Monsoon Asia Present address: 3 Rural Development Division, Japan International Research Center for Agricultural Sciences (Tsukuba, Ibaraki 305-8686, Japan) *Corresponding author: e-mail rkudo@affrc.go.jp Received 17 September 2014; accepted 3 February 2015. Introduction Rice farming is a staple industry of Asian countries and comprises 94% of the world’s rice harvest (Forster et al. 2012). Most paddy fields in the region are rain-fed without irrigation facilities. In parts of Monsoon Asia (e.g. Thailand), however, with climates characterized by rainy summers and dry winter monsoons, a number of irrigation facilities have been constructed for paddy irrigation. Although these facilities allow dry-season cropping, they alter the hydrological environment of river basins dramatically. For example, irrigation systems with large reservoirs strongly regulate river flow regimes by damming river discharges for several months, releasing stored water in response to water demand in downstream areas and diverting river flow into irrigation areas (Vörösmarty et al. 1997, Eskew et al. 2012, McCarthy et al. 2008, Forster et al. 2012). To quantify the hydrological cycle and available water resources, particularly in river basins with well-developed irrigation areas, it is therefore important to consider the effect on agricultural water use alongside natural hydrological processes. Significant fluctuations in annual rainfall in Monsoon Asia lead to highly variable rice yields, which implies that changes in precipitation characteristics caused by climate change will significantly impact on agriculture in this region. Accordingly, the impact of climate change on agricultural water use is one of the critical issues facing Monsoon Asia (Eastham et al. 2008, Kiem 2008, MRC 2009, Ministry of Foreign Affairs of Japan). The impact of

Modeling of Paddy Water Management with Large Reservoirs in Northeast Thailand and its Application to Climate Change Assessment