A large proportion of the water used in monsoon Asia is used for rice paddies, and the mechanism of use varies from area to area. Hydrological models, however, do not necessarily describe these various types of water use. Through a series of papers, we propose a distributed water circulation model that incorporates the various uses of paddy water. In this paper we propose a model for estimating cropping pattern and area; the model considers shifts in the cropping system in accordance with changes in precipitation patterns within years and between paddy types (rain-fed and irrigated). We incorporated this model into a series of water circulation models and applied it to the Mekong River Basin, using data from 1999 through 2003. We found that: i) actual evapotranspiration estimated by the model correlated highly with that calculated from observed data; ii) the cropping pattern within each year was well modeled in accordance with the influence of precipitation amount and the difference in paddy type; iii) the difference in actual evapotranspiration between our estimation model and the Penman-Monteith equation was large, particularly in cells where the proportion of rain-fed paddies was high; and iv) assessment of the impact of land-use change on water circulation was easy with this model, as the ratio of each land-use category was shown in each divided cell. In addition, the instability of planting in the rainy season was replicated through consideration given to cropping pattern according to paddy type and rice cultivar.