This paper describes the development of a distributed water circulation model that characterizes and assesses human interaction in unique water uses for agriculture in the Mekong River Basin. A high proportion of water is used for agriculture in Monsoon Asia given the various types of paddy irrigation utilized there, and the distinct dry and wet seasons. The proposed model incorporates those characteristics and reproduces the mechanism of the water cycle in that region. The use of agricultural water in rain-fed paddies of the basin is first classified as three types of practices: using only rainfall, temporarily using supplementary water, and using floodwater. Irrigated paddies are also classified into six types based on the major types of irrigation and facilities employed. Secondly, the model consists of four sub-models used to calculate potential evapotranspiration, simulate cropping patterns and planting/harvesting areas, estimate the use of agricultural water, and analyze runoff components, respectively. Thirdly, actual evapotranspiration is estimated based on estimated water content in the root zone, one of the three layers modeled. Finally, model simulation was conducted for the years 1999 through 2003 and volumes of calculated discharge were compared with observed data at key points in the basin, showing a high degree of accuracy.