Irrigation in Japan accounts for 70% of total water use, mainjly for rice cultivation, giving riseto large return fl ow. Due to the spatial extent and extended duration of return fl ow, its directmeasurement is rarely practical and only few data are available. Besides, measurementsalone cannot successfully quantify it because drainage from irrigated areas has severalcomponents that are diffi cult to separate and measure.To overcome these obstacles, this study developed a hydrological model to simulate waterinfl ow and return processes in irrigated areas. The model was used to evaluate the impactof agricultural activities on fl ow regimes. The catchment was divided into grid cells, eachrepresenting the local hydrological features. For each grid cell, the model calculates ET,overland fl ow, snowfall and snow melt, and agricultural water use. The surface runoff is routedby using a one-dimensional kinematic wave for channel fl ow. In addition grid cell-basedcomputations, water fl uxes across multiple grid cells are also considered.
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