Study on evaluation of water environment in basin by a composite model of hydrological regime and agricultural water use.

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Irrigation generally needs much water and has an historically established priority over other uses. It also affects river discharge, and in drought periods their relation influences the basin management. To evaluate the water environment in a basin, which is strongly affected by the relationship, a simulation model was developed, based on the "Complex Tank Model". And it was used to analyze agricultural water demand and supply, a water re-use systems of irrigation, and return flow from farmland in a river basin in Japan. Results of analysis made clear that irrigation systems have built up effective re-use systems and that return flow forms the greater part of the river discharge. It was also noted that, in drought periods, the critical point deciding the water supply from the reservoir should be focused on and maintaining the adequate discharge at the point leads to the utilization of water resources. Pollutant run-off sub-model was set in the model and also used to estimate the nutrient concentration and load of the river water.