In order to attain a sound and sustainable water environment, it is important to carry out the environmental management of the watershed. For this purpose, knowledge on the pollutant runoff mechanism from non-point sources becomes very important especially under rainy conditions. At Isahaya, Nagasaki, Japan, a big project of construction of sea-dyke and reclamation is now going on, so reducing the pollutant runoff, especially from non-point sources, becomes more important. Some runoff models of rainwater are developed to predict the rate of pollutant loads from the non-point sources, and their results are compared with each other to investigate the accuracy of prediction. In this paper, runoff analysis of both rainwater and pollutants has been carried out using three models: the tank model, the kinematic wave (K-W) model, and a model using the digital elevation model (DEM). For precise estimation, it becomes necessary to identify the parameters included in these models. Here, total nitrogen has been considered as pollutants, and detachment rates are evaluated, correlated with a class of land use, soil type, and moisture content. Finally, it has been shown that pollutant runoff from non-point sources can be predicted fairly well, identifying the model parameter appropriately.

Sea-dyke is sometimes constructed, so one part of bay has been enclosed. This kind of situation is also observed at Isahaya of Nagasaki, that is a big project of construction of sea-dyke and reclamation. In order to make clean the enclosed water bodies, integrated management based on rivers and watershed should also be emphasized. On this viewpoint, pollutant runoff analysis by means of the Digital Elevation Model (DEM) has been carried out, and how to make available the water environmental management has been discussed, considering the environmental changes around at the enclosed sea area.Following to the purpose described above, in this paper, runoff rate of nitrogen from the watershed has been estimated, using the two-dimensional model of pollutant runoff by DEM. Even though parameter included in the model should be identified based on a lot of field data, it has been shown that proposed model might be useful for the prediction and control of non-point pollutant sources distributed over the watershed.

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