Modeling of nitrate runoff from agricultural watershed

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In Shibetsu watershed that is located in eastern Hokkaido, about 25% of net nitrogen input (NNI) was nitrified into nitrate form and discharged into rivers without removal by plant uptake and denitrification. The nitrate runoff causes eutrophication in coastal and brackish area and often results in degradation of environment of these area and severe damage on fishery. Therefore, we need to establish the management to minimize the nitrate runoff.In this study, we aimed to make model that predicts rate of nitrate removal in deeper soil horizon in two dimension with watershed scale. In this model we assumed that the nitrate removal reaction (mainly denitrification reaction) mediated by microbes obey first order reaction. The time term in this kinetic reaction was replaced by the ratio of distance between the point of nitrogen load and nearest river by using Hydrology Extension in ArcGIS 9.0. From field observation, we found empirical rule that nitrate concentration in rivers negatively correlated with slope of rivers.This model explained 94% of spatial variation in nitrate concentration in rivers in Shibetsu watershed. The predicted rate of nitrate removal by denitrification amounted from several percent to fifty percent. The rate of removal depended on size of watershed and average slope in the watershed.Further effort is needed to give sound theoretical proof to the empirical relationship between nitrate concentration in rivers and the slope of the river. In this study, we tried to prove the relationship by using Manning's law and Leopold's empirical law but failed to give final answer.