The Estimation of Total Nitrogen and Total Phosphorus Outflow Loads from Watersheds Located behind the Closed Water Area

In this study, the amounts of the outflow loads of nitrogen and phosphorus from the Kahoku-gata lake at Ishikawa Prefecture in Japan is estimated, and measures for the load reduction are examined. The composite reservoir model is used to estimate the runoff discharge from watersheds. This model consists of four kinds of tank models to suit the different land use categories; forest, paddy field, farmland and city. The simulation results show the good fitting of the curve representing between observed and calculated discharges. The amounts of the outflow loads of T-N and T-P are estimated by using two methods (the pollutant load factor method and the LQ equation method). Based upon the loading estimates from 1998 to 2002, Kahoku-gata lake received approximately 661 t/yr of T-N and 31 t/yr of T-P. It is clear that the largest contributor of loads was from paddy fields. The reduction measures for effluent loads will focus on influence of reuse of irrigation water and fertilization of the slowly available fertilizers. As a result, each measure could reduce about 10% of effluent loads. It was judged that the strategy combing these measures was effective for controlling the effluent loads.