POINT AND NON-POINT-SOURCE NUTRIENT CIRCULATION MODELLING FOR THE TAKASAKI RIVER BASIN, CHIBA JAPAN

Abstract

Nutrient discharge to the Takasaki River from different Point-Sources (PS) and Non-Point-Sources (NPS) are considered in this study. Land use types: agricultural areas, forests and urban areas are considered as NPS in the study. Nitrogen (N) and phosphorous (P) based nutrient discharges from different land use types have a significant contribution to the river water quality. Limitations in technical, human and financial resources impede efficient monitoring of those nutrients at watershed scale. At this end, numerical model would be a beneficial tool to understand and predict the water quality variations under different climatic and anthropological conditions. The Water and Energy transfer Processes (WEP) model coupled with nutrient transport processes is applied to understand the impact of NPS considering the land use distribution in the Takasaki River. Mainly six land use types are prevailing in the Takasaki River basin, namely forests, crops, bare lands, paddy fields, urban areas and water bodies. Initially two rainfall events were selected to check the performances of the model. During the first event, average measured river discharge, TN loading and TP loading were 21.51 m3/s, 108.04 g/s and 11.73 g/s, while the simulated averages were 17.67 m3/s, 86.24 g/s and 7.52 g/s respectively. In the case of second event, average measured river discharge, TN loading and TP loading were 22.52 m3/s, 88.74 g/s and 8.73 g/s while the simulated averages were 17.72 m3/s, 80.84 g/s and 7.65 g/s respectively. As the next step two sub basins within the Takasaki basin which are agriculture dominant and urban area dominant were also analyzed to understand the impacts in difference land use types and their loadings to the river.