Submitted to: Journal of Hydrology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/12/2012
Publication Date: 5/23/2012
Citation: Somura, H., Takeda, I., Arnold, J.G., Mori, Y., Jeong, J., Kannan, N., Hoffman, D. 2012. Impact of suspended sediment and nutrient loading from land uses against water quality in the Hii River basin, Japan. Journal of Hydrology. 450-451:25-35. Interpretive Summary: Lake Shinji is a typical brackish lake in eastern Shimane Prefecture in southern Japan. Despite recent government attempts to improve water quality, the lake does not meet environmental standards for nitrogen and phosphorus. An extensive monitoring program was implemented to understand the source loadings and pathways of sediment and nutrients within the river basin draining into Lake Shinji. Loadings of sediment, nitrogen and phosphorus were determined from different land uses including rice paddys, upland fields, urban areas and forested areas. It was also determined that the major pathway of nitrogen to the lake was through ground water and while the majority of phosphorus was transported to the lake with sediments from rice paddys and upland fields. The study provided several implications to land management within the basin to manage water quality of Lake Shinji. Since forested areas are a major source of sediment, reduction of sediment will be difficult. In contrast, the primary sources of nitrogen and phosphorus in the basin are agricultural and appropriate fertilizer management will be instrumental in reducing loads to Lake Shinji.
Technical Abstract: Lake Shinji lies in eastern Shimane Prefecture, and is typical of brackish lakes in Japan. Water quality of the lake does not meet the expected environmental standards for total nitrogen (TN) and total phosphorus (TP), even though the national and prefectural governments have tried to improve water quality by developing maintenance scenarios for sewage, plant effluent, agricultural activity, and forestry. Consequently, detailed data of nutrient loading to the lake from river inflows is crucial to support strategies for improving the lake water environment. The Hii River contributes approximately 80% of the discharge flowing into the lake. In this study, we examine the Hii River catchment with a focus on land uses such as paddy fields, upland fields, residential areas, and forestry. Average annual discharges of suspended sediment (SS), TN, and TP loads were determined at Otsu, near the outlet of the basin into Lake Shinji. We also determined average yield per unit area of SS, TN, and TP loads from each land use. Yields per unit area from upland areas were the greatest, whereas yields from forests were the lowest. Forests were the largest contributor of SS, TN, and TP in the basin, because of its dominant land area. Upland fields had the second largest impact on these loads in the basin, because fertilizer applied to the fields is a major source of nitrogen (N) and phosphorus (P). Large differences in yields per unit area between fine and rainy day conditions were also observed, especially for SS and TP loads. Furthermore, we determined that a major pathway of N to the river was through groundwater, regardless of land use, whereas P was transported to the river with sediments, especially in paddy and upland fields. Based on these analyses, it will be difficult to reduce the SS load discharge in the basin in the future, because forestry is the major source. In contrast, N and P load reductions are straightforward, because the primary source is agricultural, and appropriate management of fertilizer application will be a key factor in reducing loads.