Impact of suspended sediment, nutrient loading from land uses against water quality in the Hii River basin, Japan

摘要

Lake Shinji lies in eastern Shimane Prefecture, is typical of brackish lakes in Japan. Water quality of the lake does not meet the expected environmental standards for total nitrogen (TN), total phosphorus (TP), even though the national, prefectural governments have tried to improve water quality by developing maintenance scenarios for sewage, plant effluent, agricultural activity, 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, forestry. Average annual discharges of suspended sediment (SS), TN, 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, 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, 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), phosphorus (P). Large differences in yields per unit area between fine, rainy day conditions were also observed, especially for SS, 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, 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, P load reductions are straightforward, because the primary source is agricultural, appropriate management of fertilizer application will be a key factor in reducing loads.