The Effects of Vegetation and Agricultural Land use of a Watershed on Water Quality

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Effects of vegetation and land use status on water quality were investigated for several watersheds in central Japan. Stream water was sampled along a small tributary of a branch, the Hida River, of the Kiso River, and that of the Namai River, a branch of the Jintsu River. Grassland used for cattle grazing are located in the source areas of the watershed of both tributaries. High concentration of total nitrogen was found during the grazing period free from snow cover. The level became low during grazing-free period. The highest level of total nitrogen was found at the uppermost stream and it decreased downstream. This result suggests a purification effect of vegetation on water quality. Field measurements were conducted along the upper streams of the Hida River to examine the mechanisms of such water purification. Extent of the brown forest soil, extent of the evergreen conifer forest, forest volume per unit area and volume increment were significant as factors affecting the concentration of total nitrogen : concentration was decreased with the increase of the values of these factors concerned with vegetation and its environment. Especially, forest volume and its increment were closely related in the amount of total nitrogen in the stream water.Further measurement was conducted along the Aratagawa River in Gifu City to examine material run-off from a watershed in relation to a land use change following urbanization. The result suggested that run-off was affected by not only the area extent of each land use type but also their spatial arrangement within the watershed.