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

研究了日本中部几个流域的植被和土地利用状况对水质的影响。沿木曾川的一个分支希达川和津津津川的一个分支纳迈川的一个小支流对河水进行了沿着采样。两条支流流域的源头地区都有放牧用的草地。无积雪的放牧期土壤总氮含量较高。在禁牧期，该水平变低。最高水平的总氮被发现在最上游和下游下降。这一结果表明植被对水质有净化作用。现场测量进行了沿着上游的希达河，以检查这种水净化的机制。棕壤面积、万年青针叶林面积、单位面积蓄积量和蓄积生长量是影响土壤全氮浓度的重要因子，其浓度随与植被及其环境有关的因子值的增加而降低。特别是，森林蓄积量及其生长量与河水中的总氮量密切相关。进一步测量了沿着岐阜市新田川流域的物质径流，探讨了与城市化后土地利用变化有关的流域物质径流。结果表明，径流量不仅受各土地利用类型面积大小的影响，而且受其在流域内的空间布局的影响。

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.