Abstract
Water quality was surveyed at 36 locations in agricultural watersheds of Shari and Abashiri region in Hokkaido to examine how the nitrate nitrogen concentration in the river water there varies with land use. The land-use categories of upland field which includes grasslands and forested land were chosen, because they are the main land-use categories of the subject areas. As land-use indexes, in addition to the generally used percentage of upland field, we used the Connections Number (CN), which expresses land-use agglomeration, and Spatial Continuity (SC), which expresses the spatial continuity of land use of the same kind. Percentage of upland field was found to strongly corelate with nitrate nitrogen concentration of river water, confirming that nitrogen from upland fields influences the nitrogen contamination of river water. A positive correlation was found between the CN and SC of upland field and the nitrate nitrogen concentration of river water, and a negative correlation was found between the CN and SC of forestted land and the nitrate nitrogen concentration of river water. This clarified that, in addition to the percentage of upland field, the distribution of land use, such as agglomeration and spatial continuity of the land use of the same kind, relate to the nitrate nitrogen concentration of river water.
