The Difference of Agricultural Land Use in Watersheds and Long Term Fluctuation on the River Water Quality

Abstract

This study was conducted to determine the relationship between the characteristics of fluctuations in river water quality at the normal water level and the proportion of agricultural land in watersheds of Eastern Hokkaido that have different agricultural land use, based on the 20 years observations. The investigations were carried out during late August to early September in 1992, 2003 to 2006, and 2012 at 35 watersheds in two areas, and NO₃-N, EC of river water and river discharge were measured. The Tokachi area (24 watersheds) is located in the northwestern part of the Tokachi General Subprefectural Bureau, the main land uses are upland and dairy farming. The Nemuro area (11 watersheds) is located in the western part of the Nemuro Subprefectural Bureau, the main land use is large-scale dairy farming. In the 20 years since 1992, NO₃-N concentration and EC have shown increasing tendency in some of the investigated river watersheds. The river water quality in some watersheds investigated in 2004, immediately before the full implementation of “The Law on Animal Waste Regulation”, greatly differed from the observation results of several years before and after 2004. When the two areas had the same proportion of agricultural land, the NO₃-N concentration in the river water tended to be higher in Tokachi area than in Nemuro area. This is attributed to the proportion of upland fields, which require large inputs of chemical fertilizer in the Tokachi area. EC, however, tended to be high in the Nemuro area, even for watersheds with similar proportion of agricultural land. This suggests that the river water in the Nemuro area contains many materials of geological origin, in addition to containing NO₃⁻.