Effect of Land Use Agglomeration on Nitrogen Concentrations in River Water in the Tokachi River

Abstract

Since in the nineties, contamination of river water by nitrogen from cultivated land has been an issue in major agricultural region of Hokkaido. This study examines the effects of the ratios of cropland or forestland to the watershed area on the nitrogen concentration in the river water, taking as its survey area the Tokachi River basin in eastern Hokkaido, Japan. The study also investigates the impact of cropland and forestland agglomeration on the nitrogen concentration in river water. Survey samples were taken at 37 locations on the Tokachi River. Nitrogen concentrations in the river water were measured at all locations in June, August, and October of 2007, when water levels were normal. In addition, a GIS land use analysis was conducted using a land use map of the region with a grid resolution of 100 m. This analysis determined that cropland and forestland accounts 90 % of the total area in the study watershed. Moreover, spatial continuity (SC) was calculated as an index of cropland and forestland agglomeration. SC represents the average area of a contiguous patch of cropland or forestland in this study. Based on the accumulated data, the followings were found; 1) nitrogen concentration in river water are positively correlated with the proportion of cropland, but are negatively correlated with the proportion of forestland. It is clear that the proportion of each land use is a factor affecting nitrogen concentrations in the river water. 2) The correlation between river nitrogen concentrations and SC values is positive for cropland, but negative for forestland. This indicates that nitrogen concentrations in river water likely decreases if the land use pattern is changed: scattering cropland to reduce its patch size, or connecting forestland together to make the forest patch size larger.