Influences of Riparian Land Use on Nitrogen Concentration of River Water in Agricultural and Forest Watersheds of Northeastern Hokkaido, Japan

抄録

This study examines the relationship between nitrogen concentration in river water and agricultural land use in a large-scale upland farming region, with particular emphasis on the influences of riparian land use in agricultural watersheds on the nitrogen concentration in river water. In 21 watersheds in Northeastern Hokkaido nitrogen concentration was surveyed during normal river flow. Cropland (upland and grassland) accounted for 1% to 88% of each watershed, and forestland for 7% to 98%. The survey was conducted six times during the summer and autumn of 2006 and 2007. There was a close correlation between the percent of cropland in each watershed and nitrogen concentration in river water. Use of land for agriculture was shown to elevate the nitrogen concentration in river water. Next, using the buffering function of ArcGIS software, we set three buffer zones demarcated from the channel centerline outward: BZ₂₀, BZ₆₀, and BZ₁₀₀. The subscripts indicate the width of the zone in meters, with the riverbank as 0 m. The ratio of percent of forestland in BZ₂₀ to the percent of cropland in the watershed was defined as the Land Use Index (LUI), and its relationship with nitrogen concentration was investigated. This revealed a strong negative correlation between LUI and total nitrogen concentration, a correlation that could be approximated by linear regression. This suggests that even when cropland area accounts for much of a watershed, establishment of much riparian forest can reduce the nitrogen concentration in a river.