Delineating nitrogen-vulnerable zones within a catchment in the region around Lake Kasumigaura, Japan, by using a modified GIS model

Abstract

Agriculture, including livestock husbandry, is a major source of nitrogen (N) in surface waters. The difficulty of detecting source areas of N on a catchment scale makes it a challenge to prioritize sites with N pollution of surface waters that require urgent management. This study aimed at delineating N-vulnerable zones within a catchment on the basis of pathways of N transport from each site to the river. Topographic data (differences in elevation between adjacent cells with a threshold value of 3 m) were used in a GIS model to create a spatial distribution of gaps in shallow groundwater in a catchment. Areas between the river and the first gap were considered as less vulnerable zones (LVZs), where loss of N originating in LVZs by denitrification in riparian subsoil was expected. The other areas were considered as more vulnerable zones (MVZs). The area ratio of MVZ (R_MVZ) varied between 6.8% and 52.4% among catchments. In both zones there was a linear relationship between potential N loads and N concentrations in effluents among catchments. The MVZ/ LVZ ratio of both slopes was 6.25. The N concentration in river water was calculated from concentrations in effluents from both zones and R_MVZ. These results indicate that in catchments with R_MVZ>13.4%, N load reduction in MVZs would be most efficient at lowering N concentrations in river water, and in catchments with limited MVZs, reduction in LVZs would be also helpful.