Mapping the potential annual total nitrogen load in the river basins of Japan with remotely sensed imagery

The increase of nutrient loads such as nitrogen and phosphorus to a river due to land cover changes in surrounding areas has been one of the major sources of water pollution or eutrophication. Monitoring the influent nutrient load from river basins to rivers is now crucial in the management of river basin environments. The monitoring is not easy, however, because it requires spatial and temporal measurement tools for land cover changes in the river basin and water qualities, and also it requires models relating them.In this study, we first analyzed the relation between the land cover types estimated from monthly maximum Normalized Difference Vegetation Index (NDVI) imagery calculated from NOAA Advanced Very High Resolution Radiometer (AVHRR) imagery and the annual total nitrogen load discharged from river basins. We found that the runoff load factor from urban areas is higher than those of forested areas. We also found that the impacts of land cover such as plantation and field weed communities on the total nitrogen load of each river are higher than the impacts of other land cover types such as Beech and Camellia japonica community type.Finally, we produced two advanced maps of the potential annual total nitrogen load (PTNL) index and the potential annual total nitrogen load for each river basin area (PTNL/area) index by considering the relationship between the land cover types and the annual total nitrogen load discharged from river basins in Japan. The PTNL map will be useful for the risk assessment of total nitrogen load impact on lakes and the sea through rivers from each basin. The PTNL/area index, which considers the effects of river basin areas, will allow evaluation of the state of river basins.