Water Quality Preservation Effect of Riparian Forests in Watersheds with Dairy Farming Areas in Eastern Hokkaido

Abstract

The effects of riparian buffer zones consisting of forest and wetland on water quality functions were investigated by comparing the river water quality in two watersheds with dairy farming grassland in a large-scale dairy farming area of Eastern Hokkaido. The nitrogen, phosphorus and suspended solids (SS) components from the two watersheds, one without riparian buffer zones (MB) and the other with riparian buffer zones (HY), were compared at normal flow and at rainfall runoff periods. Total nitrogen (T-N) and NO3−N concentrations were lower in the water from the river with riparian buffer zones. For the water sampled during the rainfall runoff period, all the water quality indicators showed lower concentrations for the river with riparian buffer zones than for the river without riparian buffer zones. The amount of nitrogen that flowed out of land into the river (nitrogen load) in the watershed with riparian buffer zones was 70% of that of the watershed without riparian buffer zones. The differences in water quality between the two watersheds are assumed to have resulted from water purification by the riparian buffer zones on one of the rivers. It is assumed that the riparian trees adsorbed nitrogen and that the riparian wetlands removed nitrogen and captured suspended solids, both of which contributed to purifying the runoff water. Based on the above findings, it can be determined that the conservation of riparian buffer zones is effective in preserving the river water quality on a watershed-wide scale in watersheds consisting of dairy farming grassland.