Abstract
River water quality under normal flow conditions was investigated on two watersheds with complex agricultural landuse in Hokkaido, Japan. The main forms of agriculture are upland farming and livestock farming. If all livestock manure is composted and returned to the field, its amount will exceed the required amount of fertilizer in both watersheds. The excess fertilizer may therefore cause water quality degradation. It is highly probably that the location of livestock yards and riverbank landuse strongly influenced the discharge rate of livestock manure generated in the watershed to the river. The river water quality in the watershed, including rice paddies, showed apparent influences of irrigation water. During the irrigation period, the concentration of nitrogen and phosphorus of the river water itself was diluted and lowered by irrigation water. However, the output of the load of total nitrogen from the rice paddy was larger than the input, because of the good quality of irrigation water.
