Abstract
Recent years, nitrogen effluent load from a watershed with livestock farming has become serious problem. So, simulation models have been developed to manage water quality in appropriate state in the watershed. However, such a model can not be applied to a watershed where no hydrological and water quality data in the past are available. From this reason, we developed a nitrogen mass balance model depending on present hydrological and water quality data from Sept. 30, 1998 through Sept. 15, 1999 to estimate nitrogen effluent load from a watershed with cow barn and forage field. With this model, we calculated total nitrogen load and concentration in drainage out of the watershed from 1987 through 2030. As a result, the load and concentration in 1998 were estimated to be five times larger than those in 1989 because much amount of raw cow dung and urine was thrown into the forage field since 1989. However, when two measures, such as composting the dung and taking manure out of the watershed, and improving feed self-sufficiency rate, are employed, the load and concentration in 2030 are predicted to reduce to 60% of those in 1998.
