Ammonia nitrogen (NH4+-N) is one of the three main forms of total nitrogen (TN). Most studies have estimated the load of TN from nonpoint sources instead of one specific form. The relationship between land use and concentrations of NH4+-N in runoff was analyzed using the hydraulic analysis functions of a Geographic Information Systems (GIS), and the annual loads of NH4+-N in the Xitiao River catchment were estimated according to model results. The results suggested that the calculated annual loads of NH4+-N agreed well with the observed ones, which was a good example demonstrating that it was possible to estimate the pollutant loads from nonpoint sources without enough monitoring data. Results also showed that the concentrations of NH4+-N in river waters of forest and arable land in the study area were 0.16 and 0.39 mg/L, respectively. The total nonpoint source load of NH4+-N from 1995 to 2001 (except 1998 and 2000) varied between 2,098 and 7,992 tons/year in the Xitiao, River catchment.