A study was carried out with the upriver region of the Taihu Basin as a subject.Land use information about the area was acquired by means of TM/ETM image interpretation,and the upriver region was divided into small catchments with the aid of the IDRISI software.Field monitoring was performed in some selected catchments for analysis of relationships between the land use and river water quality in the catchments,and estimation of total concentration of the nonpoint source nitrogen in runoff from forest land and farmland.Based on the analysis and estimation,a cellbased model for simulating nonpoint source pollution is developed by combining the distributed hydrological model with measured values of the typical catchment.The model is well calibrated and validated with the monthly runoff data in the period from 1980 to 2000 and then used to simulate spatial distribution and flux of nonpoint source nitrogen to the Taihu Lake.Results show that the mean concentration of total nitrogen is 0.778 mg L^-1 in runoff from forest land,and 2.518 mg L^-1 from farm land.The modeled runoff volume and total nitrogen fluxes match well the observed ones,respectively,in the validation area.The average error value is 13% for the former,and 11.6% for the latter.The nonpoint source total nitrogen flux from the upriver region to Taihu Lake is 7 632 t a^-1,contributing about 40.8% to the total nitrogen flux of the entire basin.The flux varies with the type of land use.For farm land,the total nitrogen flux is 4 289 t a^-1,accounting for about 24.22% of the total nonpoint source nitrogen output,and for forest land and builtup land,the total nitrogen flux is 1 849 t a^-1 and 1 270 t a^-1,for about 24.22% and 16.64%,respectively.