Land use patterns from 1970 to 2000 were analyzed to determine the changes in the study area.On this basis

long-term hydrologic impact assessment(L-THIA)model was used to evaluate the long-term average non-point source(NPS)pollution loads and to simulate their spatial distri-butions.Unit contribution index(UCI)was introduced to assess the impacts of various land-use categories to annual average NPS pollutant production.The results are as followed.(1)From 1970to 2000

farmland

forest and grassland were the primary land use types in this area

ecolo-gical land area decreased dramatically with rapidly increased nonecological lands

implying intensified human activities and the tendency of ecological recession.(2)During the past 30years

NPS pollution loads rose from 0.34million tons to 0.62million tons;the areas of high TN and TP loads distributed mainly in the middle reach but has expanded to the upper reach considerably.(3)Contribution rate(CR)and UCI were calcu-lated to analyze the relationship between land use types and NPS pollution

with UCI reflecting more closely the impacts of land use change to NPS pollution.Farmlands had the most significant impact on NPS pollutants and were identified as the main source

followed by residential and industrial lands

and forest and grasslands.