| The effect of land Use and its change on non-point source pollution are hotspot areas for rsearch domestic and overseas. The nitrogen and phosphorus nutrient in water is the important influential factor that causing non-point source pollution.The Taihu Lake iregion is economically developed,its land use changed largely and water environment problem is serious,it has great significance to research the effect of land use change on the export of nitrogen and phosphorus nutrient in Taihu Lake basin. This study selects Zhongtian river watershed which is a typical small watershed of hilly region in Taihu Lake as the study area.By analyzing the land use change of 1979,1988, 2001,2010 and 2013 and forecasting the fiiture land use situation, applied the AnnAGNPS model to simulate the export of nitrogen and phosphorus nutrient from 1975 to 2018. Then analyzed the changes and regularity of nitrogen and phosphorus nutrient export in continuoiis periods. EHscussed the effect of land use change on nitrogen and phosphorus nutrient export through the aspects of the land use type,area ratio, landscape pattern change and intensity. The main Conelusions were as foUowing:1. Analysis of land use change in Zhongtian river watershed. The main land use types in Zhongtian river watershed were forest, farm land and urban land from 1979 to 2013, all of the three types took mow than 85% of the whole area,along with the improvement of the urbanization level and ecological environment, this proportion will continue to rise and gradually being stably in the ftiture. The largest change of land use type were grass and bare land,followed by farm land and urban land,finally to forest and waters. Land use change would be mainly concentrated in the increase of urban land and water area, and the great decrease of bare land and grassland area in the future. At the same time, spatial difference of landscape would be strengthened,the advantaged lanscape would turn from the forestand farm land into forest, urban and farm land, and landscape’s evenness increased in this watershed. There was a little change of the land use intensity from 1979 to 2013 overall, 2013’s land use intensity was the largest, while 2010’s was the smallest.1979 to 1988 was the land use adjustment period,1988 to 2001 was the slow development period, from 2010 on the ecological environment began to recover better and 2013’s land use intensity increased, the increase amount in the future 2018 would be mainly from the increase of the urban land use intensity.2. The construction of AnnAGNPS model in Zhongtian river watershed. The parameter’s sensitivity analysis showed that CN, LS, K and the soil, crop and fertilizer parameters were the key parameters affecting the simulation results. Through calibration and verification of simulation results for the model showed that:Annual runoff in the calibration and validationof simulation deviations were basically controlled within 5%. The export of nutrients in the calibration of the model for TN simulated and measured deviations was controlled in less than 20%, Ens reached 0.97, R2 reached 0.9913; TP’s simulation deviations are all between 30% to 40%, but the model for simulation efficiency coefficient Ens TP was 0.83, R2 was 0.8676. In the validation period, the model simulated TN’s Ens of 0.88, R2 was 0.8906, TP’s R2 was 0.8149. Which showed that AnnAGNPS had good adaptability in Zhongtian river watershed, and the model could be used to analyze the regularity of non-point source pollution situation and the nitrogen and phosphorus nutrient export of Zhongtian river watershed. At the same time, because the Zhongtian river watershed is a typical small watershed of hilly region in the Taihu Lake watershed, its climate, soil and other condition is similar to other small watershed of Taihu Lake watershed, therefore it can provide reference for AnnAGNPS’s popularization and application in the taihu lake watershed.3. Applying AnnAGNPS model combined with the main historical years’land use situation, simulated the nitrogen and phosphorus nutrient export in 1975-2018. Results showed that the annual export loadings of TN and TP change with rainfall and runoff in the 44 years from 1975 to 2018, the amount of change is high in May to September and the rest months (1-4,10-12) is low. The maximum export of TN is in July(1255.79kg), while the minimum is in December(412.41kg). The export loadings of TN and TP changes with runoff obviously in the past years, TN’s maximum export is in 1991(111138.34kg) and the minimum is in 1997(7262.80kg); Export levels of TP is at a relatively reduced level in 1975-2013, in addition to larger than the individual basic runoff year maintained at 7000～ 8000 kg/year with the same maximum value in 1991(19363 kg), the minimum value is 1997, only 982kg. While the loadings of TN and TP decreased distinctly than the past in recent years, which benefit from environmental regulation and the increase of the residents’ environmental awareness. The export loadings of TN and TP yet will be significantly reduced in the future periods (2014-2018).4. The land use change had great influence on the nutrient export. The spatial distribution of land use types influenced the nitrogen and phosphorus nutrient export. The areas of high nutrient export intensity was concentrated in the farm land and urban land areas on the either side of the river channel, which showed that the farm land and urban land were the key area of nitrogen and phosphorus nutrient export, the two land use types’ average annual export intensity was significantly higher than other land use types in each period. Land use types’ TN export from strong to weak order was farm land, urban land, forest, bare land, grassland and water, TP’s order was urban land, farm land, bare land, forest, grassland and water; in the furure (2014～2018), with the decrease of farm land, the use intensity and the improvement of urban development, etc., the farm land and urban land’s TN intensity would be increased slightly, TP would reduce a little, while other land use types would have different degree of reduced. Through the multiple regression analysis method, we got that the TN and TP export concentration had little relationships with a single land use type but various land use types. The concentration of TN and TP had obvious correlations with landscape pattern change which showed that the impact of landscape pattern change on TN and TP was the same with land use area proportion, not a single factor, but complex internal influence mechanism. Land use intensity had a clear correlation with the concentration of TN and TP export, the greater the intensity of land use, the higher the concentration of nitrogen and phosphorus export. Nowadays with the acceleration of development process of urbanization and the improvement of ecological environment, it should be taken into account about land use changes in the watershed, timely adjust policy and plan, focusing on maintaining and improving the ecological environment, control of non-point source pollution from the fundamental. |
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