| Taihu basin is developed regions and population gathering area in China.Keeping the water quality good is significant for economic development and residents living condition.Water quality of rivers inflowing into the Taihu Lake have important effects on the lake water quality in the basin.Along with the social and economic rapid development,land use and land cover of Taihu Basin change dramatically.This leads to the changes on non-point source pollution and then impacts water quality of major rivers inflowing into the lake.This research taked the upstream Xitiaoxi watershed as the research object and simulated the region water quantity,water quality status with the SWAT model.Effects of rainfall and land use changes on water quality were analysed to provide reference for land use planning and water quality water environment treatment in similar watersheds in Taihu Basin.The main results are as follwos:1.The Soil and Water Assessment Tool model for Xitiaoxi River was established based on DEM,land use,soil and meteorological data.Monitoring data of the river runoff and water quality between 2002 and 2012 was used to calibrate and validate the model parameters.The Re values were less than 11%and correlation coefficient R2 were greater than 0.8 and the Ens were greater than 0.75 for river runoff simulation.The Re values were less than 10%and correlation coefficient R2 were greater than 0.65 and the ENS were greater than 0.60 for TN simulation.The model simulation accuracy met the needs of study in this area.2.According to the land use data obtained from image interpretation in 2002 and 2008,the urban land use has increased 44.9km2,the farmland area has reduced by 11.78km2 and the forest land has decreased 18.31 km2.The change of land use in the river basin is mainly changed from farmland to urban land,followed by conversion of forest land into farmland.The sub basins with higher proportion of the urban and the higher proportion of farmland are mainly concentrated in the northern part of the basin,and the urban development is dominated by the sub basins with higher proportion of urban land use,while the farmland is more growing in the sub basins with higher proportion of farmland.3.Time variation characteristics of runoff and water quality index:DO,NH3-N,TN,TP were analysed based on the monitoring data.The relationship between runoff and water quality was figured.In addition,spatial distribution characteristics of TN and TP concentration were figured based on short-term monitoring data of 13 sites in the watershed.Precipitation was on the rise from 2003 to 2012.Precipitation value high in June,July and August within the year.Runoff had similar characteristic with precipitation value.Monthly rainfall and runoff peak increased year by year between 2002 and 2012.Considering water quality up to the standard Ⅱ according to the Surface water environmental quality standard(GB383 8-2002)with DO,NH3-N and TP,river water quality was not bad and getting better from 2003 to 2008.But when take TN into consideration,the rate of reaching the standard went down terribly.TN is the main pollutants in this watershed.When the runoff was less than 20m3/s,the concentration ranges of TN and NH3-N were large and there was not any obvious regulation.When the runoff was greater than 20m3/s,concentration of TN was less than 3mg/L and concentration of NH3-N was less than lmg/L.TN concentrations increased gradually from upstream to downstream.TP concentration near watershed outlet was high than that inside the watershed.4.The change of annual runoff depth of the river basin had a good response to the change of urban land use.There was a good linear relationship between them.The larger the urban area changed in sub-basin,the larger the annual runoff depth changed.There was a certain relationship between TN loadings changes and farmland changes.The larger the farmland area changed,the greater the TN loading changed.5.Precipitation in the catchment had a great influence on TN output.With the increase of precipitation,the output of TN in the catchment increased.In the non-growing period,TN output increased more with the increase of precipitation.Precipitation also has a certain influence on the TN concentration of the river.As the precipitation increased,the TN concentration increased.It increased more during the non-growing period.6.After the land-use changed,the flow of the river basin increased.Annual average TN output also increased.During the growth period,the monthly average TN output increased,and the monthly average TN output during the non-growth period decreased.The overall TN concentration increased.TN concentration increased during the growth period and decreased during the non-growth period.The change of TN output is mainly affected by the change of farmland area.When the area of farmland decreased,the increase of urban area would reduce the decrease of TN output. |
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