| The shortage of water resources and deterioration have been the severe water problems around the world nowadays.At the same time,land use and climate change and the consequent effects on water environment have come to the forefront in the political and academic community,and become the frontier and hotspot area of studying effect of changing environments.The Taihu Lake region is economically developed,and has a strong demand for water resources of high qulity.However,the region's water environmental problem has been seriously amplified due to the dramatically change of land use as well as effect from climate change.Consequently,it has great significance to research the effect of land use and climate change on water environment for the purpose of developping measures of flood control and waterlogging prevention,as well as nutrients load reduction in the future.In this study,the Xitiaoxi watershed in the main triburary of Taihu Lake were selected as the study area.Flow and nutrient component output processes were firstly simulated and validated by the HSPF(Hydrological Simulation Program-Fortran)model.The future land use and climate change scenarios were predicted by climate change prediction model and land use change prediction model,respectively.Finally,the predicted scenarios for land use change and climate change were integrated with the calibrated HSPF model by seperate and coupled ways to analyze the separate and coupled effect of land use and climate change on runoff and nutrients load from annual and monthly scale as well as different flow region.The main conclusions were as follows:Established HSPF model in the Xitiaoxi watershed and identified the key prameters for flow and nutrients simulation in HSPF model by using the DSA(diffetrential sensitivity analysis)method.The results showed that flow was mostly affected by parameters related to groundwater and evapotranspiration,including DEEPFR(fraction of groundwater inflow to deep recharge),LZETP(lower zone evapotranspiration parameter)and AGWRC(base groundwater recession),and most of the sensitive parameters had negative and nonlinear effects on flow.Additionally,nutrient components were commonly affected by parameters from land processes,including MON-SQOLIM(Monthly values limiting storage of water quality in overland flow),MON-ACCUM(monthly values of accumulation),MON-IFLW-CONC(monthly concentration of water quality in interflow)and MON-GRND-CONC(monthly concentration of water quality in active groundwater).Besides,parameters from river systems,such as KATM20(unit oxidation rate of total ammonia at 20? had a negative and almost liner effect on ammonia concentration.MALGR(maximal unit algal growth rate for phytoplankton)had a negative and nonlinear effect on ammonia and orthophosphate concentrations.After calibrating these sensitive parameters,the deviation of simulated and observed annual flow during calibration(2002-2007)and validation(2008-2010)periods at the Gangkou station was-2.1%and-15.3%,respectiovely.R2 and Ens(Nash-Sutcliffe efficiency)were both greater than 0.76 and 0.58 for monthly and storm event scale,respectively.In terms of nutrients simulation,calibration and validation were carried out at the Hengtang and Ancheng station in the same periods(2009.06-2010.06),and R2 were greater than 0.73 and 0.58 for calibratin and validation period,respectively,and Ens were greater than 0.65 and 0.53,respectively.The statistical results for model performance illustrated that HSPF model is capable of simualtion flow and nutrients in the Xitiaoxi watershed,therefore,it can be used to explore the effect of land use and climate change on flow and nutrients.Future land use change scenarios were predicted by the CA-Markov(celluar automata-markov)model based on natural and social economic factors of the study area,and the predicted scenarios were integrated with the calibrated HSPF model to analyze effect of land use change on flow and nutrient load output.Results indicated that future land use categories will be dramatically changed in Xitiaoxi waterhsed,and the changes mainly focus on dramatical decrease of forest and agricultural land,and increase of urban,grass and garden land.The effect of land use on water environment showed that,land use change will result in increase of annual flow in Xitiaoxi watershed,the increased proportion are 7.6%and 10.9%for the two predicted land use compared with base scenarios,respectively.Land use change tend to increase more on the month which has a higher runoff output,and this may further lead to flood happen.Land use change effect on flow during storm event was very distinct,especially for peak flow.Additionally,effect of land use change on different flow level tend to be consistent,making all of them increase,and shows highest impact on the high level of flow.Future land use change in Xitaoxi watershed will decrease ammonia and nitrate load due to the decreasement of forest and agricultural land,and increase orthophosphate load due to the increasement of urban.Compared with base scenario,annual mean nitrate load will be reduced by 9.7%in 2020,and 13.6%in 2030.In parallel,annual mean ammonia load will be reduced by 9.5%and 7.8%.Annual mean orthophosphate load will be increased by 11.4%and 31.1%.Further,effect of land use on monthly nutrients load output show significant differences,and these results can be used to govern pollution.Furthermore,effect of land use change on nutrient components during storm event was also very distinct.In total,future land use change will result in more flow and orthophosphate load,less nitrogen load,and increase the probability of flood happening.Downscale processes were carried out for BCC CSM1.1 dataset in CMIP5(Coupled Model Intercomparison Project Phase 5 multi-model dataset)model to establish the future climate change scenarios,and then the predicted scenarios were integrated with the calibrated HSPF model to analyze the effect of climate change on flow and nutrient load output.Future temperature and rainfall of Xitiaoxi watershed will be increased,the annual mean maximum temperature will be increased by 0.6-1.0?,and annual mean minimum temperature will be increased by 0.4-0.9?,annual mean rainfall will be increased by 3.6-12.6mm.Among different scenarios,rangeability of temperature in RCP8.5 is the biggest,and this scenario has the least rainfall in the future.RCP2.6 scenario has the most abundant rainfall.Climate change will lead to the flow increased by 19.5%-27.4%,and the increasement mainly concentrated in high flow level.Flow in RCP2.6 will be the largest due to the highest rainfall in this scenario,and be the least in RCP8.5 scenario.Change tendency of monthly flow in the future is consistent with base scenario,and the biggest flow will appear in July of every year.In term of effect of climate change on nutrients,compared with the base scenario,nutrients load will be increased in the future.Among these nutrient components,orthophosphate load shows the most sensitive to climate change,increased by 94.8%-114.9%,follow by nitrate load,increased by 32.8%-42.7%,and ammonia increased by 14.7%-18.2%,these different responses between nutrients load to climate change are mainly caused by the difference that they response to temperature and rainfall.In monthly,nitrate load increase dramatically in May,June and July,while other months increase a little.Ammonia load increase slowly in every month,its change tendency is similar with base scenario.Orthophosphate load shows obvious seasonal fluctuation.Land use and climate change scenarios were integrated with HSPF model by coupled ways to analyze the effect of them on flow and nutrient load output.Effect of coupled climate and land use change will result in flow significantly increase in all scenarios,with the range of 30.3%-38.4%,and RCP2.6 is the highest scenario,RCP8.5 is the lowest.Monthly flow in each scenarios is consistent with base scenario.Furthermore,the increased flow in flood periods is considerable,and thus,the watershed may face a severe flood threat in the future.In terms of nutrient components,the effect of coupled cimate and land use change scenarios on each nutrient load will be significant different.Orthophosphate will be increased by 114.8%-138.6%,nitrate will be increased by 18.6%-23.2%,and ammonia will be increased by 8.8%-18.3%.The effect on monthly nutrients load will be simimar with influence on flow,making them show more variation characteristics affected by climage change alone.Additionally,the peak of nutrient components load show hysteresis or advance compared with base scenario due to they mainly affect by climate change.The effect of climate and land use change on flow and nutrient components is different.The coupled scenarios result in the highest increase for flow,followed by separate climate change,and the two types of change will amplify one another's effects.The effect of climate chang on nutrient components is larger than that of land use,and the two types of change will ameliorate one another's effects. |
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