| Since 2002,ecological projects have been implemented in karst areas in southwest China.However,the restoration effect at ecological barrier Karst fault depression basin in the upper reaches of Pearl River is not clear.Moreover,the drastic climate changes in this area since the 21st century,especially the once-in-a-century drought in 2009,make the water and sediment in the basin difficult to predict.Nandong Underground River Basin is a typical basin of faulted basin,which is located in the economic development center of southern Yunnan.It is under great pressure of population,fragile ecological environment and great harm of soil and water loss.To study the response of water and sediment to climate and land use/cover in the Nandong Underground River basin and predict the future trend is not only conducive to understanding the dynamic characteristics of water and sediment in the faulted basin,but also can provide scientific basis for ecological restoration and soil loss control and restoration.(1)Based on multi-time series analysis,this study analyzed the climatic and hydrological characteristics of the basin and the response of water and sediment to climate change.The characteristics of land use/cover change in the basin were analyzed based on multi-period land use,and vegetation-topographic factor(VTF)and vegetation-topographic landscape index(VTLI)were constructed to analyze the response of water and sediment to land use/cover change.The Budyko model framework was improved to quantitatively assess the contribution rate of climate and land use/cover to watershed water and sediment change.Statistical downscaling(SDSM)and Ca-markov method were used to predict the future climate and land use/cover change.On this basis,SWAT model was used to reveal the response mechanism of water and sediment to climate and land use/cover change in the future.The main research conclusions are as follows:(2)Analyze the response of runoff and sediment transport to climate change.The standardized rainfall evapotranspiration index(SPEI)and standardized runoff index(SRI)showed that the climatic drought and flood tended to be humid,while the hydrological drought and flood tended to be arid.SPEI has a 1-year lag for SRI on the annual scale,with a maximum lag of 3quarters on the seasonal scale and 4 months on the monthly scale.The rainfall erosivity showed no significant upward trend,and the sediment transport had a lag time of 1 year on the annual scale,no lag response on the seasonal scale,and a lag of 2 months on the monthly scale.Analyze the response of runoff and sediment transport to climate change.The mean values of standardized rainfall evapotranspiration index(SPEI)and standardized runoff index(SRI)at all time scales were around 0.The larger the scale,the less obvious the drought and flood situation,climate drought and flood tend to be humid,while hydrological drought and flood tend to be drought.The annual sediment transport has a lag time of 1 year for rainfall erosivity,and the monthly sediment transport has a lag time of 2 months for rainfall erosivity.The correlation coefficient between seasonal sediment transport and rainfall erosivity is the maximum 0.693,but there is no lag response.(3)Based on multi-period land use data,land use/cover characteristics of the basin were analyzed.After 2000,land use conversion became active,mainly for arable land and grassland to urban land,while the area of woodland and water body changed little.Except that the center of gravity of cultivated land shifted from northwest to southeast and forest land shifted from west to south,the center of gravity of grassland,water body and construction land shifted from east to northwest.The area of grassland and cultivated land in low altitude area decreased,while the area of construction land in low altitude area increased significantly.The conversion of grassland and cultivated land into construction land is mainly located in the low-slope area.The development trend of construction land is northwest slope and cultivated land is east or southeast slope.(4)VTLI and VTF were constructed to analyze the responses of runoff and sediment transport to land use/cover change.Except for LPI and DIVISION index,VTLI factors were significantly correlated with water and sediment(p<0.05).VTLI’s PLSR interpretation of runoff increased from0.639 to 0.778,and that of sediment transport decreased from 0.809 to 0.613.The influence of underlying surface variation on runoff yield increased,while that of sediment yield decreased.VTF restoration sustainability accounted for 31.05%,mainly located in the basin mountain fringe area.The unsustainable VTF is only 2.48%,mainly distributed outside Mengzi City.The ecological restoration project of the Nandong underground river basin has achieved remarkable results.The change of underlying surface increases the runoff yield and reduces the sediment yield.(5)Improve the Budyko framework to quantitatively assess the contribution of climate and land use/cover to water and sediment change in the basin.Tmax is the main climatic factor affecting the underlying surface,and rainfall is the main climatic factor affecting the sediment concentration.Climate change led to a 77.6 percent drop in runoff;Ecological restoration resulted in a 51.3 percent reduction in sediment.The sensitivity of runoff to the maximum temperature was the highest(3.61).The sensitivity of sediment transport to rainfall and NDVI was 5.37 and 3.26,respectively.Climate change is the main factor of runoff reduction and ecological restoration is the main factor of sediment transport reduction.(6)SDSM and Ca-Markov methods are used to predict climate and land use/cover change in future scenarios.RCP4.5/RCP8.5 Mean temperature,maximum temperature,minimum temperature and average annual rainfall increased by 2.0℃/3.5℃,2.3℃/3.9℃,1.6℃/2.9℃and 28.8mm/74.1mm compared with 2018,respectively.The variation of temperature and rainfall in each season was spring>winter>summer>autumn.The intermonthly temperature increase was mainly concentrated in January to May,and the intermonthly rainfall increase was mainly concentrated in July and October.During 2015-2030,the ecological development scenario is mainly for building land and cultivated land to be transformed into water and forest land.During2030-2050,the decrease of cultivated land area under economic construction scenario is much greater than that under suitable development scenario.In the2050-2080 period,the suitable development scenario lays more emphasis on water protection and balanced development of construction land.(7)SWAT model was adopted to reveal the response mechanism of water and sediment to climate and land use/cover change in the future.The model was calibrated and verified,and the monthly runoff R~2and NS were both above 0.6,and the monthly sediment transport R~2and NS were above 0.55.The simulation results met the requirements.Two climate scenarios(RCP4.5and RCP8.5)and three land use/cover scenarios(2015,2030 and 2050)were set to simulate future runoff and sediment transport.Based on the land use in 2015,the average annual runoff from 2021 to 2100 under RCP4.5 and RCP8.5 scenarios increased by 0.01%and 1.69%compared with the change period(2003-2018).The average annual sediment discharge changed by-2.8%and 4.68%,and the runoff and sediment discharge changed little under the two discharge scenarios.The partial runoff with increasing rainfall and partial runoff with decreasing temperature cancelled each other out.Under the land use/cover scenarios in 2030 and 2050,the annual runoff will increase by 16.8%and 18.6%compared with 2010.The average annual sediment discharge increased by 37.4%and 32.7%.In the future scenario,land use/cover change has a far greater ability to change water and sediment content than climate change. |
PDF Full Text Request