Research On The Response Pattern Of Hydrology And Water Quality In The Three Gorges Reservoir Area Under Changing Environment

Under the background of significant global climate change,the complexity and uncertainty of the hydrologic environment of the Three Gorges Reservoir Area(TGRA)are increasingly evident due to the complex underlying surface of the basin and the increasing human activities since the establishment of the Three Gorges Reservoir(TGR).It is of great theoretical and practical significance to explore the law of hydrological and water quality response in the TGRA under the changing environment,so as to maintain the utilization of water resources and fine management of water environment under the changing environment,and better cope with the huge water environment challenge brought by global change.In this paper,based on the fine simulation framework of non-point source pollution,a numerical model of watershed hydrology and water quality was constructed to serve the changing environment.The spatial and temporal characteristics of hydrological processes and water quality in the changing environment were investigated.The main research achievements are as follows:(1)Land use/cover change and its impact on hydrological processes and water quality,and climate change and its impact on hydrological processes and water quality were systematically reviewed.A large number of research achievements have been made in the study of the changing environment and its influence on the hydrological process and water environment of the watershed.Model-based simulation is one of the most widely used methods.At present,most studies only focus on the impact of climate change or land use/cover on a certain aspect of the hydrological process or water environment in the basin,and there are few studies to quantitatively distinguish the difference between the impact of land use/cover and climate change on the hydrological process and water environment in the basin scale.There is no systematic quantitative analysis of land use/cover and climate change and their hydrological processes and water quality response laws for the TGRA.(2)Based on the passive attitude model of land use/cover,regression analysis model,land use/cover transfer matrix,spatial analysis and mathematical statistics methods,the differences of land use/cover distribution and change trend characteristics of the TGRA were analyzed from the perspectives of spatial distribution and temporal variation.With the acceleration of urbanization,the construction and operation of the TGR and the promulgation and implementation of ecological protection measures such as "conversion of cropland to forest and grassland",the construction land area and water area of the TGRA have significantly expanded,and the overall cultivated land area and grassland area have shown a declining trend,while the proportion of forest area has shown an increasing trend.The rate of change in construction land and water area suddenly changed in the mid-2000 s.The general trend of land use/cover change was the transfer of cultivated land to forest,grassland and construction land.The expansion of construction land mainly came from the occupation of cultivated land,forest and water area.(3)Using Mann-Kendall test,distance average analysis,method of moving average,Morlet wavelet function and Kriging interpolation,the trend change,periodic variation and spatial distribution characteristics of precipitation,air temperature(daily maximum air temperature and daily minimum air temperature)and relative humidity in the TGRA from 1980 to 2018 were studied respectively.The intensity variation of extreme climate events and characteristics of extreme climate change of the TGRA were analyzed.In the past 40 years,the interannual variation and annual distribution heterogeneity of local climate in the TGRA have been gradually enhanced,and the periodic variation is obvious.Precipitation showed a decreasing trend,and the difference of annual distribution pattern was gradually enhanced.The periodic change of precipitation showed quasi-16-year change and quasi-22-year change,and the spatial distribution showed a trend of “low-high-low” from west to east.The overall air temperature showed an increasing trend,and the daily maximum air temperature and daily minimum air temperature had obvious abrupt changes,which were located in 2001 and 1998 respectively.The periodic changes of air temperature were mainly in the quasi-23-year and quasi-29-year periods,and the temperature in the tail region of the TGRA was higher than that in the middle and tail regions.The relative humidity of the TGRA was relatively large,and it tended to increase after the impoundment of the TGR.Extreme precipitation event and extreme hot weather were increasing,while extreme cold weather events were decreasing.(4)Based on the technical framework of fine simulation of non-point source pollution,a numerical model of the response law of hydrological and water quality in a watershed under changing environment was constructed.LH-OAT method was used for parameter sensitivity analysis.Daily runoff monitoring data from 10 hydrological monitoring stations and monthly total nitrogen and total phosphorus water quality monitoring data from 8 water quality monitoring stations in the TGRA from 2010 to 2013 were selected for model calibration validation.The determination coefficients and Nash efficiency coefficients of the monitoring value and the simulated value of each hydrological station and water quality monitoring station in regular period and verification period were mostly higher than 0.5,and the matching degree between the simulated value of runoff and water quality and the measured value basically met the requirements of model simulation accuracy.(5)Based on the established hydrological and water quality numerical model of the TGRA,8 landscape land use/cover maps and daily meteorological data of 22 meteorological stations from 1980 to 2018 were used as input conditions for the model.The effects of land use/cover changes on runoff depth and nitrogen and phosphorus loads of land surface units in the TGRA were quantitatively analyzed from the spatial and temporal perspectives of different scales,and the effects of different land use/cover types on runoff and nitrogen and phosphorus pollution loads were analyzed.The land use/cover change had a small impact on the total runoff of land surface unit in the TGRA,but a large impact on the total nitrogen(TN)load and total phosphorus(TP)load.Affected by the construction and operation of the TGR,the abrupt change point of hydrologic and water quality factors was located around 2005.In terms of spatial variation,the runoff depth in the head(407.2 mm)and the middle region(359.3 mm)of the TGRA was higher,and the average runoff depth in the left bank region(358.9 mm)was higher than that in the right bank region(353.9 mm).The load of nitrogen and phosphorus was higher in the middle and tail region of the TGRA,and the TN load was lower in the head region of the TGRA.The contribution rate of different land use/cover types to runoff was forest(55.82%)> cultivated land(32.60%)> grassland(11.00%)> construction land(0.58%),and the contribution rate to nitrogen and phosphorus loads was cultivated land > forest > grassland >construction land.The runoff generation ability showed the law of grassland(487.9 mm)>forest(425.3 mm)> construction land(385.0 mm)> cultivated land(262.7 mm),The TN load generation ability showed the law of cultivated land(31.3 kg/ha)> forest(22.3 kg/ha)>construction land(5.8 kg/ha)> grassland(5.1 kg/ha),while the TP load generation ability showed the law of cultivated land(0.312 kg/ha)> grassland(0.176 kg/ha)> construction land(0.124 kg/ha)> forest(0.035 kg/ha).(6)The land use/cover in the current year of 2018 and the daily meteorological data of 22 meteorological stations from 1980 to 2018 were used as input conditions of the model.From the perspective of time variation trend,the variation trends of land surface runoff and nitrogen and phosphorus load in the TGRA in different years,seasons and water periods were quantitatively analyzed.From the perspective of spatial variation characteristics,the effects of climate change in the 1990 s,2000s and 2010 s on land surface runoff,total nitrogen and total phosphorus were quantitatively analyzed based on meteorological conditions in the 1980 s.Under the influence of climate change,the variation trend of annual average runoff depth and TP in the land surface unit of the TGRA was similar to that of rainfall,presenting a "W" type fluctuation trend,while that of TN presented an upward trend.Runoff depth and nitrogen and phosphorus loads had obvious seasonal distribution difference.In terms of spatial distribution characteristics,influenced by climate change,the runoff depth of the TGRA at the head region,middle region and on the left bank region showed a decreasing trend,while the runoff depth at the tail region showed an increasing trend.Except for few areas of the middle region of the TGRA,TN load in most areas of the TGRA showed an increasing trend.the TP load in most areas of the TGRA was decreasing Except for few areas at the head and tail regions.The influence of climate change on TP load in the middle region and the left bank region of the TGRA was higher than that in the head and tail regions and right bank region.According to Pearson correlation coefficient analysis,in the first decade of this century(2000s),climate change had the most significant impact on the runoff depth and TP load in different regions,while in the past 20 years,the impact of climate change on TN load in different regions has been increasing significantly.(7)The comprehensive influence and contribution degree of land use/cover and climate change on the variation trend of runoff depth and nitrogen and phosphorus pollution loads in the whole basin and local areas of the TGRA were quantitatively analyzed.The comprehensive effects and contributions of land use/cover and climate change on the variation trend of runoff depth and nitrogen and phosphorus pollution loads in the whole basin and local regions of the TGRA varied greatly.Under the comprehensive influence of land use/cover and climate change,the runoff depth of the river basin showed a decreasing trend(-19.9 mm,-5.50%),and the influences of climate change on the change of land surface runoff of the whole TGRA and local regions were dominant.Under the comprehensive and separate influences of land use/cover and climate change,TN load showed an increasing trend(6.750 kg/ha,92.10%)in the whole basin and local areas of the TGRA,and the impact of land use/cover change and climate change on TN load change of the whole basin was similar.The TN load in the head region and left bank of the TGRA was greatly affected by land use/cover,while the TN load in the middle region,tail region and right bank of the TGRA was greatly affected by climate change.Under the comprehensive influence of land use/cover and climate change,TP load in the TGRA showed an increasing trend(0.064 kg/ha,83.21%),while in some regions(middle region and right bank region)showed a decreasing trend.The influence of land use/cover change played a leading role in the change of TP load in the whole basin.In terms of regions,TP load in the head region,tail region and the left bank of the TGRA was significantly affected by land use/cover,while that in the middle region and the right bank region was significantly affected by climate change.This study revealed the law of the hydrological water quality response of the TGRA under land use/cover and climate change by constructing a numerical model of the hydrological water quality,and was innovative in clarifying the internal relationship between the hydrological water quality and the changing environment.In terms of scientific questions,the main influencing factors of hydrological process and water quality change in the TGRA were answered.In terms of application management,this study can provide scientific support for watershed water resource utilization and fine management of water environment in response to changing environment.