| In recent years,due to the combined influence of meteorological factors and human factors,the eco-hydrological processes of the watershed have undergone significant changes.People's attention to the ecohydrological issues driven by meteorological factors and human factors is continuously increasing.Climate change will intensify the hydrological cycle process,and human activities will change the underlying conditions,both of which will change the natural hydrological cycle in the river basin.The study of runoff response under the influence of climate change and human activities has gradually become a hot topic in hydrological cycle research.As an important part of hydrological cycle,evapotranspiration is a coupling process of hydrology and meteorology,and also a connecting process of hydrology and ecology.Accurate calculation of actual evapotranspiration is very important for runoff forecasting,water resources management and hydrological process simulation in the basin.The upstream of Han River is the water source area of the Middle Route Project of South-to-North Water Diversion and Hanjiang-to-Weihe river valley water diversion project in Shaanxi Province.It is of great value to study the evolution law of runoff and the actual evapotranspiration for the stable operation of the water diversion project.The upstream of Shiquan hydrological station in the upper reach of Han River was taken as the research object.Firstly,the evolution law of eco-hydrological and meteorological variables in the research area was analyzed.Secondly,the impact of human activities and climate change on runoff was quantitatively analyzed by SWAT model and elastic coefficient method,and the runoff response under different climate change and land use change scenarios was analyzed.Finally,based on the theory of evapotranspiration complementarity,the model of evapotranspiration complementarity that is suitable for the study area was selected,the actual evapotranspiration was estimated,and the attribution analysis of the evolution of the actual evapotranspiration was conducted.The main achievements are as follows:(1)For precipitation,potential evapotranspiration,runoff,vegetation index and land use,the linear regression,cumulative anomaly,ordered cluster analysis and related analysis methods are used to reveal the evolution of ecological,and hydrometeorological variables.The results show that,the annual precipitation,potential evapotranspiration,and annual runoff in the study area are all decreasing.The runoff at Hanzhong,Yangxian,Lianghekou and Shiquan showed a decreasing trend while runoff at Youshuijie shows an insignificant upward trend.The regional precipitation and runoff both changed suddenly in 1990.The annual average NDVI value showed a significant increase trend,and the NDVI value continued to change from low level to high level The ascending order of NDVI values of different vegetation type is shrub<grassland<evergreen coniferous forest<agricultural land<mixed forest<deciduous broad-leaved forest.Land use transfer mainly occurs among forest land,cultivated land and grassland.The area of forest land,construction land,water area and unused land is increasing,and the area of grassland and cultivated land is decreasing from 1980 to 2015(2)Based on the meteorological data and underling condition data of Hanzhong upstream area and Hanzhong-Shiquan area,SWAT hydrological model was applied,and the parameters were calibrated and verified.The results show that,the runoff sensitivity parameters of the Hanzhong and Shiquan hydrological stations are slightly different.The monthly runoff simulation results of Hanzhong and Shiquan hydrological stations are good in the calibration and validation periods.The Nash-Sutcliffe efficiency coefficient NSE,deterministic coefficient R2 and KGE coefficient are all greater than 0.8.The absolute value of the relative error is less than 14%,which meets the accuracy requirements of the hydrological model.It shows that the SWAT model has good applicability in the area above the Shiquan hydrological station and can be used for the study of runoff response under changing environment.(3)Using six elastic coefficient formulas and the SWAT hydrological model constructed,the quantitative analysis of the contribution rate of the regional climate and human activities to the runoff changes of Hanzhong Station and Shiquan Station was conducted.The contribution rates of climate change to the reduction of runoff at Hanzhong Station and Shiquan Station were found in the range of 79.50%-80.90%and 59.41%-73.50%,respectively,and the contribution rate of human activities to the reduction of runoff at Hanzhong Station and Shiquan Station is in the range of 19.10%-20.50%and 26.50%-40.59%,respectively.Climate change is the main cause of runoff reduction,which is much more sensitive to precipitation than potential evapotranspiration(4)When the land use remains unchanged,different climate change scenarios are set,and the hydrological process under different scenarios is simulated by the SWAT model.The results show that,when the temperature is constant,the response of runoff to the decrease of precipitation is slightly greater than the response to the increase of precipitation.When the series of precipitation is unchanged,the magnitude of runoff change caused by temperature change is smaller than that of precipitation change when temperature series is unchanged.This also shows that the sensitivity of runoff to precipitation is higher than that of air temperature,and precipitation is the dominant climatic factor affecting runoff changes at two hydrological stations Without considering climate change,different land use scenarios are set,based on the SWAT hydrological model,the hydrological processes under different land use scenarios are simulated(5)The parameters of three models of complementary relationship of evapotranspiration were determined,and the actual evapotranspiration process in the study area was analyzed.The complementary relationship of evapotranspiration in the study area was verified,and the applicability of the AA model,B2015 model and H2018 model in the study area was compared The B2015 model estimated the actual evapotranspiration best.The results show that,the average annual actual evapotranspiration in the study area shows a decreasing trend from south to north,and decreases with the increase of latitude.The change law of actual evapotranspiration among the seasons is summer>spring>autumn>winter.The main reason for the increase of annual actual evapotranspiration is the significant increase of temperature,the decrease of precipitation and wind speed.The reason for the significant increase in actual evapotranspiration in spring is the significant increase of temperature,the increase of sunshine hours and the significant decrease of wind speed.The decrease of sunshine hours is the main reason for the decrease of actual evapotranspiration in summer.The increase of actual evapotranspiration in autumn is mainly due to the significant increase of maximum temperature and the decrease of precipitation.The significant increase of the actual evapotranspiration in winter is due to the significant increase of minimum temperature and relative humidity... |
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