| Evapotranspiration is a key link and process of surface water cycle.Rapid urbanization leads to the rapid increase of impervious surface,which changes the structure and thermophysical properties of the regional underlying surface,and affects the variation characteristics of urban surface evapotranspiration on the spatiotemporal scale.At present,how impervious surface changes affect urban surface evapotranspiration has not been clearly revealed.By analyzing the influence of urban impervious surface change on evapotranspiration,the temporal and spatial variation law of urban surface evapotranspiration in the process of rapid urbanization can be explored and revealed,which provides a theoretical basis for the allocation of water resources in urban areas and dealing with climate change.it is of practical significance to explore the mechanism of urban landscape on urban climate regulation.Based on Remote Sensing,GIS,and machine learning technology,six cities in humid and semi humid climate areas are selected as the research area.The impervious surfaces are extracted by multi feature classification method of remote sensing,and the long-time series evapotranspiration data are retrieved by gee SEBAL model.Based on the theories and methods of economics and landscape ecology,this dissertation reveals the temporal and spatial evolution characteristics of urban impervious surface and urban surface evapotranspiration,and quantitatively analyzes the impact of impervious surface change on Urban Surface Evapotranspiration in the study area from 1986 to 2019.The research results and conclusions as follows.(1)Using spectral features,texture features,spectral index features and component features,Relief F,Grid Search CV and Recursive Feature Elimination(RFE)method are used for feature reduction and feature selection,and the impervious surface information of the study area is retrieved based on random forest algorithm.The results show that this method can effectively reduce the phenomenon of "salt and pepper",the shadow of buildings and the misclassification of bare,and the method has high accuracy and universality.(2)Using the methods of spatial pattern,distribution density,spatial agglomeration and landscape pattern,the temporal and spatial characteristics of impervious surface in the study area are comprehensively analyzed.The impervious surface area of each city increased rapidly from 1986 to 2019,and the impervious surface showed a trend of outward expansion from the central area.The impervious surface density of each city has changed greatly.The growth rate of high-density and medium high-density areas in Shanghai,Beijing and Guangzhou is faster than 10%,and the growth rate of high-density and medium high-density areas in Wuhan,Zhengzhou and Xi'an is between 5% and 10%.In Zhengzhou,Wuhan,Shanghai and Guangzhou,the proportion of medium density,medium and low density of impervious surface areas increased rapidly,about 20%,while Beijing and Xi'an grew relatively slowly,11.66% and 5.33% respectively.The spatial distribution heterogeneity of impervious surface in each city is weakening,the spatial agglomeration diversity is gradually decreasing,and the spatial distribution of impervious surface tends to be balanced.The patch data,patch density,maximum patch index and other landscape pattern indexes of impervious surface have changed significantly,and the fragmentation of each urban landscape has increased significantly.The impervious surface landscape has an obvious aggregation effect,and the values of patch combination degree and aggregation degree have little change.(3)Based on geeSEBAL model and reference evapotranspiration ratio invariant method,the monthly and annual evapotranspiration data of cities in the study area from 1986 to 2019 were calculated.Using trend analysis,Theil Sen regression analysis,Mann Kendall mutation analysis,coefficient of variation and Hurst index,the temporal and spatial variation trend of surface evapotranspiration in each city is comprehensively analyzed.From the perspective of the overall change trend,except that Wuhan has an upward trend,all cities such as Beijing,Shanghai,Guangzhou,Xi'an and Zhengzhou show a downward trend.The areas with significant decrease in evapotranspiration are mainly distributed in the areas with high urban impervious surface coverage,and the areas with significant increase in evapotranspiration are mainly distributed in the areas with high vegetation coverage in mountainous areas.There are differences in the change trend of urban evapotranspiration between humid climate area and semi humid climate area.The area of urban evapotranspiration in humid climate area is relatively high,and the area of urban evapotranspiration in semi humid climate area is relatively high.The catastrophe analysis of each city shows that the change of urban evapotranspiration is closely related to urban development.In terms of the stability and sustainability of urban evapotranspiration change,cities have high fluctuation and sustainability in areas with high impervious surface coverage.(4)This dissertation analyzes the influence of impervious surface on surface evapotranspiration in various cities from 1986 to 2019.The results show that the actual daily evapotranspiration of vegetation cover area is significantly higher than that of impervious surface cover area,and the difference of evapotranspiration in humid climate area is greater than that in semi-humid climate area.There is also an obvious difference between the average annual evapotranspiration impervious surface and the vegetation surface.The difference of evapotranspiration in humid climate region is large,between 200-300 mm,and there is little difference in semi-humid climate region.The correlation between the area proportion of impervious surface and evapotranspiration is analyzed.the results show that in six cities in humid area and semi-humid climate zone,except Wuhan,the proportion of impervious surface has a negative correlation with urban surface evapotranspiration.The impervious surface density has a great influence on the urban surface evapotranspiration,and the surface evapotranspiration decreases with the increase of impervious surface density.According to the catastrophe analysis of area proportion evapotranspiration,it is found that the characteristics of evapotranspiration catastrophe points of cities in different climate regions are different,and cities in humid climate regions have no obvious characteristics;the mutation points of urban evapotranspiration in semihumid climate regions show similar consistency,and the location of abrupt change is between 10% and 14% of impervious surface.Through the spatial correlation analysis of impervious surface density and annual evapotranspiration,it is found that impervious surface and evapotranspiration have obvious spatial aggregation characteristics of high and low values.Through the Local G* hot spot analysis,there are hot spots and cold spots on the impervious surface and evapotranspiration in each city.The hot spots of evapotranspiration in each city are mostly distributed in the areas with low density of impervious surface and high vegetation coverage,while the areas with high density of impervious surface are mostly cold points.(5)There is no fixed relationship between the effect of impervious surface landscape pattern index on evapotranspiration,in which NP and PD mainly show positive correlation with cities in humid climate area,negative correlation with cities in semi-humid climate area,negative correlation between SHDI,SHEI and all cities,positive correlation between LPI and each city,and negative correlation between DIVISION and each city.Through the correlation analysis of landscape pattern index and urban evapotranspiration,it is found that the impact of impervious surface landscape fragmentation on urban surface evapotranspiration is different in different climate regions.Evapotranspiration increases with the fragmentation of impervious surface in humid climate area,while evapotranspiration decreases with the increase of fragmentation in semi-humid climate area.From the correlation of LPI,there is a positive correlation between the increase of maximum patch index and surface evapotranspiration in all cities.From the correlation between all landscape indexes and each city,urban surface evapotranspiration in humid climate area is relatively less affected by the change of impervious surface landscape pattern. |
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