| Mountains are widely distributed in China and around the world,and slope is a key landscape feature for generating terrestrial water resources.Valley bottoms are wetter than mountain tops,and sunny slopes are warmer and drier than shady slopes,resulting in systematic differences in soil and vegetation between valleys and mountain tops,and between sunny and shady slopes,making slopes critical to understanding the structure and function of water and terrestrial ecosystems.As a major mode of water consumption on land surface,evapotranspiration is an important component of the global water cycle and a key link between ecological and hydrological processes.Therefore,slope-scale evapotranspiration monitoring is beneficial to the better understanding of the water and heat fluxes on slopes and effectively improves water cycle analysis and water resource management.The existing evapotranspiration models are too coarse in resolution to reveal the mechanism of the influence of slope structure on hydrothermal fluxes,and the existing evapotranspiration data products are also low in spatial resolution and insufficient to reflect the heterogeneity of evapotranspiration under complex topographic conditions.In this study,we firstly develop a data fusion model to improve the accuracy of evapotranspiration data in mountainous areas from the uncertainty analysis of existing evapotranspiration data products,and then develop high spatial resolution slope-scale evapotranspiration models from the perspectives of key parameters of evapotranspiration process and main influencing factors of evapotranspiration.The theoretical support for water resources management and allocation in mountainous areas is provided according to the research results.The main conclusions and innovations of this thesis are as follows.(1)Uncertainty analysis and integrated fusion research based on multi-source evapotranspiration data.The uncertainty analysis of 13 evapotranspiration datasets from the perspective of time series trend and time series development consistency was conducted.The comprehensive assessment of each evapotranspiration data with 8evaluation indices based on flux observation data under different underlying surface types,climatic zones and elevation range was carried out.And then the weighted evapotranspiration fusion method based on assessment sheet and Taylor skill score(TSS)was developed.PML＿V2,ETWatch and SSEBop datasets were utilized to construct the synthesized evapotranspiration dataset from 2003 to 2017,and GLEAM3.3a,CR＿ET and NTSG datasets were utilized to construct the synthesized evapotranspiration dataset from 1982 to 2002.This part of the study shows that the uncertainty of evapotranspiration data is higher in mountainous areas than that in nonmountainous areas.(2)Evapotranspiration model incorporating the net radiation results considering topographic factors.From the perspective of energy-driven evapotranspiration process,based on the high spatial resolution Sentinel-2 remote sensing data and digital elevation model data,we obtained high spatial resolution surface information,improved the calculation process of solar radiation,developed the calculation model of net radiation considering topographic factor,and obtained the monitoring results of evapotranspiration with high spatial resolution at the slope scale.The validation accuracy shows that the coefficient of determination reaches 0.84 in Huairou and 0.86 in Baotianman;the root mean square error values are 0.59 mm and 0.82 mm,respectively.The validation results show that the model is applicable to areas with complex topography and the spatial distribution of evapotranspiration results can capture a certain topographic texture.From the perspective of slope orientation,the evapotranspiration of south-facing slopes in both study areas was higher than that of north-facing slopes.The average annual evapotranspiration was 616.93 mm for the sunny and semi-sunny slopes of Huairou and 596.22 mm for the shady and semi-shady slopes;for Baotianman,the average annual evapotranspiration was 761.58 mm and655.53 mm for the sunny and semi-sunny slopes and the shady and semi-shady slopes,respectively.In addition,the accuracy of the evapotranspiration model results is improved than that of the evapotranspiration results combining net radiation without considering topographic factors.(3)Evapotranspiration disaggregation model based on slope units.Based on the topographic features,different slope units were firstly delineated based on the digital elevation model data with high spatial resolution.Considering the main influencing factors reflecting the difference of evapotranspiration under different topographic conditions,including radiation factor,vegetation factor and moisture factor,with Sentinel-2 data and digital elevation model data with high spatial resolution,an integrated indicating factor reflecting the difference of evapotranspiration between slopes was constructed.ETWatch data with 1 km resolution was utilized to develop a slope-scale evapotranspiration disaggregation model based on the integrated indicating factor.10-meter spatial resolution,reliable evapotranspiration data with reasonable spatial distribution was obtained,which can reflect the variability of slope-scale evapotranspiration.The validation shows that the coefficients of determination in Huairou and Baotianman reach 0.9 and 0.91,respectively,and the root mean square errors are 0.45 mm and 0.47 mm,respectively,indicating that the evapotranspiration disaggregation model based on slope units can achieve good performance in both study areas.Meanwhile,compared with the input 1-km resolution evapotranspiration data,the accuracy of the slope evapotranspiration disaggregated results in both study areas was improved to some extent,with the coefficients of determination in Huairou and Baotianman improved by 0.01 and 0.02,respectively,and the root mean square error decreased by 0.12 mm and 0.07 mm,respectively.This model also has a wide application prospect in water resources management and allocation in mountainous areas. |
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