| Evapotranspiration is a key process and vital component of the water circulation process.60% of the land precipitation returns to the atmosphere in the form of evapotranspiration,which has an important impact on the amount of surface available water resources.Karst landforms account for 10% of the global land area,and the karst landforms are widely distributed with porous media such as cracks and pipes,which lead to rapid water loss.So water is the main limiting factor for the karst ecosystem.In order to restore and rebuild the degraded karst ecosystem,a large number of ecological projects,such as returning farmland to forest and grassland and comprehensive control of rocky desertification,have continuously and profoundly affected the underlying surface.Meanwhile,changes in temperature and precipitation have further exacerbated regional water cycles.Therefore,in-depth understanding of the actual ET change and its influencing factors in the karst area of southwest China under the changing environment is of great significance to the regulation of water cycle and the allocation of water use for agriculture and forestry in the karst area.At the regional scale,due to the lack of ET data for a long time in the southwestern region,by comparing and evaluating the suitability of five commonly used ET remote sensing products,ET remote sensing product with the best relative accuracy was selected.Because of the complex surface morphology and cloudy and rainy weather conditions in the study area,ANUSPLIN software TPS interpolation method was used to interpolate the meteorological station data,and MODIS double-star data was used to collaboratively construct a high-quality vegetation EVI index.From the perspectives of different geographical differences,different vegetation types and different karst landforms,the spatial and temporal distribution characteristics of ET and its components Ei,Ec and Es were analyzed using Theil-Sen Median trend analysis and Mann-Kendall test.Correlation analysis and normalized multiple regression analysis were used to reveal the influence of temperature,precipitation,wind speed,relative humidity,sunshine hours and vegetation EVI on ET changes.The main research results are as follows:(1)The accuracy of five ET remote sensing products is evaluated,and the results show that PML V2 has the highest accuracy in the southwest region.The verification results of site observation data show that the correlation coefficient,root mean square error and mean absolute error are 0.82,18.64 mm and 15.73 mm,respectively.The verification results of water balance data show that the absolute error is 2.58 mm and the relative error is 0.34%.Based on the analysis of long time series ET remote sensing products,the mutation year was 2003.Therefore,the follow-up work of the paper used the PML V2 product with the best relative accuracy to carry out the ET change study from 2003 to 2017.(2)The average ET in Southwest China is 708.56 mm.The multi-year average ET decreases with the increase of latitude(or decreases from south to north);decreases with the increase of longitude,but is significantly affected by the different landforms within the region and the characteristics of the underlying surface coverage,showing a "W"-type change trend;with the altitude Increase first and then decrease and then decrease.The multi-year average ET value of different vegetation types is ranked as follows: woodland > grassland > cultivated vegetation > shrub > meadow > alpine vegetation.The multi-year average ET values of different geomorphic areas are ranked as follows: non-karst region > peak forest plain > peak-cluster depression > karst basin > karst gorge > moderate-high mountains > karst plateau > karst trough valley.Compared with cultivated vegetation,the proportion of transpiration of forest shrub and grass in Karst area increased by an average of 3.72%.Therefore,the inefficient water consumption of the ecosystem is reduced,which further indicates that ecological projects such as returning farmland to forests and grasses promote the effective consumption of evapotranspiration.(3)From 2003 to 2017,ET in Southwest China increased year by year,with an average increase rate of 5.08 mm/a.The increasing trend of ET is mainly caused by the increase of Ec and Ei,and the corresponding contribution rates are 84.25% and 57.87% respectively.The order of interannual ET change rate of different vegetation types is: meadow > grass > woodland > shrub > alpine vegetation > cultivated vegetation > 0.Except that the increase in ET of alpine vegetation is mainly caused by the increase of Es(contribution rate is 55.39%),the increase in ET of other vegetation types is caused by the increase of Ec and Ei and the contribution rate of Es is negative.The interannual ET change rate of different geomorphic areas is ranked as follows: karst gorge > nonkarst region > moderate-high mountains > karst basin > peak forest plain > peak-cluster depression > karst plateau > karst trough valley > 0.The increase of ET in different landform areas is mainly caused by the increase of Ec and Ei.Compared with cultivated vegetation,the average transpiration rate of woodland and shrubs and grasses in the Karst area was accelerated by 0.47 mm/a.The increase in effective water consumption further indicates that ecological projects such as returning farmland to forests and grasslands have exerted better material production and carbon fixation and oxygen release benefits.(4)From 1960 to 2017,the temperature in southwest China increased significantly,the relative humidity,sunshine hours and wind speed decreased significantly,and the precipitation did not change significantly.The change rates were 0.19 ?/10 a,-0.54 %/10 a,-2.99 h/10 a,-0.04 m/s/10 a and 3.29 mm/10 a,respectively.The abrupt changes of multiple meteorological elements all occurred before the period from 2003 to 2017.Compared with the period from 1960 to 2017,in the recent period after the abrupt changes(from 2003 to 2017),except the wind speed changes slowed down,the change amplitude of temperature,precipitation,relative humidity and sunshine hours all increased significantly.Recently,under the background of climate change and ecological engineering construction,the vegetation growth in the study area has obviously improved,and 91.30% of the regional vegetation has been restored.Compared with non-karst region,the vegetation recovery rate in peak-cluster depression and peak forest plain is faster.Even after artificial intervention in karst plateau and karst trough valley with poor habitat,the vegetation recovery situation is more optimistic.(5)There are strong regional differences in the dominant factors of ET variation in southwest China.In the whole region,correlation analysis showed that temperature,precipitation,relative humidity,sunshine hours and EVI of vegetation were positively correlated with ET on the whole,and EVI of vegetation in 81.63% of the area was positively correlated with ET.However,normalized regression analysis showed that temperature was the dominant factor of ET change in the whole region.Comparing all karst region as a whole with non-karst region,vegetation EVI is the dominant factor of ET change in karst region,while temperature is the dominant factor of ET change in non-karst region.The normalized regression analysis after the karst region is divided into different geomorphologic subareas shows that air temperature is the dominant factor of ET change in moderate-high mountains and karst basin,wind speed and sunshine hours are the dominant factors of ET change in karst gorge and karst plateau,and vegetation EVI is the dominant factor of ET change in the peak-cluster depression and peak forest plain. |
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