Assessment Of The Impact Of Climate And Vegetation Change On Evapotranspiration In Northwest China

Evapotranspiration is the primary process of water-energy exchange between the surface and atmosphere and plays a decisive role in the terrestrial water cycle,carbon cycle,and energy balance.Ecosystem evapotranspiration mainly consists of evaporation and transpiration.Component separation is a prerequisite for obtaining the total elements of evapotranspiration,and it is also an important way to understand the water cycle process deeply.With global warming,the global climate and environment have undergone significant changes.As a sensitive area to climate change,the water cycle in Northwest China has changed dramatically.On the one hand,evapotranspiration research in Northwest China is conducive to an in-depth understanding of the impact of global climate change on the water cycle in Northwest China.On the other hand,it also provides the scientific basis for rational allocation and sustainable development of water resources in Northwest China.Based on the observation data of 175 surface meteorological stations,reanalysis of meteorological data,soil moisture data,and remote sensing data of vegetation,firstly,this paper analyzed the spatiotemporal variation characteristics of climate,soil moisture,and vegetation in Northwest China from 1982 to 2020.Secondly,the terrestrial evapotranspiration dataset across China was used to verify the accuracy of the GLEAM evapotranspiration product.Based on good verification,evapotranspiration and its components（including evaporation and transpiration）in Northwest China and different land use types were further analyzed.Finally,the PLS-SEM model of climate,vegetation,and evapotranspiration in Northwest China was established to evaluate the influence of climate and vegetation changes on evapotranspiration in Northwest China.The results show that:（1）From 1982 to 2020,the average annual temperature and vapor pressure deficit in Northwest China showed an increasing trend,and the increasing rates were 0.43℃/10a（p<0.001）and 0.03 k Pa/10a（p<0.001）,respectively.The precipitation showed an increasing trend,and the change rate was 10.64 mm/10a（p<0.05）.The relative humidity and sunshine duration showed a decreasing trend,and the change rates were-0.62%/10a（p<0.01）and-9.59 h/10a（p>0.05）,respectively.The wind speed showed little change and showed a slight decreasing trend,the decreasing rate was 0.03 m·s^-1/10a（p<0.01）.The spatial distribution of meteorological elements showed obvious spatial heterogeneity.（2）From 1982 to 2020,the soil moisture in Northwest China showed a downward trend,and the average annual soil moisture was 0.18 cm³/cm³.The soil moisture in forest land was the largest,while the soil moisture in unused land was the smallest.There were great differences in spatial distribution,with an overall trend of increasing soil moisture from north to south.（3）From 1982 to 2020,NDVI in Northwest China showed a fluctuating growth trend,with a change rate of 0.01/10a（p<0.001）.The annual variation of NDVI in all regions showed an increasing trend except for unused land.The change range of NDVI of cultivated land was the largest,and the average annual NDVI value of forest land was the largest（0.61）.The spatial distribution of NDVI was different,and the overall distribution was decreasing from southeast to northwest.The high values of NDVI were concentrated in areas with a good ecological environment,while the low values were concentrated in deserts and other places.（4）The change rates of evapotranspiration,evaporation,and transpiration were 1.68mm/a（p<0.001）,0.04 mm/a（p>0.05）,and 1.38 mm/a（p<0.001）,respectively.The annual average evapotranspiration was 263.7 mm.There were differences in evapotranspiration among different regions.The areas with high evapotranspiration value were concentrated in the areas with good forest and grass cover,while the areas with low evapotranspiration value were concentrated in desert and basin,that is,the evapotranspiration of forest land is the largest and that of unused land is the smallest.The area of significant increase in evapotranspiration was mainly in the Tianshan Mountains,the Altai Mountains,the Qilian Mountains,and the northern and southern regions of Shaanxi Province.The monthly variation of evapotranspiration and its components showed a unimodal trend of first increasing and then decreasing.On the seasonal scale,the maximum evapotranspiration was in summer and the minimum evapotranspiration was in winter.（5）The direct effects of meteorological elements,NDVI,and soil moisture on evapotranspiration in Northwest China were 0.830,0.193,and 0.117,respectively,and the contributions were 93.13%,5.02%,and 1.85%,respectively.The evapotranspiration in Northwest China and different land use types were mainly affected by meteorological factors,among which precipitation and vapor pressure deficit were the dominant factors of evapotranspiration and its component in Northwest China.