Assessment Of Multi-Evapotranspiration Products Over China And Their Mechanism Study

Even if evapotranspiration(ET)is an essential variable in linking water,carbon,and energy cycles,ET measurement limitations at a large scale hinder related mechanisms study.Remote sensing,reanalysis,and land surface model-based ET products offer alternatives at different spatial-temporal intervals.The multi-ET products have already been evaluated and analyzed across the global and regional scales,but few researchers pay more attention to validating their performances and analyze their mechanisms across China.In this study,therefore,our aim is to study spatial-temporal variations of ET products and their mechanisms.The structure is composed of three parts;the first one is to evaluate the performances of several common ET products over China;the second part is spatial-temporal analysis of these products;the last one is the mechanism of ET based on those products and respective forcing data sets.We selected four ET products in here: the Global Land Evaporation Amsterdam Model version 3.0a(GLEAM3.0a),the EartH2 Observe ensemble(EartH2Observe-En),the Global Land Data Assimilation System version 2.0 with the Noah model(GLDAS2.0-Noah),and the Modern Era Retrospective-Analysis for Research and Applications-Land(MERRA-Land)project.The major results were summarized below:(1)Validation results by FLUXTOWER data: we comprehensively evaluated the performance of these products over China using a stratification method,six validation criteria,and high-quality eddy covariance measurements at 12 sites.Results showed that,within one stratification,the performance of each ET product based on a certain criterion differed among classifications of this stratification.In short,no product consistently performed best,according to the selected validation criterion.(2)Spatial-temporal variations of ET products: regardless of the mean values,all ET products had similar distribution characteristics that multi-year mean values decreased from humid regions to arid regions.Interestingly,the regions in arid,semi-arid,and plateau(BB,SA,DW,and QTP climatic zones)almost demonstrated the same distributed patterns in ET,mostly ranging from 0 to 400 mm/yr,while obvious differences between ET products existed in humid regions(i.e.CF and CW).Intuitively,mean values of MERRA-Land andGLEAM3.0a in CF and CW were evidently larger than others and some regions were of more than 800 mm/yr and the maximum values with 1200 mm/yr(MERRA-Land).As to ET changes,we can find that significant increase for all ET products were mainly located in main region of the temperate humid region(CF),the eastern temperate semi-humid region(CW),middle part in the Qinghai-Tibet plateau(QTP),some regions of the arid region(BB).(3)Mechanism of ET: to quantify the impact of each meteorological factor on ET changes and identify the dominant factor,we built a new method,based on Budyko theory,that built a direct relationship of ET,potential ET and precipitation.By comparing the contributions of five driving factors for each product,we found that annual changes of ET across China were almost controlled by precipitation(Pre),net radiation(Rn),and vapour pressure deficit(VPD).Except near-surface air temperature(Tem),all driving factors(i.e.Pre),Rn,Tem,VPD,and Wind)for each product were positively linearly related to ET while the sensitivity of each forcing factor to ET change was different,and vice versa.Tem,which had a significant upward trend across China,would led to increased ET in the southern,however,resulted in ET decline in the northern.Pre was the dominant foctor in the water-limited regions(i.e.BB,SA,and QTP);VPD was the dominant factor in most of the energy-limited regions(i.e.CF,CW,and DW),and the dominant factor in a few regions was Rn.