Uncertain effect of component differences on land evapotranspiration

Abstract

Study region

Global and 28 large river basins

Study focus

Actual evapotranspiration (ET_a) plays a key role in the redistribution of water, carbon and energy. The emergence of many ET_a products has made uncertainty assessment increasingly important. The FLUXNET2015 dataset and 28 large watershed water balance datasets were used in this study. The monthly scale products of the ERA5-Land reanalysis data (ERA5), Global Land Data Assimilation System (GLDAS), Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA) and Penman-Monteith-Leuning Model Version 2 (PML) terrestrial evapotranspiration models were evaluated from 2001 to 2017. The differences in composition (vegetation transpiration (T), soil evaporation (E_s), canopy interception loss (E_i) and other components (open water and ice and snow sublimation) (E_o)) and vegetation among the four ET_a products were compared.

New Hydrological Insights for the Region: At the site scale, the ERA5 and MERRA products significantly outperformed the GLDAS and PML products, with the latter exhibiting poorer reliability in site validation. The PML product's basin-scale water balance KGE metric overall outperformed those of the ERA5, GLDAS, and MERRA products, with KGE > 0 in 23 basins. The use of basin-scale data mitigates the impact of local outliers on the simulation results, leading to KGE validation metrics at the basin scale that are overall superior to those obtained from site-scale validation. There were large errors in the estimates of T and E_s in the ERA5 product, related to the overestimation of E_s and underestimation of T. The boundary between sea and land (used to divide marine evapotranspiration and land evapotranspiration) was unclear in the PML products. E_o/ET_a was overestimated, and there were clearly high values at the land margin (E_o peaks as high as 3803 mm/yr). The difference in evapotranspiration components had a considerable influence on the uncertainty of ET_a. The vegetation types in the 4 ET_a products for DBF***, EBF***, ENF***, MF***, GRA***, and CRO*** all exhibited significant differences at the P<0.001 level. This study contributes to product uncertainty analysis and the determination of ways to improve ET_a products.