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RESEARCH PRODUCT

Evapotranspiration simulations in ISIMIP2a-Evaluation of spatio-temporal characteristics with a comprehensive ensemble of independent datasets

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Actual land evapotranspiration (ET) is a key component of the global hydrological cycle and anessential variable determining the evolution of hydrological extreme events under different climate change scenarios. However, recently available ET products show persistent uncertainties thatare impeding a precise attribution of human-induced climate change. Here, we aim at comparing arange of independent global monthly land ET estimates with historical model simulations from theglobal water, agriculture, and biomes sectors participating in the second phase of the Inter-SectoralImpact Model Intercomparison Project (ISIMIP2a). Among the independent estimates, we use theEartH2Observe Tier-1 dataset (E2O), two commonly used reanalyses, a pre-compiled ensembleproduct (LandFlux-EVAL), and an updated collection of recently published datasets thatalgorithmically derive ET from observations or observations-based estimates (diagnostic datasets). Acluster analysis is applied in order to identify spatio-temporal differences among all datasets and tothus identify factors that dominate overall uncertainties. The clustering is controlled by several factorsincluding the model choice, the meteorological forcing used to drive the assessed models, the datacategory (models participating in the different sectors of ISIMIP2a, E2O models, diagnostic estimates,reanalysis-based estimates or composite products), the ET scheme, and the number of soil layers inthe models. By using these factors to explain spatial and spatio-temporal variabilities in ET, we findthat themodel choicemostly dominates (24%–40%of variance explained), except for spatio-temporalpatterns of total ET, where the forcing explains the largest fraction of the variance (29%). The mostdominant clusters of datasets are further compared with individual diagnostic and reanalysis-basedestimates to assess their representation of selected heat waves and droughts in the Great Plains,Central Europe and western Russia. Although most of the ET estimates capture these extreme events,the generally large spread among the entire ensemble indicates substantial uncertainties.