HESS Opinions "A perspective on isotope versus non-isotope approaches to determine the contribution of transpiration to total evaporation"

摘要

Current techniques to disentangle the evaporative fluxes from the continentalsurface into a contribution evaporated from soils and canopy, or transpiredby plants, are under debate. Many isotope-based studies show thattranspiration contributes generally more than 70% to the totalevaporation, while other isotope-independent techniques lead to considerablysmaller transpiration fractions. This paper provides a perspective onisotope-based versus non-isotope-based partitioning studies. Somepartitioning results from isotope-based methods, hydrometric measurements,and modeling are presented for comparison. Moreover, the methodologicalaspects of the partitioning analysis are considered, including theirlimitations, and explanations of possible discrepancies between the methodsare discussed. We suggest sources of systematic error that may lead to biasesin the results, e.g., instruments inaccuracy, assumptions used in analyses,and calibration parameters. A number of comparison studies using isotope-basedmethods and hydrometric measurements in the same plants and climaticconditions are consistent within the errors; however, models tend to producelower transpiration fractions. The relatively low transpiration fraction incurrent state-of-the-art land-surface models calls for a reassessment of theskill of the underlying model parameterizations. The scarcity of globalevaporation data makes calibration and validation of globalisotope-independent and isotope-based results difficult. However, isotope-enabled land-surface and global climate modeling studies allow for the evaluationof the parameterization of land-surface models by comparing the computedwater isotopologue signals in the atmosphere with the available remotesensing and flux-based data sets. Future studies that allow for this evaluationcould provide a better understanding of the hydrological cycle in vegetatedregions.