HESS Opinions "A perspective on isotope versus non-isotope approaches to determine the contribution of transpiration to total evaporation"
- 1Institute for Marine and Atmospheric Research Utrecht (IMAU), University of Utrecht, Princetonplein 5, 3584 CC Utrecht, the Netherlands
- 2Research Center for Water Resources, Ministry of Public Works, Jl. Ir. H. Djuanda 193, 40135 Bandung, Indonesia
- 3KNMI, P.O. Box 201, 3730AE De Bilt, the Netherlands
- 4George Mason University, 4400 University Drive, Mail Stop: 2B3 Fairfax, VA 22030, USA
- 5Institute for Atmospheric and Climate Science, ETH Zurich, CHN N11, Universitätstrasse 16, 8092 Zurich, Switzerland
- 6National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307, USA
- 7Centre for Ecology and Hydrology, Wallingford, Oxfordshire, OX10 8BB, UK
- 8UNESCO-IHE, Department of Water Engineering, P.O. Box 3015, 2601 DA Delft, the Netherlands
- 9Delft University of Technology, Water Resources Section, P.O. Box 5048, 2600 GA Delft, the Netherlands
- 10LSCE-Orme, point courier 129, CEA-Orme des Merisiers, 91 Gif-Sur-Yvette Cedex, France
Abstract. Current techniques to disentangle the evaporative fluxes from the continental surface into a contribution evaporated from soils and canopy, or transpired by plants, are under debate. Many isotope-based studies show that transpiration contributes generally more than 70% to the total evaporation, while other isotope-independent techniques lead to considerably smaller transpiration fractions. This paper provides a perspective on isotope-based versus non-isotope-based partitioning studies. Some partitioning results from isotope-based methods, hydrometric measurements, and modeling are presented for comparison. Moreover, the methodological aspects of the partitioning analysis are considered, including their limitations, and explanations of possible discrepancies between the methods are discussed. We suggest sources of systematic error that may lead to biases in the results, e.g., instruments inaccuracy, assumptions used in analyses, and calibration parameters. A number of comparison studies using isotope-based methods and hydrometric measurements in the same plants and climatic conditions are consistent within the errors; however, models tend to produce lower transpiration fractions. The relatively low transpiration fraction in current state-of-the-art land-surface models calls for a reassessment of the skill of the underlying model parameterizations. The scarcity of global evaporation data makes calibration and validation of global isotope-independent and isotope-based results difficult. However, isotope-enabled land-surface and global climate modeling studies allow for the evaluation of the parameterization of land-surface models by comparing the computed water isotopologue signals in the atmosphere with the available remote sensing and flux-based data sets. Future studies that allow for this evaluation could provide a better understanding of the hydrological cycle in vegetated regions.