Stable isotopes ¹⁸O/¹⁶O and ²H/H are naturally present in water and can nowadays be measured more easily with regular lab equipment. Stable isotopes can be used to partition evaporation because non-productive evaporation fractionates (i.e., changing the ratio of heavy and light isotopes), while productive evaporation does not change this ratio. By taking water samples of all components of the soil-vegetation-atmosphere interface, total evaporation can be partitioned.

We started this research with several lab experiments (see Giuditta et al., 2018) and are currently sampling in La Selva (Costa Rica) and Speulderbos (the Netherlands), where water and vapour samples are taken from the soil, plant and atmosphere.

Nice visual story on Cesars work in Costa Rica:

Papers on this topic:

Hrachowitz, M., Stockinger, M., Coenders-Gerrits, M., van der Ent, R., Bogena, H., Lücke, A., and Stumpp, C.: Reduction of vegetation-accessible water storage capacity after deforestation affects catchment travel time distributions and increases young water fractions in a headwater catchment, Hydrol. Earth Syst. Sci., 25, 4887–4915, https://doi.org/10.5194/hess-25-4887-2021, 2021

Jiménez-Rodríguez, C. D., Coenders-Gerrits, M., Wenninger, J., Gonzalez-Angarita, A., and Savenije, H.: Contribution of understory evaporation in a tropical wet forest during the dry season, Hydrol. Earth Syst. Sci., 24, 2179–2206, https://doi.org/10.5194/hess-24-2179-2020, 2020

Jiménez-Rodríguez, C. D., Coenders-Gerrits, M., Uhlenbrook, S. & Wenninger, J. What Do Plants Leave after Summer on the Ground?—The Effect of Afforested Plants in Arid Environments. Water 11, 2559 (2019).

Giuditta, E., Coenders-Gerrits, A.M.J., Bogaard, T.A., Wenninger, J., Greco, R., Rutigliano, F.A. (2018): Measuring changes in forest floor evaporation after prescribed burning in Southern Italy pine plantations. Agricultural and Forest Meteorology, 256-257, p516-525. https://doi.org/10.1016/j.agrformet.2018.04.004

Coenders-Gerrits, A.M.J., Van der Ent, R.J., Bogaard, T.A., Wang-Erlandsson, L., Hrachowitz, M., and Savenije, H.H.G. (2014): Uncertainties in transpiration estimates, Nature, 506, E1-E2.

Sutanto, S.J., Wenninger, J., Coenders-Gerrits, A.M.J., and Uhlenbrook, S. (2012): Partitioning of evaporation into transpiration, soil evaporation and interception: a comparison between isotope measurements and a HYDRUS-1D model, Hydrology and Earth System Sciences, 16, 2605-2616. + corrigendum