EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Estimating root zone storage capacity from flux measurements

Francesco Giardina1,5, Pierre Gentine2,3, Alexandra G. Konings4, and Benjamin D. Stocker1,5
Francesco Giardina et al.
  • 1Department of Environmental Systems Science, ETH Zürich, Zürich, Switzerland (
  • 2Department of Earth and Environmental Engineering, Columbia University, New York, NY, USA
  • 3The Earth Institute, Columbia University, New York, NY, USA
  • 4Department of Earth System Science, Stanford University, Stanford, CA, USA
  • 5Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Birmensdorf, Switzerland

Terrestrial evapotranspiration (ET) is a key factor in the global energy and water cycles. It is constrained by the transport of moisture from the soil and from vegetation to the atmosphere. The water storage capacity in the root zone (Sr) is an important parameter in land-atmosphere water exchanges, defining how long vegetation is able to transpire during drought. However, Sr is hard to measure directly, being associated with the depth of plant roots actively involved in water uptake and the potential of tap roots accessing deep water and enabling sustained transpiration during drought.


In this study, we present a method to estimate Sr from flux measurements, based on a deep neural network approach trained on eddy covariance (EC) data, multiple soil moisture datasets and a remotely sensed index of vegetation greenness. We derive a soil moisture stress function (fET) that isolates the control of soil moisture on ET. We then use EC data to estimate Sr by investigating how it relates to the climatology of the maximum cumulative water deficit (CWD, defined as the cumulative difference between actual ET and precipitation) experienced by the vegetation across different sites. We hypothesize that plants exposed to high CWD develop higher Sr (acclimation to water stress) and that maximum CWD is thus a good estimator of Sr. To identify root zone water storage from flux measurements, we regress the output of fET against CWD and estimate the maximum CWD for stress by calculating the intersection of the regression line with the x-axis. The apparent sensitivity of fET to CWD and its correlation with the maximum CWD across sites are indicative of adaptation to the prevailing climate and drought regime.


We find that for many sites, particularly in seasonally dry climates, fET does not exhibit a continuous decline with increasing CWD, but follows a step-change and levelling off. That is, at the high end of the CWD spectrum, fET no longer appears sensitive to further increases in CWD. This suggests that plants may have access to deep water reservoirs during the unfolding of a drought event, indicating that plant access to water becomes decoupled from water use.


This study highlights the need to investigate the representation of plant access to deep water reservoirs during drought in terrestrial ecosystem models. These findings could improve our understanding of land-climate interactions, particularly under water-limited conditions.

How to cite: Giardina, F., Gentine, P., Konings, A. G., and Stocker, B. D.: Estimating root zone storage capacity from flux measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9248,, 2021.

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