Tsutomu Yamanaka1), Takeo Kimura2), Xinchao Sun3), Hiroaki Kato1) 4), Yuichi Onda1) 4)
Received 2017/05/30, Accepted 2017/07/24, Published 2017/09/01

Tsutomu Yamanaka1), Takeo Kimura2), Xinchao Sun3), Hiroaki Kato1) 4), Yuichi Onda1) 4)

1) Faculty of Life and Environmental Sciences, University of Tsukuba
2) Graduate School of Life and Environmental Sciences, University of Tsukuba
3) Institute of Surface-Earth System Science, Tianjin University, China
4) Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba

The root water uptake profile (RWUP) reflects a plant’s survival strategy and controls evapotranspiration and carbon fluxes. Despite its importance, there is still no reliable method for reconstructing this profile. In this study, we applied and compared two possible approaches to a case study in a conifer plantation: an isotope-calibrated mechanistic model and a mixing model with a bell-shaped approximation. Our results show that, after calibrating the hydrologically-active root density profile, the mechanistic model gave a good estimation of the xylem water isotope delta (δx); even though the measured root density was greater in shallower soils, water uptake occurred throughout the entire soil profile, with more uptake in deeper soils. The RWUPs estimated by the mixing model were different from those estimated by the mechanistic model and were unrealistic. However, when we constrained the minimum thickness of the water uptake zone, there was good agreement between the RWUPs from the two approaches. We can therefore conclude that the mechanistic model calibrated with isotopes gave better results, and that sole use of the mixing model is not recommended unless appropriate constraints are applied.

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