Impact of the representation of water transfer in the unsaturated zone on water flux in a ecohydrological critical zone model.

An increasing number of critical zone models are seeking to capture hydrological dynamics in an integrative fashion, reconciling water dynamics at multiple scales, and capture reciprocal linkage with plant dynamics. EcH₂O-iso (Kuppel et al., 2018) is such a numerical tool, where a process based, fully distributed formulation has been balanced with computational efficiency using a simplified formulation of subsurface water dynamics: three layers where top-layer infiltration is described using the Green-Ampt approach, while vertical water travel to deeper layers is gravity-driven, all using a saturation dependent hydraulic conductivity. This approach is sequential within grid cells and along the lateral drainage network, providing a fast, robust, and stable water budget. While this formulation has been successful in capturing ecohydrological dynamics (including that of isotopes tracers) in a variety of critical zone settings, gravity-driven percolation has failed to reproduce finer dynamics in some critical zone observatories displaying arid conditions and thick vadose zone (several tens of meters).

In this work, we add the possibility of considering a vertical dynamical water fluxes exchange between the layers using the Richards equation. The simulations are performed on a single pixel in order to focus on the importance of the subsurface water flux dynamics on the vertical axis only. The implementation of the Richards equation is based on the numerical resolution of Ross (2003). The resolution makes use of the Kirchhoff transform to increase the speed and the stability of the solution. The Brooks and Corey retention curve parameters are used for the resolution as it is in the original EcH₂O-iso model. The resolution of Ross (2003) is also usable for heterogeneous soils and provides a solution for the advection-dispersion equation for solute transport. The latter features paves the way for future work, including the tracer module implemented isotopy tracking in EcH₂O-iso. The fact that both the original (sequential) and current (dynamical) vertical routines are available as options in the same critical model allows for a direct benchmarking of performances and computing in a flexible comparison of the consequences of such different approaches of the subsurface flux modelling. We first validated the implementation of unsaturated zone representation thanks to standard 1D benchmarks. The impact of the new vertical routing scheme in EcH2O-iso is then evaluated in a deeply weathered profile in a dry tropical forest where a calibration of hydrodynamic parameters had been previously carried out with the sequential routing version of the model. Finally, at the same site, the newly-implement dynamical approach is used to perform a sensitivity analysis and a calibration of the parameters over a large number of simulations, and compared again to the performances of the sequential-base model.

References

Kuppel, S. et al : EcH2O-iso 1.0: water isotopes and age tracking in a process-based, distributed ecohydrological model, Geosci. Model Dev., 2018.

Ross, P. J.: Modeling soil water and solute transport - Fast, simplified numerical solutions, Agronomy Journal, 2003.