Abstract
Soil hydraulic conductivity (ksoil) drops significantly in dry soils, resulting in steep soil water potential gradients (ψs) near plant roots during water uptake. Coarse soil grid resolutions in root system scale (RSS) models of root water uptake (RWU) generally do not spatially resolve this gradient in drying soils which can lead to a large overestimation of RWU. To quantify this, we consider a benchmark scenario of RWU from drying soil for which a numerical reference solution is available. We analyze this problem using a finite volume scheme and investigate the impact of grid size on the RSS model results. At dry conditions, the cumulative RWU was overestimated by up to 300% for the coarsest soil grid of 4.0 cm and by 30% for the finest soil grid of 0.2 cm, while the computational demand increased from 19 s to 21 h. As an accurate and computationally efficient alternative to the RSS model, we implemented a continuum multi-scale model where we keep a coarse grid resolution for the bulk soil, but in addition, we solve a 1-dimensional radially symmetric soil model at rhizosphere scale around individual root segments. The models at the two scales are coupled in a mass-conservative way. The multi-scale model compares best to the reference solution (−20%) at much lower computational costs of 4 min. Our results demonstrate the need to shift to improved RWU models when simulating dry soil conditions and highlight that results for dry conditions obtained with RSS models of RWU should be interpreted with caution.
Highlights
Most functional-structural root architecture models used to calculate root water uptake (RWU) consider root system architectures (RSA’s) as networks of discrete cylindrical tubes embedded in 3D soil domains
We found a rather large overestimation in our RWU calculations compared to the numerical reference solution computed on a fine adaptive soil grid meshed around an explicitly modeled 3D RSA
Overestimation of RWU under drying conditions using root system scale (RSS) modeling concepts was found to depend on soil discretization for all soils analyzed in the scenarios (Figure 2)
Summary
Most functional-structural root architecture models used to calculate root water uptake (RWU) consider root system architectures (RSA’s) as networks of discrete cylindrical tubes embedded in 3D soil domains. We refer to those macroscopic models as models on the root system scale (RSS) (Schroeder et al, 2009a). If the soil becomes dry due to RWU, ksoil becomes very low, leading to the formation of steep microscopic gradients in s around the roots These gradients are often not spatially resolved by the numerical grid used to simulate the soil water flow (Schroeder et al, 2008; Carminati et al, 2020; Rodriguez-Dominguez and Brodribb, 2020). The ksoil drop leads to an earlier onset of drought stress, while inaccurate representation may lead to the overestimation of simulated RWU
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