Root System Scale Models Significantly Overestimate Root Water Uptake at Drying Soil Conditions

Area of Science:

• Soil Science
• Hydrology
• Plant Physiology

Background:

• Soil hydraulic conductivity decreases significantly in dry soils, creating steep water potential gradients near roots.
• Root system scale (RSS) models often use coarse grids that fail to resolve these gradients, leading to overestimated root water uptake (RWU).

Purpose of the Study:

• To quantify the overestimation of RWU in drying soils by RSS models with varying grid sizes.
• To develop and validate a computationally efficient multi-scale model for simulating RWU in dry conditions.

Main Methods:

• A benchmark RWU problem in drying soil was analyzed using a finite volume scheme with different grid sizes.
• A continuum multi-scale model was implemented, coupling a coarse bulk soil grid with a fine rhizosphere-scale model around root segments.

Main Results:

• Coarse grids (4.0 cm) overestimated cumulative RWU by up to 300%, while finer grids (0.2 cm) showed 30% overestimation at higher computational cost.
• The multi-scale model achieved high accuracy (-20% difference from reference) with significantly reduced computational cost (4 min vs. 21 h).

Conclusions:

• Existing RSS models may yield inaccurate RWU simulations in dry soils, necessitating cautious interpretation of results.
• The developed multi-scale model provides an accurate and computationally efficient alternative for simulating RWU, especially under dry soil conditions.