Scales in single root water uptake models: a review, analysis and synthesis

Scales in transport of water to roots are compared with the length and volume scales by using the concepts associated with the representative elementary volume (REV). The possibility of a mismatch between model scale and system scale when using a Darcy-Buckingham-based model to describe soil water transport to a single root is evaluated. In the absence of a mismatch, the replication requirements for evaluating the Darcy-Buckingham-based model near a single root are discussed by using a synthesis of the elementary scales involved, including those for soil, plant and roots, and of the measurement device. By using REV scales from lattice-Boltzmann simulations, the effective half-root mean distance and the available measurement techniques, the evaluation of Darcy-based single root uptake models is possible in roughly 50% of the combinations of soil- and root-system properties. On the basis of an assessment of the scale characterizing natural soil variability, the number of replicates required to assess the average root water uptake profile near a single root is large, and either requires miniaturization of the measurement methods for the hydraulic transport characteristics, or very homogeneous (artificial) growing media with little variability. Variability of water uptake per unit root length will increase the number of samples required.