Influence of three root spatial arrangement on soil water flow and uptake. Results from an explicit and an equivalent, upscaled, model

Soil water uptake by plant roots results from the complex interplay between plant and soil which modulates and determines transport processes at a range of spatial and temporal scales: at small (single root) scale, uptake is determined by local soil and root hydraulic properties but, at the root system scale, these local processes interact with the macroscopic water flow in soil and the spatial arrangement of roots in the soil. Recent modeling approaches, such as 3D functional architectural models of root systems, are becoming useful tools for integrating plant processes and studying their interactions/responses with the environment. However, integrating efficiently the microscopic flow towards roots is at stake in the coupling of such 3D root models with soil water flow models. The required fine meshing of soil with (large) 3D root systems for the flow solution would result in huge, impracticable, simulations. We show here a way to estimate the "microscopic" gradients around a root, interacting with other adjacent roots, based on the superposition principle of linear PDEs, resulting in an "equivalent model" of root water uptake that avoid the fine meshing of the soil for the flow problem. We test this approach by comparing a detailed finely meshed, explicit modeling of soil and roots, with this equivalent approach using a coarse mesh. The example application is a 2D case (root impacts) which shows how root spatial arrangement (regular, clumped or heterogeneous root distribution) impacts temporal pattern of root water potential, compared to the mean soil water potential, but also the decrease of actual evapotranspiration and the use of available soil water by plants. The good agreement between the equivalent and explicit modeling makes this equivalent approach promising for 3D functional architectural modeling of root systems in soil. (C) 2013 The Authors. Published by Elsevier B.V