Steady Infiltration from a Surface Point Source into a Two-Layered Cylindrically Confined and Unconfined Soil Region with Root Water Extraction

Abstract

Steady infiltration from surface point sources into two-layered cylindrically confined and unconfined soil regions with roots extracting water is analyzed with a linearized form of the Richards equation. In the upper layer, from which plant roots withdraw water with a uniform extraction coefficient (λ), the hydraulic conductivity at saturation decreases (−1 ≤ β 0) exponentially with depth. The lower soil layer is homogeneous, and the soil texture coefficient α is the same for both layers. The evaporation loss is taken to be proportional to the matric flux potential (MFP) at the soil surface, with a proportionality coefficient m . It is shown that the point-source solutions for the modeled systems are physically relevant, i.e., consistent with an exponential hydraulic conductivity function, not only for positive but also for negative β if λ ≥ 0.5α 2 (1 − β). Also, the fractions of water lost via evaporation at the soil surface, extraction by plant roots, and deep percolation are found to be identical for both laterally confined and unconfined regions. Numerical examples illustrate the behavior of the solutions for various combinations of the coefficients λ, β and m , the root-zone depth, and the lateral extent of the soil domain. The water uptake rate ( q up ) increases with increasing λ and root-zone depth and the wetted regions contract as the water uptake increases. Without evaporation, the water uptake rate decreases in the order q up (β q up (β = 0) > q up (β > 0). Evaporation losses significantly reduce both the water uptake rate and the extent of wetting. For β q up is larger than for β ≥ 0.