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Summary of the Article on Soil Water:
Soil Water Modeling
The article discusses the modeling of soil water in a multi-layer system, where the vertical soil moisture transport is governed by various processes, including infiltration, surface and subsurface runoff, gradient diffusion, gravity, and canopy transpiration through root extraction.
Conservation of Mass
The conservation of mass for one-dimensional vertical water flow in soils is described by the equation:
∂θ/∂t = -∂q/∂z - e
where θ is the volumetric soil water content, t is time, z is height in the soil column, q is the soil water flux, and e is a soil moisture sink term representing evapotranspiration loss.
Darcy's Law and the Richards Equation
The soil water flux, q, is described by Darcy's law:
q = -k (∂ψ_h/∂z)
where k is the hydraulic conductivity, and ψ_h is the hydraulic potential, consisting of the soil matric potential (ψ_m) and the gravitational potential (ψ_z).
Substituting Darcy's law into the conservation of mass equation yields the Richards equation:
∂θ/∂t = ∂/∂z [k(∂ψ/∂z + 1)]
This equation is numerically solved to predict changes in soil water content, as described in the section on the numerical solution (Section 2.7.3.2).