Richards equation for unsaturated water flow
In a rigid porous medium with effectively incompressible water and fixed air pressure, local water conservation with unsaturated Darcy flux gives θ_t=∇·[K(ψ)(∇ψ+e_z)].
Key knowledge
In a rigid porous medium with effectively incompressible water and fixed air pressure, local water conservation with unsaturated Darcy flux gives θ_t=∇·[K(ψ)(∇ψ+e_z)]. Here z is elevation upward and total head h=ψ+z. On a differentiable single retention branch, θ_t=C(ψ)ψ_t. The gravity term therefore has a definite sign fixed by the chosen vertical coordinate.
Values and equations
θ_t=∇·[K(ψ)(∇ψ+e_z)]=C(ψ)ψ_t
- θ
- volumetric water content 1
- ψ,z
- pressure head and upward elevation m
- K
- unsaturated hydraulic conductivity m s⁻¹
- C
- retention-branch moisture capacity m⁻¹
- e_z
- upward unit vector 1
- t
- time s
Conditions and scope
- Use a suitable continuum with local retention equilibrium, specified K(ψ), initial condition and hydraulic/flux boundaries; omit water sources only when they are absent.
- The equation neglects air-flow dynamics, density change and skeleton storage compared with changing moisture content. Saturated coupling, hysteresis, preferential flow, phase change and dynamic capillarity need an adapted model.
Sources · 1
References
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Evidence and review
Read actual§2.11 equations2.78–2.80 and h=ψ+z conversion with decoded original math. Retained the author's explicit discarded storage terms and boundary-data requirement; excluded1979 commentary about what future theory must await.
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Entry details
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