Richards Equation
Unsaturated water flow through soil — the wetting front
Soil & Rain Parameters
Soil type
Sandy soil
Loam
Clay
Rainfall rate
5
mm/h
Initial moisture
0.1
Layer depth (cm)
25
Reset Soil
Pause
Surface flux: 0.00 mm/h | Wetting front: 0 cm
Richards equation
(1931) describes unsaturated flow:
∂θ/∂t = ∂/∂z[K(θ)∂ψ/∂z] + ∂K(θ)/∂z
where θ = volumetric water content, K(θ) = hydraulic conductivity (highly nonlinear), ψ = matric potential.
van Genuchten model:
K(θ) = K_s·S_e^{1/2}·[1-(1-S_e^{1/m})^m]²
Notice how
clay
has high water retention but slow infiltration, while
sand
drains rapidly. The denser layer creates a perched water table.