リチャーズ式
∂ θ ∂ t = ∇ ⋅ D ( θ ) ∇ θ {\displaystyle {\frac {\partial \theta }{\partial t}}=\nabla \cdot D(\theta )\nabla \theta }
ここで D(θ) [L/T] は(土壌)水分拡散係数で、このように定義される。
D ( θ ) ≡ K ( θ ) C ( θ ) ≡ K ( θ ) ∂ h ∂ θ {\displaystyle D(\theta )\equiv {\frac {K(\theta )}{C(\theta )}}\equiv K(\theta ){\frac {\partial h}{\partial \theta }}}