where ( ) HPg ρ ′= is atmospheric pressure head, P is atmospheric pressure, Kxx(w), Kyy(w), Kzz(w) is the permeability coefficient of the soil in 3 directions respectively, S(s0) is water uptake rate of soil, s0 is moisture content of soil before rainfall, ks is Saturated permeability coefficient, λ is porosity of soil, t is duration of rainfall.