The quantity n(x) prescribes the porosity of the soil, K(x) is the permeability of the soil, μ is just the constant viscosity of water, and d(x) := d(x_1,...,X_d) = Qgx_d with the constant water density q and with the gravitational constant g. The nonlinear parameter function θ describes the saturation of the soil in dependency of the pressure p. k is the relative permeability of the soil which depends on the saturation. There are several models available which describe the shape of θ and k.
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