Theory
Water flow in the soil is assumed to be laminar and, thus, obey Darcy’s law as generalised for unsaturated flow by Richards (1931):
where kw is the unsaturated hydraulic conductivity, ψ is the water tension, z is depth, cv is the concentration of vapour in soil air, Dv is the diffusion coefficient for vapour in the soil and qbypass is a bypass flow in the macro-pores described below. The total water flow, qw, is thus the sum of the matrix flow, qmat, the vapour flow, qv, and the bypass flow, qbypass. The general equation for unsaturated water flow follows from the law of mass conservation and eq.(2.1):
where θ is the soil water content and sw is a source/sink term. Under over saturated periods the flow of water in the upper soil compartment can be directed up-wards, and that water is then added to the total surface runoff (see section Surface Water).
The transit time for water flow through the soil profile can be calculated for each soil layer separately and also for the whole simulated profile (see switch TransitTime Estimation).
Bypass flow in macropores