This paper presents an inter-compartment boundary condition for the simulation
of surface runoff, soil moisture, and soil air as a coupled system of partial differential
equations. The boundary condition is based on a classic leakance approach to balance
water between differently mobile regions such as the land surface and subsurface. Present
work applies leakances to transfer water and air simultaneously through the land surface
for soils, which are connected by an air flux with a steady atmosphere. Shallow flow and
two phase flow in a porous medium are sequential calculated in an iteration loop. General
criteria are stated to guarantee numerical stability in the coupling loop and for leakances
to control inter-compartment fluid fluxes. Using the leakance approach, a numerical model
captures typical feedbacks between surface runoff and soil air in near-stream areas. Specifically,
displacement of water and air in soils is hampered at full-water saturation over the
land surface resulting in enhanced surface runoff in the test cases. Leakance parameters
permit the simulation of air out-breaks with reference to air pressures, which fluctuate in
the shallow subsurface between two thresholds