Darcy

Soil water flux at the interface between two adjacent soil layers, computed from Darcy's law with the Zeng and Decker (2009) equilibrium-potential correction (Oleson et al. 2010, CLM4 Tech Note Eq. 5.88). q = -k_interface * ((ψ_upper - ψ_lower) + (ψ_E_lower - ψ_E_upper)) / (z_lower - z_upper), positive upward. Migrated from UrbanCanopy.jl@9200edd4 src/hydrology.jl:471 (`SoilWaterFlux` @component). Observed-only: the seven inputs are parameters and q is given by a single algebraic equation.