Drainage

Sub-surface drainage out of the soil column as an exponential function of water table depth, modulated by an impermeable-fraction reduction in frozen soils, following Oleson et al. (2010, CLM Tech Note Ch. 5) Eqs. 5.140 (drainage) and 5.141 (impermeable fraction). q_drai = (1 - f_imp) * q_drai_max * exp(-f_drai * z_v); f_imp = max((exp(-α(1 - f_ice_weighted)) - exp(-α)) / (1 - exp(-α)), 0). Migrated from UrbanCanopy.jl@9200edd4 src/hydrology.jl:698 (`GroundwaterDrainage` @component). Observed-only: both inputs are parameters and both outputs are given by single algebraic equations.