Introduce a max pore radius in soil hydrology.
This is based on O. Ippisch, H.-J. Vogel, and P. Bastian, Advances in
Water Resources 29, 1780 (2006). It introduces a small, nonzero pressure
head value at which the soil saturates and hydraulic conductivity or
diffusivity no longer increase. This affects especially the conductivity
of fine-textured soils, which in the usual VG model increases sharply
for pressure heads corresponding to unrealistic pore diameters (1 cm and
larger). Removing this sharp increase at saturation would lead to an
increase of conductivity for all saturation values if the matching point
at saturation was kept. To avoid this and to confine the modification to
the region close to saturation, we must renormalize ks_usda as a
function of the cutoff. We do this by requiring that the conductivity at
a saturation of 0.5 remains unchanged.
The modification is controlled by the parameter CUTOFF_H (in meters),
which corresponds, via the capillary rise law, to the maximum pore size.
By default, the parameter is set to 0 and the modification should have
no effect. A safe setting is CUTOFF_H = .003, which corresponds to a max
pore diameter of 1 cm.
The most important feature of the implementation is the need to
renormalize ks_usda. That is because we change the shape of the
conductivity function, but
Some notes on the implementation
- mvg_K takes avan in m-1, hence a change of units is necessary when
when using avan_usda.
- The locations of the declaration and of the reading of cutoff_h are
inspired by how similar parameters (e.g. soilc_max) are treated.
