As can be seen from figure 6a, tdep is virtually insensitive
to changes in qo in the range where Zmin occurs at the inlet end
of the border. In addition, for a given Zr and infiltration
parameter set, req(Zr) is a constant. Thus, equation 22 shows
that if the requirement Zmin = Zr is to be met, then tco(qo) also
needs to be nearly constant