Note that tco can also be insensitive to changes in L when
border lengths are very short (figs. 1c and 1d). Here as well,
it is the combined effect of a constant
req(Zr) and a nearly
insensitive tdep(L) (fig. 6b) that renders tco nearly insensitive
to changes in L for short borders. In addition, it can be seen
from figures 5a and 5b that if L is increased beyond Lt the
cutoff ratio decreases steadily. However, if L becomes
excessively high, then the consequent progressive steepening
of the advance curve and the final infiltration profile near the
downstream end of the border make the cutoff distance very
sensitive to changes in L. As a result, the inflow cutoff
distance begins to grow at a faster rate than L; hence, R begins
to back up (fig. 5b). Depending on the range of L considered
in the analysis, R may back up to 100% (fig. 5b). This
suggests that a second threshold border length may exist. In
general, the question of a second threshold border length is
pertinent only when extremely long borders are considered
(fig. 5b). Such border lengths are physically unrealistic, and
hence the issue of a second threshold border length is of no
practical design and management significance