Abstract:
Well water levels and ditch flow were used to evaluate road-induced changes in
surface and subsurface flow for two sites (Trocadero and Polk) in southeast Alaska. At
the Trocadero Site, low water levels before and peak water levels during a storm were
used to test for differences in subsurface water levels above and below an existing road.
Low water levels at the Trocadero Site were significantly different in the wells above
verses below the road (p = 0.02). However, peak water levels above and below the road
showed no significant difference (p = 0.25).
Low water levels before and peak water levels during a storm were used to test
for changes in subsurface water levels pre- and post-road construction at the Polk Site.
Four storms were measured prior to road construction. Thirty-one storms were measured
following road construction. Using an Antecedent Precipitation Index, fifteen storms
from the post-road construction dataset were selected that were within similar moisture
conditions as the pre-road storms. After road construction there was no significant change
in low water levels. There was a small (0.08 m and 0.05 m), but significant (p = 0.01 and
0.03), decrease in the peak water levels following road construction.
Ditch flow measured on two forested hillslopes showed that road cutbanks
intercepted roughly 100% of the area precipitation from upsiope contributing areas. Due
to shallow soils, and excessive amounts of precipitation during the late summer and fall,
much of the precipitation falling on the forest floor becomes shallow subsurface flow
between the organic and mineral soil interface. The amounts of road-intercepted flow do
not translate into equivalent changes in subsurface water levels, rather the changes in
subsurface water levels are typically minimal. Where changes do occur, they tend to be
concentrated immediately above the cutbank and below the road fillslope.
