decreases, bankfull flow within it tends toward
makes it highly susceptible to downslope move-
sheet flow, where width is much greater than
ment by gravity, detachment by raindrops, and
depth. Thus, the amount of hillslope erosion tak-
detachment and transport by overland flow.
ing place in such a rill could be less than that in a
The magnitude of these FT-induced effects is
rill unaltered by FT. In nature, sediment infilling
variable, however; Benoit and Voorhees (1990),
and channel alterations by FT-induced soil creep
and Kok and McCool (1990) report that FT effects
occur in conjunction with sediment transport by
are some of the least understood aspects of the soil
intermittent flows, and rill cross-sectional shape is
erosion process, even though soil FT processes
determined by hydraulic and soil processes in
have been investigated for years. In addition, cur-
northern climes.
rent erosion models still cannot predict infiltrabil-
I am unaware of studies that have compared
ity or erodibility in frozen and partially frozen
erosion on a compacted and rutted hill to that on
soils at their surface (Seyfried and Flerchinger
an undisturbed, rilled slope, but I hypothesize that
1994), even though Young et al. (1993) have devel-
the transport capacity of vehicular ruts may be
oped a method to predict soil frost depth as part of
changed by FT processes as are rills. However, the
the Water Erosion Prediction Project (WEPP) soil
ruts may not infill as rapidly as natural rills be-
erosion model.
cause the compacted soil along a rut may have
Knowledge of the role of snow cover on soil
higher shear strength, which may temporarily re-
erosion mechanics remains rudimentary as well.
tard FT-induced creep down the rut side slope.
Haupt's (1967) research showed that a snow cover
Thus, I also hypothesize that erosion on a rutted
tends to insulate and thus preserve soil frost, even
hill could be more severe. First, flows in ruts may
during spring rains, by preventing raindrops from
have higher velocity and, thus, higher stream
contacting the frozen soil. His study shows that
power than rill flows. Parker et al. (1995) found
because the soil remains frozen under snow-
that flow velocity near the soil bed increased with
covered plots and is very resistant to flow, virtu-
bulk density (compaction), because surface rough-
ally no soil is eroded, even though the vegetation
ness was less in the more compacted soil, and that
below the snow is sparse.
sediment concentration in runoff was higher on
A significant portion of previous soil erosion re-
more highly compacted soil (Fig. 7). They found
search has been directed at changes in soil bulk
that the increased velocity from compaction had
properties, not changes in surface soil structure
more effect on erosion than the increased soil
and strength, which are most important to erodi-
shear strength. Second, the volume of surface wa-
bility. Shainberg et al. (1994) point out that soil
ter would be greater on a compacted slope than on
detachment by rill or overland flows depends on
the undisturbed slope, owing to reduced infiltra-
soil-particle-binding forces at the soil/water inter-
tion. Third, the compacted soil around a rut would
face, not bulk-tensile-strength properties of the
allow less infiltration along its length than may
soil at depth. Yet, bulk soil strength properties are
occur along a rill on the same slope; thus, the ero-
often measured and used to predict soil erodibil-
sivity of rut flow could be maintained for greater
ity.
distances downslope. Research comparing rill and
Misra and Rose (1995) summarized research on
rut processes is planned as part of this project.
the relationships between soil strength, as mea-
sured in the field, and soil erodibility, as defined
by the amount of soil particles moved by rain
Seasonal soil erodibility
While FT can loosen compacted soil and
splash and runoff. However, that relationship
smooth rills, and possibly ruts, over time, it can
remains unclear, and the need for a method to pre-
also make undisturbed soils more erodible in the
dict seasonal soil erodibility over time persists,
spring than they are at other times of the year
which impedes improvements in our ability to
(Gatto 1995). When soil water freezes in the win-
predict soil erosion (Nearing et al. 1994).
ter, the ice crystals can disrupt soil grain interlock-
ing, which results in a less dense, weak soil upon
RESEARCH NEEDED AND PROJECT GOALS
thaw. In addition, the soil water content in a freez-
ing soil usually increases as moisture is drawn
Clearly, soil freezethaw cycling is a dynamic
from the unfrozen soil below to the freezing front.
process, substantially affecting runoff and soil
Thus, newly thawed soil usually has more water
erodibility during the year. And the most impor-
than before it froze (it may be temporarily satur-
tant experimental topics in the soil erosion arena
ated), which contributes to its low strength and
are the dynamic processes that determine soil
7
Previous Page
