PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

The study areas are located at several of th e
lower watersheds in the H . J. Andrews Experimental
Forest near Blue River, Oregon . Vegetation
is typical of the low-elevation Douglasfir
forest. Study slopes average about 75 per -
cent. Soil depth is variable with maximu m
depths in excess of 5 meters . Because of high
porosities (70-80 percent) and large proportions
of macropores, these soils drain rapidly .
Permeabilities of 5,000 and 900 mm per hou r
have been noted on nearby watersheds fo r
surface soil and subsoil, respectively (Dyrnes s
1969) .
Methods
Initial investigations are being directe d
toward describing the physical properties of
the porous medium through which wate r
moves on its way to a stream . These field and
laboratory investigations will indicate where
water movement most likely occurs .
Drilling with a portable power drill wil l
follow a grid pattern over a small stream-toridge
portion of slope . At each grid point soi l
depth, depth and thickness of saprolite, an d
depth to unweathered bedrock will be deter -
mined. Additional drilling between initial gri d
points will indicate in more detail the surfac e
contour of the impermeable parent material .
Aluminum tubing placed in each hole wil l
provide access for measurement of ground -
water level or soil moisture content .
In the laboratory, undisturbed soil core s
taken from various depths in soil pits locate d
over the study area are being analyzed . Such
properties as porosity, pore-size distribution ,
stone content, permeability, and moisture retention
characteristics are being evaluated .
The type and amount of measurements to
be made during and following winter stor m
events in 1972-73 will depend on the in -
formation gathered during initial field an d
laboratory investigations now underway .
Anticipated measurements include soil moisture
content, vertical and lateral extent o f
saturated flow, soil moisture tension, precipitation,
and water outflow from the base of
the slope . Drilling and tracer studies will
attempt to define the source area for this
water .
Preliminary Results
Although the study has just recently begun ,
certain observations have provided qualitative
information concerning the subsurface flow
process on steep slopes. Precipitation moves
downward under the influence of gravity until
this movement is obstructed . In some parts of
the study area this obstruction may be cause d
by rock fragments which cause shallow ,
localized saturation as evidenced in several
soil pits during a period of heavy rain . Where
rock fragments are not present, downward
movement of water continues until the relatively
impermeable parent material is reached .
Here saturation occurs, flow acquires a horizontal
component, and water begins movin g
toward the stream.
At some point on the slope this saturated
flow is concentrated into pipelike subsurfac e
channels. The cause of this concentration i s
unknown but could conceivably result fro m
the microrelief of the impermeable material ,
from bedrock fractures, or from decayed root
channels. At the toe of the slope the channels
are spaced about 1-6 meters apart . They lie on
the bedrock surface and appear associated
with surface micro-relief. Shapes of their cross
sections range from circular to flat rectangular.
Width is also variable, ranging from 1
centimeter to about a meter. Where these
channels discharge into the stream channel ,
they are separated by soil which may contai n
a shallow saturated lower layer from whic h
seepage occurs. Water velocity of the seepage
appears to be several orders of magnitud e
lower than that of the subsurface channels .
The latter accounts for the greatest portion o f
stormflow .
The subsurface channels evident at the to e
of the study slopes may be outlets of a sub -
surface drainage system much like that de -
scribed for other humid areas (Jones 1971) .
Water can be observed discharging from suc h
channels in roadcuts and recent soil slumps at
various slope positions in the vicinity of th e
study area. Such a subsurface drainage syste m
could account for the rapid hydrologic
response of these steep slopes .
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