Please Help! Evapotranspiration question, part II.

DASDLM dasdlm at aol.com
Wed Jul 12 11:21:01 EST 1995


I recently posted a note about a literature search I am conducting on the
relationship between timber harvest, evapotranspiration, groundwater
fluctuations, and slope stability.  I neglected, however, to indicate who
I am and with whom I am affiliated.  My name is Doug Smith and I am a
geomorphologist with the Mt. Hood National Forest.  The geotechnical group
there is revising their guidelines for managing "earthflow terrain." 
Currently, to determine the amount of timber that can be cut within an
earthflow complex, the management strategy relies on percent canopy
closure and tree diameter (DBH), and varies with the activity or "risk" of
the earthflow.  This risk is a qualitative rating which is assigned based
not on the likelihood of earthflow movement, but on the potential of the
earthflow to damage infrastructure or natural resources.  This strategy
has resulted in a very restrictive cutting policy, which suits some but
not others.  More importantly, the guidelines behind the strategy were put
together hastily, and appear to be arbitrary, inconsistent, and, perhaps,
altogether without scientific basis, not conceptually but in terms of the
specific application.  For example, on a "medium risk" earthflow, the
guidelines state that 90 percent canopy closure of >8 inch DBH trees must
be maintained.  Essentially, the claim is that, in this condition, the
earthflow is said to be in a state of hydrologic equilibrium or, if this
condition had been preceded by timber harvesting, then the earthflow is
said to have recovered hydrologically from the effects of that harvest. 
It is a tough claim to make and, unfortunately, there is no documentation
for how these minimum hydrologic requirements were derived.  The result is
that on a medium risk earthflow, only 10 percent of the land surface can
ever be without timber, a value that, again, suits some but not others. 
It may seem self-evident, however, that timber harvesting should be
forbidden on an active earthflow, particularly one poised to deliver
sediment to anadromous fish-bearing streams.  This brings up the next
complicating factor.  The level of mapping, of earthflow delineation, that
has occurred is very general.  In truth, individual earthflows are not
delineated;  rather, entire earthflow complexes are delineated.  The
result is that a "medium risk earthflow" may be more than a square mile in
size, but have an active zone of only a few tens of acres.  Even if the
bulk of the earthflow complex is stable, unless the entire complex meets
the minimum hydrologic recovery requirement, timber harvests cannot occur.

The geotech group is prepared to validate the existing guidelines if they
are supported in the scientific literature, or dispose of the guidelines
if something better (more scientifically justifiable) can be found.  This
is the reason behind the current literature search.  Answering the
question, "Will reduced evapotranspiration cause groundwater levels to
rise and result in greater instability?" is a simple one.  But managing
timber for a prescribed level of evapotranspiration to preserve slope
stability, particularly on slopes so disposed to failure and, in places,
currently failing, is very difficult.  So, if you have any suggestions or
know of any studies or publications that may help, please respond to me at
dasdlm at aol.com.  The first round of this work--a list of references--needs
to be completed by Friday, July 14, but more work will continue over the
next few months and I am happy to receive information at any time.  As I
mentioned in the previous posting, I am primarily concerned with
deep-seated slope stability, but references pertaining to shallow slope
failures are also welcome.  Essentially, if you have a reference you think
is remotely germane, please send it.  Lastly, I apologize for not posting
the customary information last time and hope it did not prevent anyone
from responding.  To those who have responded, thank you very much.  You
have been very helpful and I will reply to some of you directly.

Sincerely,
Douglas A. Smith
Geomorphologist
Mt. Hood National Forest



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