DISSERTATION
LOGGING EFFECTS ON SOIL MOISTURE LOSSES
Submitted by
Robert Ruhl Ziemer
Department of Earth Resources
In partial fulfillment of the requirements
for the Degree of Doctor of Philosophy
Colorado State University
Fort
Collins,
Colorado
June, 1978
COLORADO
STATE
UNIVERSITY
June, 1978
WE HEREBY RECOMMEND THAT THE DISSERTATION PREPARED UNDER
OUR
ON
SUPERVISION
SOIL
MENTS
FOR
BY
MOISTURE
THE
ROBERT
LOSSES
DEGREE
OF
RUHL
BE
ZIEMER
ACCEPTED
DOCTOR
OF
AS
ENTITLED
FULFILLING
PHILOSOPHY.
Committee on Graduate Work
Adviser
ii
LOGGING
IN
EFFECTS
PART
REQUIRE-
ABSTRACT OF DISSERTATION
LOGGING
The
an
depletion
isolated
EFFECTS
of
mature
soil
sugar
moisture
pine
and
California Sierra Nevada was
2
weeks
for
measured
5
SOIL
MOISTURE
within
the
an
LOSSES
surface
adjacent
uncut
15
feet
forest
by
in
the
measured by the neutron method every
consecutive
periodically
ON
summers.
Soil moisture recharge was
the
winters.
during
intervening
Groundwater
fluctuations within the surface 50 feet were continuously recorded
during
the
the
soil
same
period.
surface, eventually recharging the entire soil profile to
"field
capacity".
portion
of
deplete
moisture
the
early winter.
rainfall,
to
During
the
recharge
soil.
was
at
from
the
drier
"field
period,
although
capacity",
soil
below
the
the
the
trees
wetting
top
continued
front
to
into
Groundwater levels began to rise within days after
whereas
progress
Soil
Each fall, a wetting front progressed from
weeks
through
moisture
or
the
months
were
unsaturated
depletion
by
the
zone
required
above
isolated
for
the
tree
the
wetting
water
table.
was
maximum
front
at
a
depth of 8 to 13 feet and extended about 15 feet away from the tree.
The influence of the tree on soil moisture depletion extended to a
depth of about 18 feet and to a distance of about 40 feet. An
excellent linear relationship was found between the quantity of
soil moisture depleted by the tree at the end of the summer and distance
from
the
tree.
The
isolated
tree
iii
used
between
2200
and
2600
cubic feet more soil moisture than a bare portion of the plot outside
of
the influence
of the
tree.
Robert Ruhl Ziemer
Department of Earth Resources
Colorado State University
Fort Collins, Colorado 80521
June, 1978
ACKNOWLEDGEMENT
This
research, Study PSW 1603-5, was undertaken as an integral
part of the Lower Conifer Project, U. S. Forest Service, Pacific
Southwest Forest and Range Experiment Station.
received
Water
additional
funding
Resources,
forestry
from
were
the
State
of
California,
Department
of
During the 15 years since I began this study, many
students
California
by
Until 1967, the study
the
Berkeley
introduced
to
the
campus
joys
of
of
the
University
outdoor
research
of
as
seasonal
employees or as work-study students collecting field or laboratory
data
or
attempting
to
make
sense
out
of
the
mounds
of
computer
output.
The number of men and women who worked on various aspects of this
study
are, unfortunately,
too numerous to acknowledge individually, but
their assistance is greatly appreciated.
Some
Agee's
individuals,
special
recognition.
Jim
field work and perceptiveness was outstanding during his 2
years
with
soil
analysis
years
the
of
quired
ment
however, require
was
devotion
for
with
research
study,
data
this
Hutch Wood's assistance with the laboratory
indispensable.
to
developing
analysis
study,
cannot
in
some
Bob Thomas' contribution
the
multiple
be
overstated.
small
computer
way, induced
many
programs
Perhaps
each
and
re-
their
to
involve-
follow
a
career.
The
resident
staff
at
the
Challenge
Experimental
Forest
were
invaluable for their daily observations, particularly Ross Cole, who
carried
18
on
months
the
field
while I
was
work
and
attending
kept
in
classes
telephone
at
CSU.
communication
for
the
Gina
Enrique, Melinda Hamen,
and
Diane
Haase
were
very
under-
standing and provided excellent clerical assistance during the heat
of
out
manuscript
preparation.
And
finally, Ray Rice must be commended for his patience through-
the
lengthy
gestation
which
I
required
vi
before
completing
this
work.
TABLE OF CONTENTS
Page
Chapter
I.
II.
INTRODUCTION . . . . . . . . . . . . . . . . . . . .
l
l
Historical
Perspective . . . . . . . . . . . . . . .
The Soil Moisture Study . . . . . . . . . . . . . .
1
6
THE STUDY AREA. . . . . . . . . . . . . . . . . . . . .
9
Location . . . . . . . . . . . . . . . . . . . . . .
Geomorphology . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
Soils
Climate
. . . . . . . . . . . . . . . . . . . . .
Vegetation
III.
. . . . . . . . . . . . . . . . . . . . .
LOCATION AND INSTRUMENTATION OF SOIL MOISTURE SAMPLING
SITES . . . . . . . . . . . . ... . . . . . . . . . .
9
9
11
11
13
15
Plot Description and Instrumentation . . . . . . . .
Tree Growth . . . . . . . . . . . . . . . . . . . .
Soil Analysis . . . . . . . . . . . . . . . . . . .
Description . . . . . . . . . . . . . . . . . .
Texture . . . . . . . . . . . . . . . . . . . .
Water retention . . . . . . . . . . . . . . . .
Soil Moisture Measurement . . . . . . . . . . . . . .
Access tube installation . . . . . . . . . . . . .
Calibration . . . . . . . . . . . . . . . . . .
Field measurement . . . . . . . . . . . . . . .
15
16
22
25
25
28
30
30
38
38
40
SOIL WATER REGIME . . . . . . . . . . . . . . . . . . .
42
Plot
IV.
1
Selection . . . . . . . . . . . . . . . . . . .
Soil Moisture at Recharge . . . . . . . . . . . . .
Depletion Trends . . . . . . . . . . . . . . . . . .
Partially cut condition . . . . . . . . . . . .
Isolated tree condition . . . . . . . . . . . .
. . . . . . . .
Bare condition
Total Summer Soil Moisture Depletion . . . . . . . .
Surface 5 feet . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .
5 to 10 feet
. . . . . . . . . . . . . . . . .
10 to 15 feet
Total 15-foot profile . . . . . . . . . . . . .
Depletion During Fall Recharge . . . . . . . . . . .
Groundwater
Variation . . . . . . . . . . . . . . .
Recession . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .
Rise
vii
59
62
62
65
67
69
69
74
76
77
89
101
102
110
Page
Chapter
................
115
LITERATURE CITED. . . . . . . . . . . . . . . . . . . . . .
121
APPENDIX
126
V.
SUMMARY AND CONCLUSIONS
. . . . . . . . . . . . . . . . . . . . . . . . . .
viii
LIST
OF
TABLES
Table
Page
1.
Climatological
2.
Distribution of basal area by species and size class
in the study plots . . . . . . . . . . . . . . . . . . . . . 17
3
Typical profile characteristics of the Challenge Soil
Series . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.
Physical and chemical analyses of the Challenge Soil
Series collected by the California Cooperative Soil
Vegetation Survey at the series classification plot. . . . . 29
5.
Physical characteristics of the soil
plots. . . . . . . . . . . . . . . .
.
l
Summary, Challenge Ranger Station . . . . . . 12
in the study
...........
37
6. Average soil moisture by 5-foot depth classes at the end of each
summer depletion season in study plot L1
. . . .
70
7.
Average soil moisture within the surface 15 feet of
soil at the end of each summer depletion period in
study tree plot Ll for each of the six concentric distances from the study tree from 1965 through 1969. . . . . . 78
8.
Average soil moisture within the surface 15 feet of
soil at the end of each summer depletion period in
study tree plot Ll for each of the six concentric distances from the study tree relative to soil moisture
40 to 60 feet from the study tree from 1965 through
1969 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
9.
Average soil moisture within the surface 15 feet of
soil at the end of each summer depletion period in
study tree plot Ll for each of the six concentric distances from the study tree relative to soil moisture
in the plot after the tree was cut in 1969 . . . . . . . . 82
10.
Volume of soil moisture depleted within 40 feet of the
study tree in excess of that depleted 40 to 60 feet
from the study tree when adjusted for equal. soil
moisture in the plot after the tree was cut. . . . . . . . . 88
11.
Changes in soil moisture in the uncut control plots C1
and C2 during the fall recharge period . . . . . . . . . . . 91
ix
Page
Table
12 .
Daily precipitation during 1964-65, Challenge Ranger
Station............ .
... .. .. . . . .
127
13 .
Daily precipitaiton during 1965-66, Challenge Ranger
Station.........................
128
14 .
Daily precipitation during 1966-67, Challenge Ranger
Station.........................
129
Daily precipitation during 1967-68, Challenge Ranger
Station.........................
130
15 .
16.
17 .
Daily precipitation during 1968-69, Challenge
S t a t i o n . . . . . . . . . . . . . . . . . ...
Ranger
. .. .
Daily precipitation during 1969-70, Challenge Ranger
Station.........................
X
131
132
LIST
OF
FIGURES
Figure
Page
1.
Location of study plots within the Challenge Experimental Forest, California . . . . . . . . . . . . . . . . . 10
2.
Spatial
distribution of trees and neutron access
tubes in plot L1. . . . . . . . . . . . . . . . .
3.
Spatial distribution
of trees and neutron access
tubes in the uncut control. plots. . . . . . . .
l
. . .
18
. . . . . 20
4.
Concentric distance classes and location of neutron
access tubes around the study tree in plot L1 . . . . . . . 21
5.
Annual growth of the sugar pine study tree and of a
ponderosa pine from a nearby uncut stand. . . . . . . . . . 24
6.
Distribution of sand, silt, and clay in
tree plot L 1 . . . . . . . . . . . .
7.
8.
the
.
31
Distribution of sand, silt, and clay in the uncut
control plot Cl . . . . . . . . . . . . . . . . . . . . .
32
l
l
Distribution of sand, silt, and clay in the uncut
control plot C2 . . . . . . . . . . . . . . . . . . . . 33
l
9.
study
. . .
. . .
l
l
Soil moisture retention in the study tree plot Ll . . . . . 34
Soil moisture retention in the uncut control plot Cl. . . .
35
11. S o i l moisture retention in the uncut control plot C2. . . .
36
10.
12.
13.
14.
15.
Isopleths of soil moisture in the the partially cut
study plot L1 on a) August 16, b) September 14, and
c) October 19, 1965 . . . . . . . . . . . . . . . . . . .
43
Isopleths of soil moisture in the partially cut
study plot Ll on a) November 30, 1965, b) January
17, and c) February 11, 1996. . . . . . . . . . . . . . . .
44
Isopleths
of
soil
moisture
in
the partially
cut
study
plot L1 on a) May 6, b) May 19, and c) June 9, 1966 . . .
45
Isopleths of soil moisture
in the partially
cut study
plot L1 on a) June 30, b) July 15, and c) August 12 ,
1966. . . . . . . . . . . .
. . . . . . . . . . . . . . . .
46
Figure
--
16.
17.
18.
19
l
Page
Isopleths of soil moisture in the partially cut study
plot Ll on a) August 29, b) September 6, and c) October 25, 1966.. . . . . . . . . . . . . . . . . . . . . .
47
Isopleths of soil moisture in the isolated tree study
plot Ll on a) January 19, b) March 2, and c) June 22,
1967..
. . . . . . . . . . . . . . . . . . . . . . .
48
Isopleths of soil moisture in the isolated tree study
plot Ll on a) July 18, b) August 16, and c) September
7, 1967...... .
.
. . . . .. . . . .
49
Isopleths of soil moisture in the isolated tree study
plot Ll on a) September 26, b) October 13, and c) October 30, 1967 . . . . . . . . . . . . . . . . . . . .
50
l
20.
21 .
22
l
23.
24.
25.
26.
27.
28.
Isopleths of soil moisture in the isolated tree study
plot Ll on a) January 22, b) March 4, and c) May 2, 1968 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
Isopleths of soil moisture in the isolated tree study
plot Ll on a) May 29, b) June 7, and c) June 26, 1968. . .
52
Isopleths of soil moisture in the isolated tree study
plot Ll on a) August 12, b) August 28, and c) September ll, 1968 . . . . . . . . . . . . . . . . . . . .
53
Isopleths of soil moisture in the isolated tree study
plot Ll on a) September 25, b) October 9, and c) October 23, 1968 . . . . . . . . . . . . . . . . . . . . .
54
Isopleths of soil moisture in the isolated tree study
plot Ll on a) January 7, 1969 and in the bare study
plot Ll on b) March 27, and c) May 5, 1969 . . . . . . . .
55
Isopleths of soil moisture in the bare study plot Ll
on a) June 16, b) July 2, and c) July 25, 1969 . . .
56
l
Isopleths of soil moisture in the bare study plot Ll
on a) August 13, b) September 17, and c) October 2,
1969 . . . . . . . . . . . . . . . . . . . . . . . .
57
Isopleths of soil moisture in the bare study plot Ll
on a) October 29, b) November 20, 1969, and c) February 25, 1970 . . . . . . . . . . . . . . . . . . . . .
58
Average soil moisture at the end of each summer depletion season between the depths of a) O-5 feet,
b) 5-10 feet and c) 1 0 - 1 5
feet for
different distances
from the study tree and for the uncut plots. . . . . . . .
xii
71
Figure
29.
30.
Page
Relative soil moisture in the study tree plot at the
end of each summer depletion period for each of five
concentric distances from the study tree from Table
9 . . . . . . . . . . . . . . . . . . . . . . . . . .
83
Relationship between distance from the study tree
and relative soil moisture at the end of the 1965,
1966, 1967, and 1968 depletion seasons from Table 9 . . . . . .
31.
Profiles
of
soil
moisture
content
with
depth
at
each
of the six access tubes in the uncut control plots
during the 1964-65 f a l l recharge period . . . . . . .
32.
. .
92
Profiles of soil moisture content with depth at each
of the six access tubes in the uncut control plots
during the 1965-66 fall recharge period . . . . . . . . .
l
33.
85
93
Profiles of soil moisture content with depth at each
of the six access tubes in the uncut control plots
during the 1967-68 fall recharge period . . . . . . . . 94
l
34.
Profiles of soil moisture content with depth at each
of the six access tubes in the uncut control plots
during the 1968-69 f a l l recharge period . . . . . . . .
35.
.
. 95
Fluctuations in depth to water table during water
year 1966 in plots 5, 7, and 16 . . . . . . . . . . . . . 103
l
36.
37.
Fluctuations in depth to water
year 1967 in plots 5, 7, and 16
Fluctuations
in
depth
to
water
Fluctuations
in
depth
to
l
table
year 1968 in plots 5, 7, and 16
38.
table during water
. . . . . . . . . . . 104
l
during
water
. . . . . . . . . . . . . . 105
water
table
during
water
year 1969 in plots 5, 7, and 16 . . . . . . . . . . . . . .. 106
39.
Fluctuations
in
depth
to
water
table
during
a
portion
of water year 1970 in plots 5 and 7 . . . . . . .
107
40.
Relationships between precipitation, water table
depths in plots 5, 7, and 16, and average end of season
soil moisture in the uncut control plots. . . . . . . . . 108
41. .
Time lag in the initiation of water table rise in
plots 5, 7, and 16 relative to precipitation during
fall 1965.............
. . . . . . . . . .
l
xiii
112
Page
Figure
42.
Time lag in the initiation of water table rise in
plots 5, 7, and 16 relative to precipitation during
fall 1966............. .. . . . . . . . . . . .
xiv
113
CHAPTER I
Historical
Not
begin
An
Perspective
until
to
the
middle
experiment
additional
with
200
of
the
years
the
seventeenth
agricultural
passed
before
century
aspects
the
of
did
soil
importance
of
investigators
moisture.
soil
moisture
in forested areas was recognized as a regulator of tree growth. By
the 1800's,
field
trees
on
the
first
to
report
less
soil
The
studies
soil
difference
moisture
that
moisture
beech
than
was
forest
deeper
the
plantation
than
15
plantation
moisture
under
open
greatest
In 1892, Charmow
a
were
regime.
and
the
contained
all
late
the
influence
of
(1899) was among the
forests
during
during
document
Ebermayer
pine
areas
to
four
considerably
seasons
of
the
year.
summer.
measured soil moisture to a depth of a meter in
in
the
meters.
He
increased
forest
underway
Ukrainian
steppe
found soil
(Wyssotzky,
stands
from
where
moisture
the
decreased
1932).
Wyssotzky
to
and
1892
water
1899
as
table
the
studied
reported
was
age
of
soil
that
the
roots of the trees extracted soil moisture0 to a depth of about 16
meters.
He
further
showed
seasonal
isopleths
of
soil
depth and time for the 7-year duration of his study.
moisture
with
Later, he studied
the seasonal changes in soil moisture for a 2-year period, from 1928
to 1930. Wyssotzky's studies stood alone, but have been largely unrecognized,
as
the
most
elaborate
and
extensive
soil
moisture
storage
and depletion work in forests until the advent of neutron soil moisture
meter in the mid-1950's.
2
The early studies required an enormous effort to obtain the gravimetric
soil
moisture
samples
at
these
deeper
depths.
In
addition,
since a new hole must be dug or drilled for each sample, the site is
eventually
of
rendered
numerous
bandoned
holes
his
adversely
By
useless
left
7-year
influencing
for
by
further
previous
study
sampling.
study
in
1899
because
the
site
and
his
trenching, Fricke
(1904)
and thereby isolated a quadrat
because
of
Wyssotzky
previous
the
influence
eventually
sample
removal
the
concluded
basic
driest
that
cause
months
decreased
for
of
severed
the
of soil.
the
root
increased
roots
of
surrounding
From
competition
(1926)
made
significance and spatial
By
studying
land,
detailed
Aaltonen
species
quality
of
soil.
as
the
the
border
the
growing
the
above-ground
and
the
the
soil.
trees
then
there
are
are
was
rather
the
than
the
light.
advance
stands
was
which
highest
approached.
necessary
portion
competition
moisture
Fricke
in
understanding
the
a
is
and
reproduction
definite
directly
space
in
Fin-
arrangement
dependent
upon
the
He found that in an opening in any forest, the
center
space
He
each
soil
thinning,
major
forest
that
of
in
of
concluded
members
seedlings
next
experiments,
areas
distribution of soil moisture in the forest.
charts
between
the
the
his
for
following
previously popular concept of increased
Aaltonen
trees
He found two to three times
year.
growth
was
data.
more soil moisture in the trenched areas than within untrenched
during
a-
of
existing
for
each
trees
is
between
demonstrated
oratory experiment with corn.
this
and
The
become
poorer
tree.
the
site,
smaller
the
larger
This space arrangement of
mainly
determined
them
for
very
clearly
Unfortunately,
progressively
water
by
and
by
Aaltonen
their
roots
nutrients
in
means
made
of
no
a
labsoil
3
moisture analysis to support his theory, nor did he verify his laboratory
experiments
The
soil
work
in
of
the
field
Conrad
and
moisture, carried
(1934)
to
attempt
a
out
with
trees.
Veihmeyer
with
similar
(1929)
sorghum
study
with
on
root
plants
in
forest
trees
development
California,
in
and
led
Lunt
Connecticut.
Lunt "recognized that the California type of climate, characterized by
little
for
or
no
soil
rainfall
moisture
during
studies.
the
growing
season,
Nevertheless,
is
the
ideal
condition"
he "felt that such a study
would be of value in humid New England in spite of its frequent summer
showers".
Thus,
having
in his area, Lunt
recognized
measured
the
the
drawbacks
distribution
of
imposed
soil
by
the
moisture
climate
under
iso-
lated trees by digging a trench from the base of the tree out into the
open.
Soil moisture was determined
collected
from
the
walls
from four trees --two
was 4 feet.
pines
was
found
of the tree.
ture
content
interception
the
and
trench
two
at
several
oaks.
depths
and
distances
The maximum depth measured
In one study he measured soil moisture to a distance of
41 feet from an oak.
content
of
gravimetrically from soil samples
In
nearly
immediately
all
beneath
cases, the
the
crown
lowest
and
soil
close
moisture
to
the
base
Lunt recognized that three factors influenced the moisof
by
the
the
soil
in
crown, and
his
climate,
absorption
namely,
by
the
surface
roots.
He
evaporation,
felt
further extensive experimentation was necessary to properly evaluate
the
interaction
of
these
factors.
Lunt's
figures
also
show
that
mois-
ture was being depleted below a depth of 4 feet, but he did not specifically
acknowledge
this
observation
in
the
text.
During the 1930's the literature began to proliferate with studies
related
to
soil
moisture
under
forest
stands.
The conclusions of various
4
authors
were
soil-water
extremely
soil
of
often
relationships, unlike
complex
texture
and
and
trees, both
conditions
depth,
within
(1935)
as
and
limited
ual
papers
in
in
both
well
to
was
their
as
becoming
obvious
agricultural
time
and
climate,
that
forest
counterpart,
space.
Not
variable,
but
were
only
the
was
response
species, to these variable growing
considerably.
Several
generalize
authors, such
about
the
as
Hayes
rooting
and
depth
of
tree rooting characteristics are so interrelated with
soil texture, and
are
It
between
attempted
However,
climate,
variable
differed
Stoeckeler
trees.
contradictory.
usefulness.
related
to
tree
moisture
By
regime
1955, there
root
systems
that
such
were
alone
well
classifications
over
(Karisumi
400
and
individ-
Tsutsumi,
1958).
A bibliography containing more than 800 papers related to soil
moisture
under
of
literature
forests
seems
had
to
been
compiled
repeatedly
by
Ziemer
demonstrate
that
by
1973.
soil
The bulk
moisture
de-
pletion by trees continues below the depth of measurement unless the
roots
For
are
restricted
by
truly
impervious
and
continuous
soil
layers.
example, McClurkin (1958) in Mississippi and Gaiser (1952) in Ohio
found that all available soil moisture was used throughout the 40- to
42-inch measurement depth.
McClurkin had earlier assumed the roots
would be restricted by a heavy clay layer, but later concluded that the
clay "had not seriously impeded root penetration".
Lull and Axley
(1958) measured soil moisture to a depth of 12 feet in the New Jersey
pine
barrens
occurring
and
below
concluded
their
that
deepest
depletion
by
the
trees
was
probably
measurement.
Hendrickson (1942) was among the first to propose that soil moisture
studies
could
be
used
to
determine
water
use
by
forest
vegetation.
A study using this approach was made by Rowe and Coleman (1951) in
5
woodland-chaparral
and
ponderosa
pine
in
California.
Annual evapo-
transpiration was calculated by summing soil moisture losses between
storms.
the
This
rooting
approach
depth
of
required
the
soil
vegetation
moisture
and
an
measurements
adequate
throughout
measurement
of
the
variation of soil moisture within the forest stand.
spatial
Very
few
authors
have
understand the spatial
followed
Lunt's
early
work
in
an
effort
variation of soil moisture around trees.
to
Notable
exceptions have been Giulimondi (1960), Douglass (1960), and Ziemer
Giulimondi
(1964).
tances
The
from
a
moisture
(1960) measured soil moisture at increasing dis-
Eucalyptus
lost
3
meters
shelterbelt
from
the
into
an
adjacent
shelterbelt
was
cultivated
nearly
twice
field.
that
lost at a distance of 5 meters, 3 times that at 9 meters, and 13 times
that at 17 and 25 meters.
Unfortunately,
his soil moisture measure-
ments were only made at a depth of 30 to 35 cm.
Douglass
(1960) measured soil moisture at the end of the two grow-
ing seasons following thinning a 16-year-old
in
South
Carolina.
loblolly
pine plantation
Soil samples of the surface 4 feet were taken at
2-foot intervals along a line between trees spaced about 20 feet apart.
Soil
moisture
to the trees.
depth.
In
was
highest
midway
No
mention
was
their
between
made
of
the
soil
trees
and
moisture
lowest
adjacent
distribution
with
climate, some of the differences observed by Giuli-
mondi and Douglass may have been due to a combination of rainfall interception
by
the
tree
canopy
and
soil
As Lunt had pointed out earlier, the
moisture
ideal
depletion
climate
to
by
study
the
roots.
soil
mois-
ture depletion by forests is in an area with little summer rainfall
such
as
California.
6
In the subalpine zone of the Sierra Nevada in California, Ziemer
(1964)
measured
the
pattern
of
soil
long transects running from unlogged
moisture
storage
to
a
depth
of
4
feet
depletion
a-
red fir forests into openings
which had been cut 1, 5, 10 and 12 years earlier.
measured
and
using
the
relatively
Soil moisture was
new
neutron
meter
technique. This method allowed identical locations to be repeatedly
remeasured throughout the summer depletion season, a distinct advantage
over the earlier gravimetric technique.
Ziemer found soil moisture
content
center
progressively
increased
end of the summer, whereas
toward
in
early
the
of
the
opening
at
the
spring, soil moisture was nearly
equal throughout the plot. The trees depleted soil moisture 30 to 40
feet
the
into
the
opening.
As
differences
between
soil
new
tree
moisture
seedlings
in
the
occupied
forest
the
and
opening,
opening
became
smaller.
Those differences would become negligible 15 years after
cutting.
Because of the cobbly
nature of the morainal soils, Ziemer
was unable to measure soil moisture depletion below the rooting depth
of the trees.
Thus, through
problems,
and
a
combination
of
climate,
soil,
and
study
design
we still do not h a v e an adequate understanding of the timing
pattern
of
soil
moisture
depletion
by
individual
trees
throughout
their rooting depth.
The Soil Moisture Study
The purpose of this study was to measure the quantity, timing, and
pattern of soil moisture storage and depletion throughout the rooting
depth
of
an
isolated
mature
sugar
pine
tree.
7
To be successful in such a study, it
attempt
tered
to
by
conduct
eliminate
past
problems
researchers
this
mentation,
the
These
study.
2)
and
climate,
ments
meter
were
in
made
the
is
cation.
Consequently,
ments
taken
Prior
one
can
to
mid-1950's,
been
to
identify
repeatedly
an
idealized
be
grouped
and
encoun-
site
in
which
under
1)
instru-
the
nearly
development
of
all
moisture
soil
the
to
neutron
soil
measure-
Gravimetric sampling is very time
when collecting deep soil samples.
destructive, one
at
have
select
gravimetrically,
sampling
necessary
soil, and 4) saturated groundwater flow.
3)
particularily
consuming,
to
problems
1) Instrumentation.
moisture
which
was
can
not
repeatedly
return
to
Since the
the
same
lo-
most early studies represented a few measure-
point
in
time
and
at
relatively
shallow
depths.
The neutron meter was selected for use in this study because with an
initial installation of the access tubes soil moisture measurements
can
then
be
made
rapidly
and
repeatedly
at
the
same
location
throughout
the depth of the access tube.
This is a necessary condition to in situ
measurements
soil
of
the
2) Climate.
timing
of
moisture
depletion
and
recharge.
Lunt and others discussed the problems associated
with measuring the influence of vegetation on soil moisture depletion
in
areas
Following
where
such
continued
rainfall,
it
summer
is
rainfall
difficult,
partially
if
not
recharges
impossible,
the
to
soil.
sep-
arate the components of interception losses, surface runoff, variable
infiltration,
tion
U.
of
S.
the
and
and redistribution of the infiltrated water from deplesoil
moisture
particularly
in
by
the
the
vegetation.
central
Sierra
The
climate
Nevada
of
in
the
western
California is
ideally suited for soil moisture depletion studies because a rainl.ess
period extends from spring through autumn.
8
3) Soil.
rocky
and
Forest
are
soils
often
in
underlain
the
by
It
is
out
to
understand
soil
of
rooting
depth
moisture.
the
soil
pletion
necessary
bedrock
which
measure
the ability
shallow
soil
of
is
and
easily
moisture
the tree
through-
to
extract
is
desirable
to
ease
interpretation
of
the
moisture
de-
patterns.
fringe
of
to
typically
In addition, horizontal as well as vertical uniformity
4) Saturated Groundwater.
will
are
fractured
penetrated by roots.
the
west
is
present
have
soil
a
readily
water
use
treme
case
(1964)
and Urie
of
fluctuations
forests.
error.
within
the
rooting
available
by
the
shallow
for
depth
of
the trees,
supply
of
soil
will
be
greatly
tree
water
(1966)
If a water table or its capillary
tables,
moisture
and
such
attempted
to
vegetation
any
complicated.
investigators,
example, have
the
In the ex-
as
use
estimates
Heikurainen
diurnal
of groundwater levels to estimate evapotranspiration by
This
process
requires
many
assumptions
that
are
subject
to
In areas where the saturated groundwater is at an intermediate
depth, the magnitude of the contribution of the water table to evapotranspiration
is
completely
unknown
and
in
many
correctly ignored or assumed to be negligible.
able
to
select
sites, free
well-drained
from
be
free
from
surface
ponding
during
has
It
thus,
the
is,
influence
been
of
The
ideal
site
rainfall
which
would
table and subsurface lateral saturated flow.
also
studies
in-
prefera
water
should
result
in non-uniform soil moisture recharge.
Therefore,
a
forest&d
table
in
study
a
a
substantial
site
region
on
having
a
effort
deep
long
and
was
initially
uniform
rainless
soil
summers.
expended
with
no
to
select
groundwater
CHAPTER 11
THE
STUDY
AREA
Location
The
study
site
is
located
on
the
Challenge
Experimental
Forest
in Sections 33 and 34, T.19N., R.7E., M.D.M. at an elevation of 2,600
feet
in
the
north
Sierra
Nevada.
The Experimental Forest is located
40 miles northeast of Marysville, California at latitude 39o 29' N.,
longitude 121o 14' W. (Fig. 1).
Geomorphology
The
block,
Sierra
the
Nevada
eastern
geomorphic
margin
of
The
western
flank
or
dip
120
to
feet
per
mile
neath
180
the
alluvial
fill
which
slope
the
developed
uplifted
of
toward
of
province
this
the
along
large
west,
a
a
of
faults.
block
slopes
from
eventually
passes
be-
Valley.
The
parent
this province are metamorphosed sediments and volcanics
Carboniferous
the
The
rock
of probable
area
rocks
are
tilted
was
in
upper
metavolcanics
block
of
eroded
the
to
a
Jurassic
time.
of
Jurassic
to
Sierra
Nevada
near
tableland
and
then
The rocks of the
Triassic
the
age.
Challenge
deeply
incised
Experi-
mental
Forest
major
drainages-- Feather River to the north and Yuba River to the
south.
of
age, together with granitic rocks which intruded into
metamorphosed
Challenge
tilted
series
fault
and
Sacramento
on
into
11
Soils
The
sists
soil is
of
deep
and
metamorphosed
name
given
ed.
During
of the
very
Challenge
deep
basic
igneous
metamorphism
The Challenge series con-
well-drained
andesite, commonly
to
series.
called
rocks
the
soils
greenstone.
that
original
forest
have
been
developed
from
Greenstone
is the
hydrothermally
alter-
ferromagnesian
minerals
were
largely changed into chlorite, which gives the resulting parent
material
rock
granular,
medium
non-cobbly
Challenge
a
to
very
The
strongly
are
Experimental
series
producing
color.
acid, moderately
types
Challenge
timber
green
medium
massive,
The
a
site
important
fine
acid,
recognized.
Forest
is
covers
about
of
Challenge
series
textured
clayey
has
surface
subsoils.
reddish
soils
brown,
and
Both cobbly
red,
and
The soil in many portions of the
estimated
50
to
be
thousand
the
deep
forest
1969,
the
mean
50
to
acres
soils.
and
100
feet
deep.
is
the
highest
Economically,
it
is
temperature
at
soil.
Climate
__L___.-From
1965
through
annual
maximum
the Challenge Experimental Forest was 69oF and the mean minimum temperature was 43oF;
mean
maximum
extremes of 104oF and 11oF
temperatures
ranged
were
recorded.
90oF in July to
from
Monthly
51oF in December.
Monthly mean minimum temperatures ranged from 56oF in July to 32oF in
January
measured
(Table
only
1).
Prior
to
September
1965,
air
temperature
was
intermittently.
Precipitation occurs predominantly in winter with about 90 percent
of
the
April.
annual
The
total
entire
falling
soil
in
moisture
the
6
months
profile
is
from
November
usually
through
recharged
to
"field
13
capacity"
by
January
or
February.
Soil moisture depletion starts in
spring, usually in April or May.
2
inches
of
intensity
rain
falling
convectional
The
from
summers
June
through
thunderstorms.
are
dry
with
less
September--mainly
Thus, soil
moisture
than
from
high
depletion
continues through the summer season without significant recharge until
late
from
October
1939
or
through
November.
1969.
Precipitation
Average
annual
was
rainfall
measured
is
68
at
Challenge
inches,
ranged from 94.13 to 37.20 inches in the 30 years of record.
but
has
Snow
is
rare-- only 3 or 4 days occur annually with measurable snow depth. A
summary
of
monthly
temperatures
the study is found in Table 1.
years
is
found
in
the
and
precipitation
Daily
Appendix,
for
precipitation
Tables
12
through
the
for
6
each
years
of
of
the
6
17.
VegetationThe study site is located
forest
vegetation
in
the
area
in
consists
pine (Pinus
-....--. ponderosa Laws.), 20
menziesii [Mirb.] Franco),
Dougl.),
the
mixed
of
percent
conifer
about
40
Douglas-fir
forest
percent
zone.
The
ponderosa
(Pseudotsuga
8 percent sugar pine (Pinus
_I__-- lambertiana
6 percent incense-cedar (Libocedrus
--____c__ decurrens Torr.), 3 per-
cent white fir (Abies
concolor [Gord. & Glend.] Lindl.), and 23 per---I_ P_II_
cent hardwoods composed mainly of tanoak
&
Arn.]
Rehd.), madrone
black oak (Quercus
(Lithocarpus densiflorus [Hook
(Arbutus
- menziesii Pursh.), and California
kellogii
Newb.).
The ground cover is predominantly
bracken fern (Pteridium
--L--c--- aquilinum
--- [L.] Kuhn var. pubescens
- - - - Underw.),
poison-oak (Toxicodendron diversilobum T. & G.), Sierra gooseberry
(Ribes -roezlii- Kegel.), several
species
of
California-lilac
(Ceanothus
spp. L.), and manzanita (Arctostaphylos spp. Adans.), together with
sprouts of tanoak
and madrone.
14
The area was logged extensively from 1870-1880.
stand
found
of tanoak
on
the
Experimental
Forest
ranges
from
The
nearly
second-growth
pure
stands
with little current commercial value to dense stands of pine
and fir with stems of 40 inches dbh not uncommon.
In the general study
area, total stand density, expressed as basal area, averaged about 250
square
feet
per
acre.
CHAPTER III
LOCATION AND INSTRUMENTATION OF SOIL MOISTURE SAMPLING SITES
Plot
Selection
The
manent
Challenge
Fores t staff established about 60 per-
Experimental
growth plots prior to logging in
following
1962.
The
plots
had
the
properties:
Each
plot
center
was
located such that the plot had a basal
area of about 160 square feet
per
acre
of
conifers
greater
than 11.5 inches in diameter
2) Within a one-half acre circu lar
plot
all trees larger than 11.5 inches
3)
An
around
each
plot
center,
in diameter were measured
and
tagged.
In
addition, within a concentric one-fourth acre circular
plot,
all
eter
were
after-logging
trees
between
measured
mortality
3.5 inches and 11.5 inches in diam-
and tagge
survey
In 1963, 21 of these growth
was made of all growth plots in 1962.
plots were selected for a studv of
soil moisture storage and depletion.
A 50- by 50-foot grid of 100
blocks was located at the center of each plot and 3 of the blocks were
selected
at
random.
Within each block a neutron access tube was in-
stalled to a depth of 20 feet if soil
summer
1964,
a
water
table
observation
conditions allowed.
well
was
drilled
to a depth of 50 feet using a truck-mounted auger,
in
A 2-inch
In
late
each
plot
diameter
plastic casing with 1 mm perforations in the bottom 2 1/2 feet was installed in each auger hole.
On the basis of 2 years' observations of
16
soil
moisture
depletion
and
one
winter
of
observing
the
water
table
well, 3 of the 21 plots were selected for this study--one logged plot
(Ll) and two adjacent unlogged control plots (Cl and C2). Growth and
mortality
for
measurements
the
duration
were
of
made
the
study.
annually
The
in
each
criteria
of
for
these
plot
three
selection
plots
were:
1) No water table present to a depth of 50 feet at any time
during
2)
the
Uniform
of
year.
pattern
lateral
of
or
soil.
moisture
subsurface
recharge
with
no
indication
flow.
3) Well-drained site with no surface ponding
or water runoff
concentration.
4) No unexplained anomalies in soil moist ure data dur ing depletion
or
recharge.
5) Uniform soil with all access tubes at least 15 fee t in depth.
the
These criteria were established to reduce the
variabil ity
control
of
between
Plot
and
plots
study
and
Description
plots
between
and
and
access
to
make
tubes
comparison
within
a
plot
between
depletion
data
possible.
Instrumentation
--_II
All hardwoods in the logged plot (Ll) were poisoned with 2,4,5-T
During summer 1962, 2 years prior to the be-
in the fall of 1961.
ginning
of
this
study, 88 percent of the original basal area of the
There
were
only
one-half
acre
permanent
logged plot (Ll) was cut.
dbh
left
in 1962--l
uncut
on
the
ponderosa pine (28.5-inch
inch and 27.7~inch),
12
growth
larger
plot
than
4
inches
established
diameter), 2 sugar pines (28.7-
1 incense-cedar (9.4-inch),
inches), and 4 madrones
trees
4 tanoaks
(<- 6.0 inches) (Table 2, Fig. 2).
(<- 9.1
19
The
control plots (Cl and C2) have not been treated for about 90
years
(Table
2,
Fig.
larger
than
20
inches
sugar
pine--and 87 trees larger than 4 inches dbh.
24
trees
and
1
larger
3).
dbh--5
than
California
20
black
In general, the
prior
to
cutting
for
Douglas-fir,
inches
dbh--22
5
ponderosa
ponderosa
pine,
composition
quite
of
similar
the
vegetation
1
in
in
additional
criterion
for
selection
During summer 1965, 20
additional
soil
an
and
1
sugar
pine,
oak--and 70 trees larger than 4 inches dbh.
that
was
pine,
Plot C2 contains
to
similarity
was
The one-half acre plot Cl contains 11 trees
the
the
logged
control
plots.
of these
plot
This
plots
study.
moisture
access
tubes
were
installed in the logged plot Ll to depths varying from 16 to 21 feet
in specific quadrants at six distances from the 27.7-inch
sugar
40,
pine (Fig.
4).
diameter
The placement of access tubes at 2, 5, 10, 20,
and 60 feet from the study tree assured a greater density of
sampling
points
where
the
influence
of
the
tree
was
expected
to
be
greatest.
The location, size, and
a 120-foot
species
of
each
tree
radius of the study tree (Fig. 2, 4).
was
measured
within
All trees within 60
feet of the study tree were less than 12 inches dbh, with the great
majority
less
than
4.5
inches
dbh.
There
to 90 feet from the study tree, and a
feet
southeast
of
the
were clumps of tanoak
from
the
In
of
the
1962
tree.
group
of
several
smaller
large
trees
80
trees
about
10
Scattered throughout the plot area
and madrone sprouts growing from stumps left
logging.
December
study
study
were
1966
tree
all
were
trees
cut,
and
isolating
other
the
vegetation
sugar
pine.
within
120
Sprouts
feet
and
22
herbaceous
vegetation
were
until
conclusion
of
was
the
cut.
removed
the
study.
at
least
On
monthly
March
10,
as
they
1969,
the
It measured 31.6 inches dbh and 128 feet tall.
of 95 feet the tree was 13 inches dib, at
into
two
count
1
which
point
the
stems, each having a size of 10 inches dib.
appeared
study
tree
At a height
tree
forked
A tree ring
foot above ground level indicated the tree to be 85 years old.
The plot was kept in a bare condition for the remainder of 1969
by
Tree
cutting
sprouts
and
herbaceous
vegetation
at
least
monthly.
Growth
The basal area growth of the study tree at stump height was
measured by marking the position of each growth ring on paper tape
from the center growth ring along radii spaced every 30° of arc
(12
radii).
21-year
The
annual
growth
in
square
inches
was
computed
for
the
period, 1949-1969, by:
12
c
(m
2
i
- ITR
2
i
i=l
Annual Growth =
12
Where: r
i
is the radius of the tree in the year of interest along the
th
i
R, is
1
i
th
arc,
the radius of the tree in the preceding year along the
arc.
23
The results are found in Figure 5.
The
growth
for the
period
prior to
logging, 1949-1962, averaged 11.8 square inches per year. A leastsquares
fit
of
the
data
showed:
Annual
After
the
with
a
the
heavy
harvest
substantial
removal
of
Growth
thinning
residual
vegetation
the
year
and
In
annual
after
The
a
nearby
1962
uncut
growth
data
Growth
increment
the
1962,
in
Annual
added
in
increase
tree was cut in 1969.
The
= -0.98 + 0.18 x (age of tree)
the
rate,
in
after
study
which
1967
and
1962
tree
responded
continued
through
until
study
the
showed:
= -256.82 + 3.42 x (age of tree)
prior
logging
to
1962
was
0.18
was
3.42
square
square
inches
inches per
per
stand, the growth of an 81-year-old
year.
ponderosa
pine with a diameter of 26.1 inches dbh averaged 7.0 square inches per
year for the period 1949-1962, (Fig. 5), and:
Annual
Growth
= 4.72 + 0.03 x (age of tree)
The slope of the curve of annual growth for the sugar pine study
tree for the period 1949-1962 and for the ponderosa pine for the
period 1949-1962 and 1963-1969 was not significantly different from
zero
at
the
regression
95
for
percent
the
level
of
confidence.
pine
for
the
sugar
The
slope
of
the growth
post -logging period, 1963- 1969,
was significantly different from the slope of the regression for the
pre-logging
period, 1949-1962, at the 99 p ercent
Thus, the
in
a
period
sugar
of
pine
study
tree
moderately
rapid
growth
prior to the harvest cut in 1962.
heavy
harvest
cut
in
1962.
A
could be
characterized
and competing
A perio d
second
level of confidence.
of
with
release
as
its
being
ne ighbo
followed the
perio d of release may have occur red
following December 1966 when the residual vegetation
surrounding the
25
tree was cut.
eliminated,
Thus, when
the
tree
der of the study.
into
regions
the
competition
responded
The
root
previously
with
light
accelerated
system
occupied
for
by
may
have
the
roots
and
growth
for
of
this
study
did
not
permit
The
Challenge
was
the
remain-
been
actively
expanding
of
adjacent
competing
vegetation as reported by Ziemer (1964) for red fir.
sign
moisture
observations
of
However, the de-
root
growth
or
root
expansion,
-S o i-l
Analysis
Description.
soil
the
most
extensive
Challenge
series
is located in. the same general area as the three study plots.
Seven
soil
horizons
Survey
is
One of the California Cooperative
timber producing soil. in the area.
Soil-Vegetation
series
have
classification
been
identified
plots
and
for
the
described
by
the
Soil-Vegeta-
tion Survey as typical. of the Challenge soil (Table 3):
O1 to O2
3 inches to 9, fresh
litter
of
needles.
All
oak
and
and
shrub
partially
leaves
decomposed
and
conifer
1 to 4 inches thick.
0 to 2 1/2 inches, brown (7.5YR 4/4) light clay
loam, dark reddish brown (5YR 3/3) moist;
strong
soft
fine
when
and
medium
many
very
boundary.
common
fine
fine and medium shot;
smooth
structure;
dry, very friable moist, nonsticky
and nonplastic wet;
roots;
granular
and
very
fine
fine
pores;
slightly
acid;
2 to 5 inches thick.
and
fine
common
clear
27
Al2
2 1/2
to 12 inches,
reddish brown (5YR 4/4) clay
loam, dark reddish brown (5YR 3/4) moist; moderate
very
soft
fine
to
dry, very
when
medium
granular
friable
moist,
and nonplastic wet;
many
common
medium
very
fine
to
fine
to
pores;
structure;
nonsticky
coarse
roots;
common
fine
shot; medium acid; clear smooth boundary.
6 to 14 inches thick.
B1t
12
to
22
inches,
yellow red (5YR 5/5) heavy
clay loam, dark reddish brown (2.5YR 3/5)
weak
moist;
medium
subangular
structure;
slightly hard dry, friable moist, slightly
sticky
and
roots;
plastic
common
clay films;
wet;
fine
to
medium
common
medium
acid;
fine
pores;
to
coarse
few thin
clear wavy boundary.
4 to 15 inches thick.
B22t
32
to
50
inches,
dark red (2.5YR
blocky;
moist, very
few
B3t & C
very hard dry, very firm
sticky
medium
pores;
3/7) moist; few gravels and
massive structure breaking to
cobbles;
angular
red (2.5YR 4/7) heavy clay,
roots;
and
very
plastic
wet;
few
very fine and fine
very
many moderately thick clay films;
medium
acid;
clear irregular boundary. 10
to
25
inches
thick.
50
inches
plus,
red (2.5YR 5/6) and yellowish
red (5YK 5/6) very cobbly clay; light yellowish
28
brown (10YR 6/4) to yellow (10YR 7/6)
weathered
greenstone
structure;
very
cobbles;
few
roots;
face textures are heavy loams to clay loams with
Subsoil
shot and a few small gravels.
clays
with
a
few
with depth.
from
The
soil
to
medium
slightly
strongly
acid
in
outcrops
are
very
characteristics
nearest
the
gravels
the
and
cobbles
reaction
acid
rare.
which
becomes
in
subsoil
textures
the
medium
are
light
acid
in
clays
size
with
horizons
and
Base
saturation
is
or
and
depth,
surface
layers.
acid;
fine to medium size
increase
more
massive
amount
ranging
medium
low.
to
Rock
Table 4 describes the physical and chemical
of the Soil-Vegeta
ion Survey's classification plot
Variabili
study a
y between classification
plot s is
evident by comparing Tables 3 and
Texture.
soil
well.
moisture
ples were t kennear the center of each o f the
Soil
study plo ts while dr lli ng the water
About 500 gram s of
disturbe
SO
in
plastic
for about 6 weeks.
rated by sieving.
bags
and
stored
open
observa tion
il was extracted from the drill
hole at 2.5-foot intervals to a depth of 45 feet.
placed
table
in
the
The samples were
laborato ry to air dry
The fraction greater than 2 mm
(gr
l) was sepa-
The remaining fractions were determined by the
hydrometer procedure described by Day (1965). The soil was dispersed
by shaking the sample in sealed l-liter hydrometer cylinders for 18
hours
with
a
reciprocating
shaker.
35 sec., 45 sec., 6 hr., and 24 hr.
Hydrometer
The
readings
hydrometer
were taken
readings were
at
then
30
converted
to
textural
classes
(Fig.
6,
7,
8;
Table
5).
It may be
noted the amount of gravel found in the plot samples (Table 5) is much
less than reported at the Soil-Vegetation Survey classification plot
(Table
4)
or
series
(Table
similar.
the
3)
typical
for
Comparison
profile
comparable
is
characteristics
depths.
difficult,
sampling methods and depths.
The
however,
of
the
remaining
due
to
Challenge
fractions
the
soil
are
different
The soil. in the study plots is quite typ-
ical of the Challenge soil series (Colwell, personal communication).
Water
pressure
of
Retention.
membrane
approximately
within
a
plot.
Soil
moisture
retention
apparatus
(Richards,
grams
were
25
The
each
samples
were
was
1949).
taken
placed
Duplicate
from
in
determined
each
plastic
using
soil
samples
sampling
retainer
a
depth
rings,
saturated for 24 hours and placed in a ceramic plate extractor at l/3
atmosphere
hours.
(4.9
lb. in-2) or
15
atmospheres
samples
were
phere, the
standard
deviation
moisture
weight
The
paired
mean
moisture
Soil
in-2) for 48
lb.
The moisture content of the samples was then determined by
oven drying for 24 hours (Fig. 9, 10, 11;
197
(220.5
by
deviation
by
weight
Moisture
and
run
at
between
with
of
15
excellent
the
paired
atmospheres
paired
Table
samples
was
was
5).
In
precision.
samples
1.5
0.2
this
At 1/3 atmos-
was
1.2
percent
by
and
manner
0.3
percent
weight.
percent
respectively.
Measurement
- -
The neutron scattering method of soil moisture determination was
selected
for use
in
this
study.
The neutron method is particularly
suited to a study of soil moisture depletion in that access tubes are
permanently installed and the same point may be measured as frequently
38
as
needed.
checked
The
instrument
periodically
may
for
be
recalibrated
instrument
Access Tube Installation
---
as
often
as
desired
and
error.
Aluminum
access
tubes
1
were
installed
at each measurement site to a depth of about 20 feet to allow lowering
the neutron probe into the soil.
The tubes were placed in holes auger-
ed
with
about
access
the
one-fourth
tubes
clayey
obtaining
were
inch
sealed
soils
at
tight
fit
a
oversize
at
the
Challenge
bottom
this
between
a
the
Minuteman
with
a #9
procedure
soil
and
power
rubber
proved
the
auger.
to
access
The
stopper.
be
In
effective
tube.
in
Between
measurements the tubes were covered with cans to prevent accumulation
of
rain
water
inside
Calibration.
McGuinness,
discussed
tube.
Since
meter in the 1950's,
thoroughly
the
the
development
of
the
neutron
soil
moisture
the principle of the use of the method had been
by
a
number
et al, 1961;
Van
of
authors
Bavel,
et
(Stone,
al,
et
1963).
al,
The
1955;
questionable
adequacy of the factory calibration of the instrument for accurate
field
soil
University
moisture
of
determination
California
at
led
Davis
in
Professor
D.
cooperation
Nielsen
with
of
the
the
California
Department of Water Resources to develop an independent calibration
procedure.
Dr. Nielsen made neutron observations in cylindrical tanks,
4 feet high by 4 feet in diameter, filled
content.
soil
of
known
moisture
Nielsen collected data at 10 different soil moisture con-
tents
ranging
point
was
1
with
from
determined
0.9
at
to
43
27.8
percent
percent
by
moisture
volume.
in
a
One
tank
additional
filled
with
pea
The access tubes used in this study were purchased in 21-foot
lengths of aluminum alloy 6061-T6, 1.625-inch
O.D. x 1.555-inch
I.D.
39
gravel
and
saturated
with
water.
Nielsen
then
made
counts
with
the
same probe in a small. drum containing solutions of boric acid (a
neutron
absorber)
of
then
calibrated
by
Soil
moisture
data
cated
the
Davis
various
making
concentrations.
neutron counts
collected
boric
acid
in
field
calibration
in
Additional
the
U.
moisture
to
C.
probes
boric
depletion
be
quite
was
based
upon
determined
with
the
comparisons
probe, to
of
indicated
careful
soil
plots
This
moisture
measurements
irrigation water applied to small field plots.
acid
of
The
drum.
indi-
satisfactory
in the working range of 8 to 25 percent soil moisture.
sion
were
with-
conclu-
change,
the
as
volume
pea
gravel
of
cali-
bration point was later questioned by MacGillivray (personal communication)
and
revised,
found
to
departing
yield
from
the
values
too
original
low.
boric
The calibration was then
acid
curve
at
a
moisture
content of 36 percent and extrapolating a straight line through a
field
to
calibration
the
measured
The
study
point
count
procedure
was
to
at
rate
for
49
percent.
for
line
passed
very
close
water.
calibrating-the
simultaneously
This
measure
neutron
a
large
probe
used
number
of
In
soil
this
moisture
sites at Challenge with a probe calibrated by the Nielsen-MacGillivray
method
and
calibrated.
In
October
November 1964, 1,028 paired measurements were made.
In
June
additional
one
probe
paired
which
measurements
had
not
were
been
made
to
obtain
values
at
and
1965,
the
730
wet
end of the curve.
A ratio is made of the field count at the sampling
depth
standard
to
gression
calibrated
the
was
mean
initially
probe
and
run
the
count
with
count
tain an interim calibration.
in
the
ratio
The
a
paraffin
computed
for
soil
shield.
soil
the
moisture
linear
moisture
uncalibrated
for
A
each
for
probe
of
the
rethe
to
ob-
1,758
40
paired
points
was
composed
of
soil
moisture
measurements
taken
foot increments in 88 holes having a depth of about 20 feet.
than
five
than
2
measurements
percent, it
probes
were
not
per
was
hole
assumed
equal,
The
were
removed
greatest
field
volume.
It
curve
pass
to
is
and
In
the
moisture
through
a
the
this
vertical
data
If
more
differences
positioning
from
that
consisted
observed
reasonable
near
have
1
greater
of
hole
the
were
re-
1,696
about
63
exp ect the
to
100
was
of
percent
points.
percent
by
calibration
isture
content.
Thus,
23 additional points were taken in a 55-gallon barrel of water.
calibrated
probe
contained
a
226-radium-beryllium
neutron
Significant
differences
in
soil
moisture
The
source
the uncalibrated probe contained a 241-americium-beryllium
source.
two
manner, 3 holes of the 88 holes
edited
point
to
measurements
content
theoretically
found
that
thus, all
moved from the calibration.
surveyed
were
at
and
neutron
determination
have
been found to be due to the type of source used (Goldberg,
et al, 1967;
Ziemer,
to
et
al,
1967).
These
differences
are
due
primarily
neutron energy characteristics of the different sources.
predominantly
continuity
or
evident
when
measuring
abrupt
soil
water
soil
change
moisture
such
water table or pockets of wet or dry soil.
trons
became
significant
at
with the uncalibrated probe.
linear
the
and
data (r
Field
a
2
third
=
degree
the
higher
count
Consequently,
as
in
the
Errors
regions
near
the
of
soil
are
dis-
surface,
Coincidence loss of neurates
the
which
were
relationship
equation
was
necessary
to
tubes
were
surveyed
with
obtained
was
satisfactorily
not
fit
0.992).
Measurement.
The
a
modified
P-19
Nuclear-Chicago Soil Moisture Probe and Model 2800-A Scaler.
The P-19
Probe was obtained without source.
was
An
aluminum
source-holder
41
milled locally to hold a 0.1 Curie 241-americium-beryllium neutron
5
source having a neutron flux of 2.33 x 10 n/sec.
This
meter
produced
a count rate of about 53,500 counts per minute in a water standard.
0ne minute thermal neutron counts were made at the 9- and 18-inch
depth
and
tube.
at
The
than
9
successive
shallowest
inches
biased
by
moisture
1
loss
content
intervals
measurement
because
the
foot
measurements
of
would
could
neutrons
be
less
not
made
to
the
than
to
the
the
be
depth
made
closer
of
at
to
the
atmosphere
and
actual
the
a
access
depth
surface
the
(Ziemer,
less
are
indicated
et
al,
1967).
The sites were measured at 2- to 3-week intervals beginning in
the
spring
continued
ments
soil
until
were
surveys.
the
last
first
during
heavy
heavy
the
rains,
rains
winter
of
generally
the
period
to
the
field
in
fall.
May,
Several
evaluate
the
and
measure-
progress
of
recharge.
count
data
was
plotted
in
and
compared
to
previous
Any questionable readings were repeated by repositioning the
and
surveyed
the
made
moisture
The
probe
following
taking
if
the
another
data
count.
appeared
At times an entire hole would be requestionable.
CHAPTER IV
SOIL WATER REGIME
During
soil
moisture
isolated
51
the
data
tree
times.
5-year
course
was
plot
of
the
collected.
and
6
tubes
data
tron
which
required
ruary
were
first
computer
probe
logged
tremendous
quantity
of
Each of the 23 access tubes in the
in
some
the
form
visualize and understand spatial
tabular
a
uncut
control
plots
were
measured
This translates into about 30,000 individual soil moisture
measurements
field
study,
screened
output
of
calibration.
plot
on
each
of
presentation
in
order
and time related processes.
for
obvious
errors
soil
moisture
content
Graphic
profiles
of
measurement
date
from
and
then
based
upon
soil
August
25, 1970 were constructed (Fig. 12-27).
moisture
to
The raw
reduced
the
in
to
neu-
the
16, 1965 through Feb-
The
profiles
shown
on
the left represent isopleths
of the total moisture held in a 15-foot
soil
date
depth
on
the
particular
in
the
logged
plot.
Each
contour
line is expressed in feet of soil water in 15 feet of soil.
tour
interval is
represent
and
the
0.2 feet of water.
average
distance
from
soil
the
study
moisture
tree.
The
in
The
profiles
the
logged
values
moisture by volume, expressed as feet of water
the
particular
example,
depth
two values
and
distance
from
the
are
shown
on
The conthe
right
plot
related
to
the
average
soil
per foot of soil,
tree
on
a
given
date.
depth
for
For
were averaged for each depth and date of measure-
ment 2 and 5 feet from the study tree and four measurements were
averaged for each depth and date for the 10-, 20-, 40-, and 60-foot
distances.
Twenty-three
points
provided
the
data
base
for
the
contours
59
in the left profile and 90 points were used to produce the profile on
the
drawn
ing
After the data grid was established the contours were
right.
by
eye
using
topographic
and
the
standard
isohyetal
rules
of
interpolation
for
construct-
maps.
Soil Moisture at Recharge
Soil moisture in the isolated tree study plot (L1) from late
winter
to
early
spring, the time when the soil was totally recharged,
was
quite
similar from
the
study
to
to
determine
measure
the
year
to
year.
the
maximum
of
recharge
time
Though
soil
it
moisture
during
any
was
not
held
in
year,
we
the
the
did
intent of
profile o r
measure
soil moisture within 5 days to a week after a number of major winter
storms
throughout
the
duration
of
the
study.
In 1966, the first winter of measurement in plot Ll, soil moi
ture reached 45 to 51 percent moisture by volume at all depths, wi t h
the
exception
attained
inches
by
of
of
the
the
surface
February
11
foot.
Total soil moisture recharge was
measurement,
following
more
than
The distribution of soil moisture with depth and
rainfall.
distance from the study tree was quite uniform (Fig. 13c).
discrepancies
were
a
34
zone
of
slightly
higher
moisture
about
The only
2
feet
a-
way from the tree at a depth of 4 feet and a zone of slightly lower
moisture
below
a
depth
of
about
12
feet.
In horizontal profile, soil
moisture storage around the study tree varied from 7.0 to 7.6 feet of
water in 15 feet of soil.
The 7.0-foot isomoisture lines were about
10 feet northwest and south of the tree and also 60 feet due north of
the
tree.
The
7.6-foot
profile
was
found
40
feet
in
the
northwest
60
direction
a very small
and
zone
in
the
southwest
direction about 20
feet from the s tudy tree.
the wint er
During
isolating
was
the
of
1967,
s tudy tree.
observed
th rough out
to
vegetation was cut
surrounding
On March 2, a
the
of rain had fallen p rior
the
study
plot
the
March
uniform
soil
mois ture
content
17b).
Nearly 57 inches
measurement.
The re charge had
(Fig.
progressed deeper than in February 1966.
At the 15-foot
depth, the
soil. moisture was 48 percent by volume, whereas in 1966 it had reached
45
percent.
January
1967
had
been
an
tion was about 24 inches --nearly 12
February
1967
was
less
by
the
than
been
period
moisture
moisture.
the
in
and
The
the
surface
inches
unusually
vegetation
By March,
comparable
an
1.5 inches --over 10
coniferous
month.
soil
had
soil
1966.
was
had
had
dry
of
dried
wet
above
and
month.
normal.
warm
Precipitation
below
normal.
certainly
a
probability
substantially
were
very few
the
to
area
39
contained
percent
Transpiration
during
more
zones
soil
PrecipitaIn contrast,
month.
inches
drained
There
majority
unusually
than
of
48
this
for
the
51 percent
percent soil
moisture
and
the
45 percent isomoisture line was found 3 to 4 feet in depth as in 1966.
The total amount of water held in storage in 1967 was lower than in
1966--reflecting
drainage and probably evapotranspiration.
The
driest
zone in the plot was about 20 feet due east of the study tree--an area
which contained 6.4 feet of water in 15 feet of soil.
contained
7.4
feet
of
soil
water.
The
pattern
of
soil
The wettest zone
moisture
in
winter 1967 was similar to that for 1966, except there was about 0.2foot less water per 15 feet of soil in 1967 than in 1966.
The
inches
of
remainder
rain
of
fell
spring
between
1967
March
was
and
quite
wet.
July--more
An additional 31
than
13
inches above
61
normal
end
for
of
this
June
period.
1967
Soil moisture held in the study plot at the
exceeded
that
measured
in
early
May
1966,
including
that in the surface foot of soil.
The soil moisture in storage on March 4,
gain
quite
storage
uniform
in
previous
measurement
moisture
was
at
45
was
very
years.
about
the
moisture.
than
and
41
3-foot
similar
Seasonal
inches.
depth.
to
1968 (Fig. 20b) was a-
the
rainfall
winter
to
the
soil moisture
March
1968
There was a zone of 54 percent soil
The
majority
of
the
area
was 48 percent
A larger zone of 51 percent moisture was found below 13 feet
was
observed
percent
soil
in
1967.
The
surface 2 feet of soil were less than
moisture, but the soil moisture in the plot was fully
recharged.
On March 27, 1969, after the study tree had been cut, soil moisture
in
the
About 80
early
was
of
rainfall
only
the
feet, the
earlier
appeared
quite
similar
to
previous
winters
inches of rain had fallen by the end of March.
winter
fall
ity
plot
about
study
soil
out the plot.
2
plot
moisture
years.
3 to 4 feet.
was
substantially
above
normal,
inches
below
inches--about
7
contained
48
percent
was
than 45
less
soil
Fall and
but
March
normal.
moisture.
percent,
(Fig. 24b).
as
was
rain-
The
major-
Above 2
observed
in
There was still a zone of high moisture at a depth of
There
were
several
zones
of
51
percent
moisture
through-
Uniform soil moisture content is evident throughout the
plot as can be seen in both the horizontal as well as vertical profiles.
During
this
wet
year, some
the southwest, the northeast, and
zones
reached 7.6
immediately
feet
surrounding
of
water
the
in
study
tree, but a nearly identical pattern can be seen in the March 4, 1968
profile.
As
in
prior
years, the
zone
of
low
soil
moisture
was
found
62
about
20
feet
from
the
study
tree
in
an
easterly
direction.
By
this
late in the spring there had been an opportunity for substantial evapotranspiration.
replenishing
However,
evaporated
In summary, soil
formly
recharged
tree.
This
"field
in
light
rains
surface
soil
moisture
terms
uniformity
in
of
was
periodically
throughout
March
moisture.
the
both
found
fell
logged
depth
each
study
and
plot
distance
winter
was
very
from the
during
the
uni-
study
study.
The
capacity" of the logged plot was about 48 percent by volume
with a zone of higher soil moisture about 3 to 4 feet beneath the tree
and extending to a distance of 2 feet from the tree.
soil.
moisture
attributable
evaporation
following
were
to
found
within
the
texture, organic
between
storms
storms.
before
content
The
making
surface
of
procedure
a
soil
2
feet
the
of
Areas
which
soil,
waiting
moisture
could
and
5
of
lower
be
surface
rainless
measurement
days
provided
a
substantial opportunity for surface evaporation as well as allowing internal
drainage
Depletion
form
of
the
soil
profile
to
proceed.
Trends
Each
summer
soil
moisture
depletion
profile
season
with
began
depth
with
and
a
fully
distance
recharged
from the
and
study
unitree.
Soil moisture depletion by evapotranspiration generally began in the
spring
after
the
last
continued
through
tion
atmospheric
of
the
significant
summer
rain.
without
further
evapotranspirational
vegetation available to transpire
1. -Partially
cut condition.
P-P
Depletion of soil moisture
recharge
demand
and
and
the
was
a
amount
funcof
soil water.
In 1962, 88 percent of the stand
basal area in plot Ll was removed by logging.
Consequently,
in
1965
63
and 1966, study plot L1
the
degree
and
C2,
of
1965
soil
had
was in a partially cut condition.
moisture
one
of
the
had one of the highest.
a 15-foot
depletion
in
lowest
the
uncut
Based on
control plots
evapotranspirational
uses
C1
and
1966
The average amount of soil moisture left in
deep soil profile in the uncut control plots was 6.15 feet
of water at the end of the 1965 depletion season and only 4.56 feet
of water in 1966.
Beginning
with
a
1966, by May 6, the
and
48
39
percent
percent
uniform
surface
while
moisture
the
by
soil
moisture
moisture
remainder
volume
had
of
(Fig.
profile
been
the
depleted
soil
14a).
in
early
to
remained
spring
between
between
By May 19, soil
33
45
and
moisture
within 20 feet of the tree had been depleted to a nearly constant 45
percent (Fig. 14b).
By
early
June, soil
moisture
within
20
feet
of
the tree had become about 42 percent and beyond 30 feet from the tree
it was approximately 45 percent (Fig. 14c). The surface layers had
dried to near 30 percent. The initial development of the three distinct
lobes of lower soil moisture can begin to be observed in the May 6
profiles
with
6.6
feet
of
water
at
the
driest
points.
By
early
these lobes had reached 6.0 feet of water in 15 feet of soil.
June
This
general pattern continued at progressively lower soil moisture contents
through
the
summer.
By mid-July, beyond
mained
between
42
and
30
45
feet
percent
from
(Fig.
the tree,
the
soil
moisture
re-
A zone of low soil
15b).
moisture at 36 percent developed from 2 to 15 feet from the tree at a
depth of 8 to 12 feet.
appear
of
in
early
This
June, but
region
became
of low
more
soil
distinct
moisture
by
July.
began
to
By the end
August, very distinct patterns of increasing soil moisture with
64
increasing distance from the tree had developed (Fig. 16a).
The uni-
form
the
zone
of
high
soil
moisture
remained
beyond 30
feet
from
tree,
as did the zone of low soil moisture 8 to 12 feet under the tre e to
to
distance
of
10
feet.
a
There was a region of high soil moisture abou t
2 feet from the tree and 3 to 4 feet in depth that was observed throughout
the spring
27
percent
of
the
soil
summer.
The surface foot of soil had dried to about
moisture
by
mid-August
25, the
end
of
where
it
remained
for the
rest
summer.
By
of
and
October
soil
moisture
season, except
throughout
soil
the
moisture
the
1966
plot
was
differences
depletion
similar
with
period,
to
depth
the
earlier
and
the study tree had become much more graphic (Fig. 16c).
pattern
in
the
distance
The
from
soil
re-
mained at a quite uniform 39 percent moisture content beyond a distance of about 40 feet from the tree below a depth of 4 feet.
The soil
moisture content 60 feet from the tree was less than that 40 feet from
the
tree
due
to
the
influence
of
surrounding
vegetation
removed until the next phase of the study in 1967.
The
which
was
zone
of
not
lowest
soil moisture in the plot was at a depth of 8 to 13 feet extending to
a distance of 10 to 15 feet from the tree.
had
been
depleted
to
about 24
percent
by
In this zone, soil
volume.
The
moisture
zone of high soil
moisture at a distance of 2 feet from the tree and at a depth of 3 feet
remained
throughout
the
summer.
The pattern of soil moisture found in the partially logged plot on
October
19,
1965
(Fig.
was quite similar to the pattern on Septem-
ber 6, 1966 (Fig. 16b), one month earlier.
low
tion
There
were
three
lobes
of
soil moisture around the study tree at the end of the 1965 depleseason--a southeastern
lobe
with
4.4
feet
of
soil
moisture
in
65
15 feet of soil, a northwestern lobe also with 4.4 feet of moisture,
and
a
southwestern
1 1/2 months
of
lobe
with
depletion
in
4.6
feet
of moisture.
1966, soil
moisture
by
With
an
October
additional
25
had
been
depleted to 3.8 feet of water per 15 feet of soil in the southeastern
lobe and 4.0 feet of water in the western lobes (Fig. 16c).
extreme
northern
edge
of
the
plot, 60
was a zone of low soil moisture.
In
feet
1965,
from
the
the
soil
study
At
the
tree,
moisture
there
in
this
region was depleted to 5.2 feet of water (Fig. 12c), and in 1966
reached 4.8 feet of water (Fig. 16c). The depletion of soil moisture
in this area was not affected by the study tree, but was due to a
large
tree
immediately
outside
the
plot
(Fig.
4).
Zones
of
high
soil
moisture were found 40 to 60 feet from the tree in the northwestern
and
northeastern
portions
of
the
plot
where
soil
moisture
was
6.2
to
6.4 feet of water per 15 f e e t of soil at the end of the 1965 summer.
At the end of the 1966 summer, these
areas
each
contained
6.0
feet
of
by
re-
water.
2.
Isolated
tree
condition.
The
study
tree
was
isolated
moving all of the peripheral vegetation within a 120-foot
December
1966.
The
depletion
patterns
in
1967
and
1968
radius in
were
similar
to those for the period prior to isolating the tree, but with some
notable
As the depletion season progressed, the region
exceptions.
from 20 to 40 feet from the tree no longer contained more soil moisture than the 40- to
the
were
plot
had
nearly
mid-July
been
region.
removed and
horizontal
1967, the
60-foot
beyond
main
zone
the
20
of
The
isolines
feet
soil
from
residual
of
soil
the
tree.
moisture
vegetation
outside
moisture with
For example, in
difference
within a distance of 20 feet from the tree (Fig. 18a).
depth
was found
Beyond
20
feet
66
from the tree below a depth of 4 feet, the soil moisture content was
a
uniform 45 percent by volume. At a depth of less than 4 feet, the
influence of surface evaporation and transpiration by annual grasses
and
layers,
the
soil moisture was very similar from 20 to 60
feet
from
Closer
similar,
the
herbs
was
to
evident.
the
period
Within
tree, the
before
from
the
tree.
a distance of 2
three
primary
depletion
cutting,
but
surface
pattern
differed
was
in
detail.
depletion
the
in
tree.
general,
feet from the tree was still apparent.
of
depletion
developing
in
the
depth to
No
longer
horizontal
The
area of depletion was north and west of the study tree.
Another
of
depletion
developed
southeast
of
the
study
tree.
were
pattern.
There were now only one or two main areas of depletion.
area
to
The zone of low
The zone of high moisture at the 3-foot
lobes
of
to 12 feet in depth and to a distance of 10
soil moisture remained 8
feet
these
primary
The
small
major
zone of low soil moisture in the extreme north portion of the plot 60
feet
from
The
the
tree
lowest
had
soil
been
moisture
diminished.
in
the
surface
2 feet was reached in
late September (Fig. 19a). An early October rainfall of 2.8 inches
partially wetted the surface 4
measurement
(Fig.
feet
pattern
returned
of
to
soil
that
soil
by
the
October
moisture
observed
lated study tree extended
below
on
end
a
of
depth
September
the
of
1967
2
The
26.
depletion
feet
had
influence
The
zone
of
10
the
soil
moisture
moisture
re-
had
season,
essentially
of
the
iso-
40 feet from the tree at a depth of 11 feet
and 20 feet from the study tree at a depth of 5 feet.
soil
13
19b).
By October 30 (Fig. 19c), the
the
of
depletion
been
occurred
depleted
to
at
a
depth
30
percent
of
by
greatest
feet,
volume.
where
Soil
67
moisture
from
became
this
progressively
point.
greater
upward,
outward,
mary
evaporation
zone
to 10
with
of
seemed
soil
to
extend
moisture
to
depletion
a
in
depth
the
The
from
uncut
1967--4.8
this
The influence of
of
2
feet.
horizontal
feet north and northwest of the tree.
distance
downward
The zone of highest soil moisture in the plot was
directly under the study tree at a depth of 3 feet.
surface
and
Soil
The pri-
profile
moisture
was
5
increased
region,
control
plots, C1 and C2, were drier in 1968 than in
feet of soil water remained in the 15-foot
soil profile at
the end of summer 1968, whereas 5.1 feet of water remained at the end
of
1967.
study
in
The
plot
late
soil
minimum
soil
early
October
in
September
moisture
and
moisture
1968
late
attained
attained
the
isolated
tree
(Fig. 23b), was similar to that found
October
northwest
in
of
1967.
the
However,
study
tree
in
1968
the
was
4.6
feet
lowest
of
water in 15 f eet of soil and in 1967 was 5.0 feet of moisture.
The
minimum soil moisture content was 27 percent in 1968 and 30 percent in
1967.
tree
The minimum soil moisture content found 40 feet from the study
was
about
3. Bare
42
percent
condition.
by
The
volume
study
in
tree
both
was
cut
1967
in
and
1968.
March
1969,
leaving
a bare plot which was maintained throughout the summer by cutting tanoak
sprouts
and
The
pattern
of
season
was
herbaceous
soil
vegetation
moisture
dramatically
in
different
every
2
weeks
the
study
plot
than
that
as
through
observed
in
it
appeared.
this
either
depletion
the
1965
and 1966 partially cut period or in the 1967 and 1968 isolated tree
period.
At the beginning of the depletion period in late March 1969
(Fig. 24b), the soil in the plot was very similar in moisture content
with
depth
as
was
observed
in
previous
years.
As
the depletion season progressed, the zone of the lower soil
moisture
early
in
the
July
eastern
when
that
portion
area
of
the
plot
remained
distinct
until
contained
6.2
feet
of
moisture
(Fig.
soil
25b), At that time the soil moisture content in this zone stabilized
while
to
soil
moisture
about
the
in
same
the
surrounding
moisture
content.
area
The
continued
zone
of
to
high
be
depleted
soil
moisture
at a depth of 3 feet and a distance of 2 feet from the stump of the
study
tree
also
remained
as
the
season
progressed.
By early October (Fig. 26c), the end of the depletion season for
1969,
than
soil
in
moisture
comparable
in
the
periods
plot
prior
had
to
a
dramatically
cutting
the
different
pattern
tree.
Although
study
the zone of high soil moisture 3 feet under the stump remained as it
had
feet
prior
to
under
cutting
the
tree
the
had
tree,
the zone of low soil moisture 8 to 13
completely
disappeared.
No
longer
were
there
major differences between the soil moisture within 20 feet of the
study tree stump and that 20 to 60 feet from the stump.
3 feet in depth, the
about
45
percent
soil
by
moisture
content
remained
Below
quite
about
uniform
at
volume, which is similar to that in the region 40
to 60 feet from the tree during the period before the study tree was
cut.
Soil
moisture
closest
to
the
study
at any other location in the plot.
had
been
probably
observed
due
immediately
Soil
the
under
moisture
increase
the
to
of
the
tree
contribution
the
tree
the
surface
in
toward
presence
when
the
a
stump
greater
was
foot
the
volume
was
alive.
of
which
of
This
tree
the
had
of
study
of
was
higher
opposite
to
that
than
which
The increased moisture is
zone
been
stump
of
high
observed
soil
appeared
tree.
herbaceous
This
soil
moisture
throughout
to
was
the
study.
progressively
perhaps
vegetation
such
due
as
to
69
bracken
fern
and
poison-oak
farther
from
the
tree.
Shade
and
needle
fall from the study tree probably retarded the growth of this groundcover
vegetation.
In
the
first
year
after
removing
the
study
tree,
the herbaceous vegetation had not yet invaded the plot within about 20
feet of the stump.
after
the
ful
because
difficult
of
study
tree
was
herbaceous
to
low
Attempts to keep the surface in a bare condition
control
lying
isolated
plants
by
such
in
1967
as
bracken
periodically
herbaceous
vegetation
were
cutting
was
not
fern
the
entirely
and
tops.
observed
to
success-
poison-oak
The
are
density
generally
increase
beyond a distance of about 20 feet from the study tree prior to removal
each fortnight during 1967, 1968, and 1969.
Total
Summer
Soil -Moisture
- Depletion
The average soil moisture contained in each of three 5-foot depth
classes at the end of the five summer seasons was obtained by planimetering
the area within the right-hand portions of Figures 12c, 16c,
19c, 23b, and 26c bounded by the depth and distance from the study
tree (Table 6).
The soil moisture content in each of the six distance
classes
are
for
the
years
2
plotted
the
following
2
isolating
years
the
prior
study
to
isolating
tree,
and
the
the
1
study
year
tree,
after
cutting the study tree and when the plot was in a bare condition
28).
for
soil
The
soil
moisture
content
of
the
uncut
control
plot
is
(Fig.
included
comparison,
Surface
5
at
end
the
feet.
of
Soil
moisture
depletion
season
evaporation and evapotranspiration
(Fig. 28a).
The
soil
moisture
contained
was
in
the
primarily
surface 5
the
result
feet
of
of
surface
by shallow rooted grasses and herbs
content
appears
to
be
poorly
related
to
72
distance
from
the
study
tree, but was affected by tree removal. Prior
to isolating the study tree in spring 1967, the area 40 to 60 feet
from the study tree had more soil moisture at the end of the growing
season than the regions closer to the study tree. The region 20 to 40
feet
from
the
tree
contained
slightly
less
soil
similar to that 40 to 60 feet from the tree.
moisture
The
content
lowest
was
soil
found
in
moisture
the
content
region
was
moisture,
in
of
lowest
the
soil
study, the
moisture
uncut
content
control
observed
plot
for
to
the
ranged from 5 to 20 feet from the study tree.
years
was
An intermediate soil
closest
found
but
the
two
During
depth
tree.
regions
which
these
first
the
highest
experienced
this
study
class
at
2
the
and
end
of the 1965 and 1966 summer periods, respectively.
During
was
the
next
isolated, the
nearly
identical
2
soil
to
years, 1967
moisture
the
and
1968,
content
previous
2
in
regions
feet
beyond
from
20
the
feet
study
from
tree
the
the
years.
in the O- to 2-foot and the 2- to 5-foot
study
continued
the
uncut
study tree
control
plot
was
Soil moisture was higher
distance classes than in the
tree.
to
after
The area within 5 to 20
have
the
lowest
soil
moisture.
In 1969, following removal. of the study tree, the region 40 to
60
feet
from
the
stump
of
the
study
than any other zone in the plot.
four
regions
20
feet
of
soil
moisture
lines
when
the
pletion
season
for
each
of
the
contained - less
-
soil
moisture
It is of interest to note that the
within
parallel
tree
the
5
study
tree
content at
years
are
stump
the
produced
end
connected.
of
nearly
the
de-
This is an
indication that the process of evaporating soil moisture within this
area
was
was
similar
when the
isolated, and when
plot
was
partially
cut,
the study tree was cut.
If
when
the
study
different
tree
mechanisms
73
of evaporation were present, we would expect the slope of the soil
moisture
curves
to
change
from
year
to
year
relative
to
adjacent
areas.
For example, during high potential evapotranspiration years, such as
1965
and
1967,
we
would
expect
a
larger
difference
between
each
than in low evapotranspiration years, such as 1966 and 1968.
strata
This
is
found for the regions beyond 20 feet from the tree, but not for the
regions
closer
when
the
ed.
During
than
20
was
relogged
plot
the
feet
isolated
foot and 40- to 60-foot
from
to
tree
the
tree.
isolate
the
Between
study
slightly
when
tree,
and
the
1967,
curves
cross-
phase, in 1967 and 1968, the 20- to 40-
distance class moisture curves were parallel
as in the partial-cut phase in 1965 and 1966.
verged
1966
the
study
tree
was
cut
This
in
relationship
di-
1969.
The soil moisture stored in the uncut control plots at the end of
the depletion season can be used as a form of climatic control on the
effect
soil
of
the
moisture
vegetation
in
the
treatments
uncut
control
in
the
plots
logged
was
study
within
1
plot.
percent
Average
of
that
found 40 to 60 feet from the study tree for all years except 1967, the
year
immediately
this
year
soil
following
moisture
lower than that 40
to
note
that
lines
the
in
the
isolation
uncut
of
the
control
study
plots
was
to 60 feet from the study tree.
connecting
the
end-of-season
tree.
about
During
3
percent
It is of interest
soil
moisture
are
parallel for the regions 0 to 2, 2 to 5, 5 to 10, and 10 to 20 feet
from the study tree from the beginning to the end of the study.
The
line for the region 20 to 40 feet from the study tree is intermediate
between the variation found between the 0- to
40- to 60-foot
tween
region.
The 20- to 40-foot
20-foot
region
and
the
zone is the transition be-
that area which is affected by the study tree and that area
74
which
in
is
soil
tree
outside
the
moisture
plot.
influence
of
contained in
The
roots
of
the
the
tree.
uncut
trees
and
control
essentially
high
the
soil
same
potential
moisture
moisture
to
be
herbaceous
the
content
during
in
vegetation
uncut
control
than
years
was
of
study
probably
control
plot
the
plot.
fully
For
depleted to
both
low
and
evapotranspiration.
In summary, soil
seems
in
plot
herbaceous
occupied the surface 5 feet of soil in the
this reason, the
There was less variation
due
moisture
to
surface
vegetation.
The
loss
from
the
surface
evaporation
and
evapotranspiration
vegetation
treatments
in
5
feet
the
of
soil
by
study
small
plot
had a minor affect on soil moisture storage in the surface layers at
the
end
of
the
summer
5 to 10 feet.
depletion
period.
Soil moisture at a depth of 5 to 10 feet was much
more variable and much more dependent upon distance from the study
tree
than
was
Apparently
the
surface
soil
moisture
evaporation
within
and
the
surface
transpiration
by
5 feet
(Fig.
herbaceous
28b).
vegeta-
tion and grasses had less influence on soil moisture depletion at this
depth
than
tree
roots.
The soil moisture content 40 to 60 feet from
the study tree was higher than at any other distance for each year with
the exception of 1969, the year after the study tree was cut.
and
1966,
decrease
before
in
the
soil
study
moisture
tree
was
isolated,
there
content
as
distance
from
Within 10 feet of the study tree, the
soil
moisture
was
the
a
In
1965
progressive
tree
content
increased.
was
quite
similar.
After the study tree was isolated in 1967, there was an in-
crease
soil
all
in
distances
moisture
from
the
relative
study
to
tree.
that
in
the
uncut control
plot
at
Beyond 5 feet from the study tree,
75
the
soil
control
contained
plot
connecting
distance
for
the
the
both
same
ratio
1967
and
end-of-season
region
beyond
5
moisture
1968.
soil
feet
of
That
moisture
from
the
relative
is, the
to
slopes
for
1967
and
study
tree
were
of
uncut
the
1968
lines
for
each
essentially
A similar
same as the slope for the uncut control plot.
the
the
relationship
can be observed for the 2 years prior to isolating the study tree,
except
to
the
the
quantity
uncut
of
control
soil
plot.
moisture
in
the
plot
was
less,
relative
Within 5 feet of the study tree, the slope
of these lines was not parallel to that in the uncut control plot.
In 1969, the year after the study tree was cut, a major change
occurred
in
the
relationship
the various regions.
The
between
highest
the
soil
soil
moisture
moisture
contained
content
was
in
found
with-
in 5 feet of the stump of the study tree, followed by the region 5 to
10 feet from the stump.
The
average
soil
moisture
beyond 10 feet from the stump was within 1
of
the
distance
regions
showed
a
in
1969
relative
to
period
before
the
for
the
three
regions
percent of each other.
substantial
cutting
increase
the
in
study
soil
tree
All
moisture
with
the
exception of the region 40 to 60 feet from the stump.
Lines
connecting
the
end-of-season
soil
moisture
in
the
uncut
control plot were essentially parallel to lines connecting similar data
for the 40- to 60-foot
10-foot depth class.
distance region for all years within this 5- to
However,
the
uncut
control
plot
contained
about
10 percent less soil moisture by volume than the region 40 to 60 feet
from the study tree for all years.
Lines connecting end-of-season
soil moisture for the region 20 to 40 feet from the study tree were also
parallel to those for the 40- to 60-foot
distance and for the uncut
control
was
plot
except
after
the
study
tree
removed
in
1969.
The uncut
76
control
plot
contained
2
to
3
percent --_I
more
soil
moisture
than
the
region
within 20 feet of the study tree for the 2 years before the tree was
isolated
early
in
1967.
Then
in
1967
and
1968,
the
uncut control
plot
contained 1 to 2 percent less water than the region within 20 feet of
the
study
In
tree.
1969,
after
the
study tree
was
removed,
there was
10 to 13 percent -less- soil water in the uncut control than in this area
of the study plot.
10 to 15 feet.
-in
1966
and
1968,
During
the
years
of
uncut control
high
plot
evaporative
utilized
demand,
more soil
such
water
as
at
10 to 15 feet in depth than at a depth of 5 to 10 feet (Fig. 28c).
During
periods
of
lower
demand, such as in 1965, 1967, and 1969, soil
moisture depletion was nearly identical from the 5- to lo-foot depth
and from the 10- to 15-foot
depth in the uncut control plot.
In the study tree plot, the soil moisture in the area from 20 to
60 feet from the tree at a depth of 10 to 15 feet was nearly identical
to that at a depth of 5 to 10 feet for all years of the
ever, prior
uncut
control
to
isolating
plot
at
the
the
study tree
end
of
the
contained about 3 percent more soil
feet of the
control
area
plot
within
study
After
tree.
contained
about
2 feet of the
study
1967,
1965
and
moisture
isolating
3
in
the
The
1966
than
moisture
depletion
the
study
percent -less- soil
tree.
soil
study.
area
Howin
the
seasons
within
2
tree, the uncut
moisture
than
the
parallel soil moisture
curves are a good indication that the use of soil moisture by the isolated tree was similar to that by the uncut forest.
Before
isolating
the
about 1 percent more soil
tree.
After
isolating
the
study
tree, the uncut control plot contained
moisture than the area within 20 feet of the
study
tree, the
vegetative
surface area
77
available
uncut
for
evapotranspiration
control
plot
contained
was
about
reduced
3
in
percent
the
study
less
soil
the area within 20 feet of the isolated tree.
In
study tree was cut and the plot was bare, the
uncut
tained
about
12
percent
less
soil
moisture
than
feet of the stump of the study tree.
This
on
the
by
an
uncut
relative
use
of
soil
moisture
the
a
after
control
area
the
con-
within
20
statement
tree compared
to
forest.
Total 15-foot
profile.
The
total
amount
of
soil
moisture
con-
tained in the 15-foot deep soil profile at the end of the s
pletion
the
than
plot
definitive
isolated
and
moisture
1969,
the
is
plot
periods
for
each
of
the
six
concentric
distances
r de-
from the
study tree is found in Table 7. These values were obtained by superimposing the concentric distances shown in Figure 4 on the appropriate
soil
moisture
through
27
isopleth
for
the
represented
desired
date.
in
The
the
left
portion
average
soil
of
Figures
moisture
12
within
each
concentric region was calculated by measuring the area within each isopleth.
The
pattern
three
5-foot
depth
Let
us
assume
generally
follows
that
discussed
earlier
for
the
classes.
the
soil
moisture
at
the
end
of
the
summer
depletion
season in the area 40 to 60 feet from the study tree is unaffected by
the
study
tree.
This
is
a
reasonable
assumption
based
on
the
response by this region after the study tree was cut in 1969.
subtract
the
soil
soil moisture 40
moisture
in
the
regions closer
to 60 feet from the tree,
adjustment to the soil moisture depletion
we
the
obtain
a
tree
form
of
If we
from
of
the
climatic
within the plot which is in-
dependent of vegetation treatments (Table 8).
generalized by the equation:
to
lack
This calculation can be
78
Table 7.
Date of
survey
Average soil moisture within the surface 15 feet of
soil at the end of each summer depletion period in
study tree plot L1 for each of the six concentric
distances from the study tree from 1965 through 1969.
O-2
Distance
-.
2-5
from
study tree
T
5-10 * 10-20
(feet)
20-40
40-60
feet of water
10-19-65
4.72
4.73
4.72
4.98
5.59
5.93
10-25-66
4.16
4.17
4.20
4.69
5.35
5.65
10-30-67
5.60
5.56
5.48
5.61
5.98
6.20
10-9-68
5.07
5.12
5.06
5.24
5.64
5.92
10-2-69
6.57
6.49
6.34
6.22
6.15
6.05
79
Table
8.
Date of
survey
Average soil moisture within the surface 15 feet of
soil at the end of each summer depletion period in
study tree plot L1 for each of the six concentric
distances from the study tree relative to soil moisture
40 to 60 feet from the study tree from 1965 through
1969 (from Table 7).
0-2
Distance
from -----e-pstudy tree (feet)
-----.2-5 ~ 5-10
10-20 } 20-40 1 40-60
feet
of
water
34
0
.96
.30
0
.72
.59
.22
0
86
. 68
.28
0
-. 29
-.l7
-.10
0
10-19-65
1.21
1.20
1.21
.95
10-25-66
1.49
1.48
1.45
10-30-67
.60
10-9-68
10-2-69
.64
. 80
- .52
-. 44
l
l
80
yi = x4O to 60
where
y
- Xd '
i
i
is the adjusted soil moisture in year i, x
i
40 to 60
i
is measured soil moisture 40 to 60 feet from the study tree in
is
year i, and xd
the
measured
soil
moisture
within
the
d
th
distance
i
region in the year i.
As
we
earlier, there is a general pattern of decreasing
discussed
soil
moisture
content
three
regions
within
at
10
the
tree
feet
of
is
approached.
the
study
However,
tree
contained
soil moisture at the end of each summer except 1969.
area 40 to 60 feet from the tree, there
was
about
each of the
about
Relative
twice
equal
to
the
the
moisture
use within 10 feet of the study tree in 1965 and 1966 than in 1967
and 1968, after the tree was isolated (Table 8).
For
example,
in
1965
there was about 1.2 feet less soil moisture within 10 feet of the study
tree than in the region 40 to 60 feet from the tree.
difference
between
comparison,
and
0.85
pattern
these
the
feet
same
of
reversed
regions
relationship
water
and
was
for
soil.
1967
moisture
about
yielded
and
1.5
a
1968,
content
In
feet
of
difference
water.
of
respectively.
increased
1966, the
as
In
about
In
the
0.65
1969,
the
tree
stump
was approached relative to the area 40 to 60 feet from the stump.
In
1969, the
discussed
plot
earlier, it
was
is
kept
probably
essentially
not
bare
unreasonable
of
to
vegetation.
assume
that
the
increase in soil moisture toward the tree stump in this bare plot at
the
end
tion
of
of
the
several
1969
depletion
factors
season
was
principally
due
to
a
combina-
including:
1) Variability in soil texture and moisture holding characteristics
within
the
plot.
81
2) An artifact of the neutron method of soil moisture measurement.
The neutron method measures the concentration of principally
hydrogen
atoms
atoms
within
contained
in
the
soil
influence
water,
of
root
the
neutron
moisture,
swarm.
and
Hydrogen
organic
matter
such
as root tissue are equally and indiscriminately measured as "soil
moisture".
It
is
reasonable
matter, primarily
in
structural
are
and
roots
contain
the
to
form
assume
of
roots,
particularly
substantial
amounts
the
concentration
increases
concentrated
of
hydrogen
near
of
the
tree.
near
the
base
the
form
of
in
organic
Large
of
trees
wood
and
water.
3) There may have been more persistent herbaceous vegetation beyond
the
influence
removed.
These
of
the
tree
which
was
herbs
may
have
extracted
difficult
a
to
greater
amount
moisture as the distance from the tree increased.
minor
factor,
however,
and
the
relative
infl
control
This
and
of
is
keep
soil
probably
a
e would remain con-
stant from year to year.
In any case, the
and
after
similar
cutting
influence
exact
cause
of
the
the
study
tree
is
would
have
been
present
not
"moisture"
greatly
prior
difference
important,
to
cutting
before
because
the
a
study
tree. Certainly, vegetation conditions throughout the plot were
essentially
study
the
identical in 1967, 1968,
tree
study
was
absent
tree
were
in
1969.
and
relative
throughout
average
the
soil
soil
with
the
exception
that
In 1965 and 1966, several trees near
removed, but the region from 40 to 60 feet from the
Thus,
study tree was relatively unaffected.
the
1969,
moisture
from
study
plot
in
moisture
in
each
Table
1969
8
to
(Table 9,
distance
region
we
could
reflect
Fig.
found
further
equal soil
29).
That
is,
in
Table
7
adjust
moisture
the
was
82
Table
9.
Average soil moisture within the surface 15 feet of
soil at the end of each summer depletion period in
study tree plot L1 for each of the six concentric
distances from the study tree relative to soil moisture
in the plot after the tree cut in 1969 (from Table 8).
I
Date of
Survey
Distance
0-2
2-5
from
study
5-10
feet
of
tree
10-20
(feet)
j 20-40
40-60
water
10-19-65
1.73
1.64
1.50
1.12
.44
0
10-25-66
2.01
1.92
1.74
1.13
.40
0
10-30-67
1.12
1.08
1.01
.76
.32
0
10-9-68
1.37
1.24
1.15
.85
.38
0
10-2-69
0
0
0
0
0
0
84
first
climatically
adjusted
moisture
content
within
at
end
each
the
values
of
found
in
to
the
region
summer
Table
8
equalize
40
to
depletion
were
the
in
the
feet
from
the
study
(Table
8).
60
season
further
differences
adjusted to
soil
tree
Then the
equalize
the
soil
moisture differences between each distance from the stump of the study
tree at the end of the 1969 depletion season (Table 9). This calculation
can
be
generalized
s
by
the
= (x
i
equation:
- x ) - (x40 to 60 - xd ),
d
i
i
69
40 to 60
69
= (x
40 to 60
69
- x
d ) - yi,
69
is
40 to 60
69
the measured soil moisture 40 to 60 feet from the study tree in 1969,
where
x
d
s
is the adjusted soil moisture in the year i, x
i
is
the
69
1969, and x
adjustments
soil
measured
40 to 60
allow
moisture
soil
, x
us
and
d
i
to
i
moisture
within
, and yi
more
distance
region
removal
see
and
the
relationships
between
soil
in
These two
are as defined earlier.
clearly
vegetation
th
the d
between
moisture
and
distance from the study tree. For example, if soil moisture depletion
throughout
the
from
stump, then, a t the
the
plot
in
1969
is
considered
end
of
the
to
1965
be
equal
at
depletion
all
distances
season, there
was 1.73 feet more soil moisture depletion within 2 feet of the study
tree than in the region 40 to 60 feet from the tree.
Whereas
soil
moisture was about equal within 10 feet of the study tree in Tables 7
and
plot
8,
in
when
1969,
we
we
account
now
for
find
the
a
soil
moisture
progressive
variability
decrease
in
in
soil
pletion with increasing distance from the tree (Fig. 29, 30).
the
moisture
bare
de-
86
A sigmoid curve can be fitted by least squares to the adjusted
soil
moisture
at
the
midpoint
of
each
distance
strata,
that
is,
at
1, 3, 7.5, 1.5, 30, and 50 feet from the study tree, for each year of
the
study, with
The
excellent
general
form
results.
of the
sigmoid
equation
is:
d
e
Y=
-e
BO+f31
d
l-e
where 6
and p
0
parameters,
shape
1
and
parameters
Homeyer
are
e
the
regression
coefficients,
c
and
d
are
shape
is the base of the natural system of logarithms.
were
obtained
using
the
method
described
by
The
Jensen
and
(1970).
At least squares fit to the 1967 data (Table 9) yields a simplified
form
to
the
general
;I 1967
where
Y
is
the
predicted
equation,
= 0.0775 + 1.1965 e
adjusted
soil
- /o.o357x11'7
moisture
at
the
end
of
summer
1967 in feet of water and x is the midpoint of each distance strata in
feet.
be
The
developed
explained
for
the
variance (r2) is
other
years.
0.9995.
Similar
equations
could
However, we can approximate the sig-
moid relationship to a high degree for the area within 40 feet of the
study
tree with
a
linear
least
squares
fit.
87
2
A
'1965
= 1.792 - 0.045 x
r
i
= 2.086 - 0.057 x
r 2 = 0.990
= 1.176 - 0.028 x
r
= 1.377 - 0.034 x
r
1966
h
'1967
*
'1968
For
most
influence
of
curve
the
is
at
of
adjacent
minor
1966,
the
applications
of
trees
extreme
limits
interest.
For
year
of
estimating
upon
one
of
= 0.998
2
= 0.993
2
= 0.995
water
another,
use
the
and
the
subtle
degree
shape
of
the
influence, that is as Y approaches 0,
example, using the linear relationships,
greatest
of
soil
moisture
depletion,
we
would
in
predict
the influence of the tree to extend to about 37 feet and in 1967, the
year
of
least
soil
moisture
depletion,
to
42 feet.
2
Thus, though it
is artistically and theoretically preferable to use a sigmoid relationship, in practice a linear fit to those data points where Y is positive
(not 0) is just as good. Given only six data points and explained
variances
of
respectively,
form
should
0.9995
for
be
and
the
0.993
1967
for
data,
as
the
sigmoid
and
linear
a
general
rule,
the
relationships,
simpler
linear
selected.
The total water use by the vegetation within 40 feet of the study
tree
can
now
be
estimated, relative to the soil moisture depletion in
the bare area 40 to 60 feet from the tree (Table 10).
2
The
difference
It is not statistically correct to assign a value to the dependent
variable, Y, and calculate the value of the independent variable, x.
One should recalculate the least squares fit to the observed data, reversing the dependent and independent variables. However, in the case
where the explained variances are quite high, an adequate approximation can be made even though the statistical rules are violated. For
example using the 1967 data (Table 9), if distance is the independent
variable, the calculated influence of the tree extends to 41.82 feet,
and if distance is the dependent variable, the calculated influence
extends to 41.60 feet.
88
Table
10.
Volume of soil moisture depleted within 40 feet of the
study tree in excess of that depleted 40 to 60 feet
from the study tree when adjusted for equal soil moisture in the plot after the tree was cut.
Distance from
study tree (feet)
0-2
2-5
Area in
(ft2)
13
66
region
5-10
10-20
236
942
20-40
Total
3770
, 5027
. -
Date
of
survey
cubic
feet
of
-
water
10-19-65
22
108
354
1055
1659
3198
10-25-66
26
127
411
1064
1508
3136
10-30-67
15
71
238
716
1206
2246
10-9-68
18
82
271
801
1433
2605
89
in the volume of
within
water
depleted
each con centric
region
by
in that region from Table 9.
isolating
the
is
calculated
the
average
by
multiplying
the area
soil
moisture
difference
In 1965 and 1966, the period prior to
tree, the area within 40 feet of the study tree
study
used 3198 and 3136 cubic feet more soil moisture than the area 40 to
After removing all vegetation surrounding the
60 feet from the tree.
study tree, these
water
for
1967
values
and
were
1968,
reduced
order
from about 550 to 900
cubic
feet
tree
year
of
Depletion
With
on
the
During
the
Fall
2246
and
2605
Thus,
the
residual
isolate
the
study
respectively.
which was removed in 1967 in
depending
to
to
more
soil
moisture
cubic
feet
vegetation
tree
than
of
used
the
isolated
measurement.
Recharge
beginning
of
the
rainy
season
in
the
fall,
a
distinct
wetting front could be observed which eventually progressed through
the
soil
initiation
profile.
The
of
fall
the
presence
of
recharge
this
period.
wetting
Summer
front
defined
rainfall
was
the
not
large enough to produce an observable or persistent wetting front. As
fall rains con tinued to wet the surface soil
front progr
ed
tinued
depleted
to
be
not
designed
to
the
literature,
that
the
below
a
recharge
move
soil
water
it
is
deeper,
measure
often
the
of
soil
wetting
from
vegetation.
depletion
models, and
extraction
m oisture below
soil
by
during
students
moisture
front
these
layers and the wetting
of
does
because
deeper
and
th e we tting
Unfortunately
the
recharge
front con-
this
period.
evapotranspiration
not
more
drier
occur
energy
in
is
levels.
study
implied
drier
required
The
Most of
have
the
was
to
soil
re-
vegetation,
argued, would preferentially satisfy its evapotranspirational
90
requirements
these
from
the
more
moist
of
soil
moisture
observations
complete.
above
the
depletion
wetting
during
the
and
following
comments
In
no
some
was
direct
must
years
too
measure
great
be
of
soil
viewed
the
period
to
observe
water
with
potential
fall
However,
below
the
front
progressed
in
wetting
most
years
between
successive
depletion
front
in
on
soil
below
moisture
successive
depth.
was
Thus,
are
in-
were
made,
the
caution.
soil
the
measure-
front
be-
depth of measure-
depletion
measurements
moisture
wetting
cause recharge had progressed throughout the 20-foot
ment.
front.
Since no tensiometers or thermocouple psychrometers
installed
ments
zones
as
was
observed
the
wetting
The depth of recharge at the time of meas-
urement was quite variable between years as well as between access
tubes.
The
duration,
rate
of
recharge
was
some
complex
function
of
the
and intensity of antecedent rainfall and the characteristics
of the location of each of the soil moisture access tubes.
the
access
In most of
tubes, soil moisture recharge and depletion progressed con-
sistently each fall.
For
example, in the uncut control plots Cl and
C2, tube 42 gained some moisture throughout the entire 20-foot
tube
depth
amount,
following
the
first
major
storm
of
the
fall--even
access
in
1968
when only 3.16 inches of rain fell between the October 8 and October 22
measurement
(Table
11,
Fig.
31,
32,
33,
34).
In
other
access
tubes,
such as tubes 8 and 64, there was an abrupt wetting front with an increase
of
previous
soil
moisture
measurement.
above
and
a
decrease
below relative
to
the
In a third case, such as in tubes 2 and 22, there
was a distinct wetting front with increased soil moisture above, an
intermediate zone where soil moisture neither
and
a
deeper
zone
where
soil
moisture
increased nor decreased,
decreased.
None
of
the
access
96
tubes
showed
an
measured
rainfall,
two
the
of
3.16
increase
in
soil
moisture
except
in
the
October
access
tubes
showed
inches
of
rainfall.
Because
of
the
the
access
data
if
variable
tubes, it
one
is
is
22,
increase
depth
of
meaningful
to
in
following
the
was greater than the
1968 measuremen t when
greater
soil
not
interested
pletion with depth.
an
which
than
moisture
average
measured
recharge
the
progress
the
soil
of
among
moisture
wetting
and
de-
In the fall of 1964, 11.26 inches of rain fell
between the October 22 and November 20 measurement (Table 12). The
principal
zone
of
soil
moisture
recharge
was
found
to
be
within
the
surface 6 to 8 feet for all six access tube locations (Fig. 31).
Soil
moisture
depletion
was
measured
at tube 8 to below 19 feet-- the
The
variability
related
scure
to
in
the
any
the
depth
actual
depth
of
soil
bottom
of
the
below
of
of
depth ranging
the
measured
region
moisture
a
access
tube--at
soil
moisture
partial
wetting,
depletion
by
the
from
trees.
7
feet
tube
depletion
which
42.
is
would
Thus,
ob-
the
quantity of soil moisture depletion in the fall can not be taken as the
total water use by the trees, but
moisture
dynamics
below
the
zone
is
simply
wetted
by
front
October
18
can
and
be
December
observed
on
8, 1965,
two
13.23
indication
the
of
infiltration
the soil
of
rainfall.
34), the progress of the
In 1965 (Fig. 32) and in 1968 (Fig.
wetting
an
successive
inches
of
measurements.
rain
fell
Between
(Table
13).
The principal wetting front can be found at a depth of about 7 feet
in all tubes, although
there
was
7 feet in some of the tubes.
showed
some
depletion
soil
most
moisture
certainly
some
soil
moisture
recharge
below
All of the tubes, except tube 42,
depletion
occurred
in
at the
the
20
deeper
depths.
rainless
days
Some
of this
following
the
97
October
18
measurement
and
before
the
moderate
storm
of
November
8.
Thus, this observed depletion may simply be a residual of evapotranspiration
prior
to
wetting.
The
measured
depletion
ranged
from
1.04
inches in tube 64 to 0.18 inches in tube 2 (Table 11). The average depletion
for
the
six
tubes
was
charge was 10.08 inches.
The average measured re-
0.44 inches.
Measured recharge ranged from 8.48 inches in
tube 42 to 12.37 inches in tube 64.
A calculation of daily evapo-
transpiration based on the Thornthwaite method indicated a potential
evapotranspiration
for
the
period
of
2.59
inches
and
a
calculated
in-
crease in soil moisture of 10.64 inches (Table 11).
Another
An
soil
additional
moisture
17.12
measurement.
measurement
inches
of
rain
was
had
inches
ranged
for
from
the
period also.
0.53
inches
six
8.22
January
10,
1966.
since
the
December
8
to
feet
from
13.50
the
inches
surface.
and
Measured
averaged
10.72
Soil moisture depletion was observed for this
Measured depletion below the wetting front ranged from
to
0.82
inches
evapotranspiration
period.
Soil
based
inches
tubes.
Potential
inches
fallen
on
Within this interval the principal wetting front pro-
gressed 3 to 4 feet deeper--about 11
recharge
made
moisture
on
the
and
was
storage
averaged
0.55
calculated
to
be
0.33
calculated
to
increase
was
Thornthwaite
calculated
as
runoff
for
the
depletion
below
the
wetting
water
period.
front
inches
balance
and
for
the
six
inches
8.27
tubes.
for
by
the
8.52
inches
was
For this measurement period,
cannot
be
attributed
to
a
remnant
of
evapotranspiration which occurred prior to wetting the surface soil
layers.
A
In this case, depletion
similar
scenario
occurred
was less than that in 1965.
did
occur
below
in
1968,
although
Between
the October
the
8
wetting
the
and
front.
initial
October
wetting
22
98
measurement,
3.16
inches
of
(Table
This
storm
was
16).
rain
fell
at
the
characterized
by
Challenge
a
number
Ranger
of
Station
localized
convection cells which resulted in a highly variable precipitation
pattern.
plots
Perhaps
than
at
an
the
additional
Ranger
The
and
averaged
principal
2.89
zone
of
of
Station.
ture recharge for the 14-day
inches
inch
rain
fell
Nevertheless,
in
the
measured
uncut
soil
control
mois-
period ranged from 1.07 inches to 5.47
inches
for
wetting
the
six
following
access
this
within the surface 2 to 3 feet, although
tubes
moderate
small
soil
(Table
storm
moisture
11).
occurred
increases
were found throughout the 20-foot measurement depth in four of the six
access
tubes
crease
in
rainfall
(Fig.
soil
wetted
34).
Only in tube 8 was there a substantial de-
moisture
only
during
the
this
surface
6
period.
In
this
access
tube,
the
feet.
No additional soil moisture measurements were made until January
10,
1969.
rain
was
through
was
Within this 80-day period, an
recorded.
the
24.49
inches
of
A distinct wetting front had progressed partially
profile.
essentially
additional
The pattern of soil moisture content with depth
identical
with
that
observed
on
January
10,
1966.
This
would be expected since about the same amount of rain fell prior to the
January
1966
and
January
inches, respectively.
1969
As
measurements--30.35
in
fall
1965,
measured below the wetted zone --ranging
and
tion
averaging
for
this
0.40
inches
period
was
for
the
six
calculated
to
soil
from
moisture
0.16
tubes.
be
inches
and
27.65
depletion
inches
to
was
1.16
inches
Potential evapotranspira-
1.96
inches.
Thus, soil moisture depletion continued below the wetted soil
created
by
substantial
rains
in
which measurements were made, a
the
fall.
second
soil
In 2 of the 4 years in
moisture
survey
was
made
99
before
the
Depletion
wetting
front
soil
moisture
of
explanation
for
continued
this
progressed
was
continued
below
again
by
the
depth
of
measurement.
The
most
plausible
observed.
depletion
evapotranspiration
the
is
that
trees.
it
An
is
the
result
alternative
of
hypothesis
is that this depletion is due to continued drainage rather than evapoThe
transpiration.
very small.
The
probability
soil
of
moisture
continued
retention
drainage
data
for
seems
these
to
be
plots
indicates
an average retention of 69 percent moisture by volume at l/3 atmospheres
and
average
field
21
bulk
percent
density
soil
moisture
of
moisture
1.7
content
by
for
volume
these
below
15
at
depths
feet
for
15
atmospheres
(Table
5).
these
using
The
tubes
the
average
was
40,
43,
41, and 38 percent by volume in October 1964, 1965, 1967, and 1968,
Gravitational water held in the soil at this depth
respectively.
following
30
cent
volume.
In addition, at
content
generally
by
moisture
soil
moisture
rainfall
soil
The
be
a
to
soil
be
drainage
depletion
clearly
of
seen
a
rain
the
moist
below
water
without
increases
more
depletion
of
the
drainage
seems
moisture
disprove
of
toward
input
measurement
can
days
end
with
soil
very
the
appears
to
of
depletion
depth.
after
remote
was
soil
moisture
figures
profiles
showed
season,
soil
more
days
without
explanation
for
the
measured
front.
made,
by
published
Pinus pinaster
native woodland and -w
The
per-
However,
we
can
since no
neither
prove
nor
hypothesis.
in
however, did
52
Thus, drainage of
200
vegetation
by
not
progress
a
and
wetting
Have1
the
these
deep profiles under
changes
wetting
front
(1976).
stands in Western Australia.
identify
of
below
Butcher
Profiles of soil moisture were shown for 7-meter
authors,
about
or
wetting
potential
the
be
front
nor
at
discuss
monthly
The
the
process.
intervals
100
through
one
winter
at Challenge.
under
open
woodland
thinned
under
to
a
greatly
closely
and
in
maintain
densely
2
of 24.6 m /ha, the
was
and
resemble
the
pattern
observed
Butcher and Have1 found almost identical patterns
native
periodically
parison,
season
accelerated
a
basal
stocked
wetting
the
pine
area
pine
front
in
low-density
of
stands
7.1
m
2
which
/ha.
had
By
been
com-
stand, maintained at a basal area
was
slightly
summer.
Soil
delayed
and
soil
drying
moisture
was
exhausted
to
a depth of 7 m by mid-November in the dense stand as compared to March
under
the
more
could
be
no
the
winter
exhausted
and
wetting
to
be
of the
The
of soil
wetting
front
many
the
below
the
into
open
early
data
depletion
by
vegetation
moisture
below
the
below
had
wetting
Unfortunately,
the
authors
was
a
wetting
front
below
stand, there
been
stands, some soil moisture
winter.
by
stocked
the
soil
vegetation
the
moisture
available
more
the
densely
depletion
the
the
to
moisture
existed.
is
the
nearly
water
of
wetting
This
also
cases, entered
hypothesis
depleted
In
the
very
in
front
interlimited.
the
fall
course, much slower than earlier in the summer before the
evapotranspiration
in
soil
moisture
since
available
pretation
was, of
front
continued
Thus, in
soil
midsummer.
depletion
rate
stands.
continued
in
continued
open
lower
winter
drainage
front
all
potential
of
is
most
in
be
expected
the
fall,
dormancy.
accounting
can
the
to
be
for
clearly
available
certainly
In
soil
would
since
and
the
the
the
soil
moisture
have
potential
vegetation
Butcher
rejected.
not
the
and
moisture
Have1
has,
data,
depletion
The vegetation had
in
the
profile
permitted
and
drainage.
101
Groundwater
The
Variation
response
of
groundwater
levels
to
precipitation
in
forested
mountainous terrain of the western U. S. is an area of study in which
little
significant
progress
The
has been conducted.
mous
variations
in
has
occurred
blocks
groundwater
to
though
progress
depth
and
a
are
great
deal
related
response
of
to
observed
work
the
enor-
within
a
small geographical area imposed by steep slopes, shallow soils, and
fractured
bedrock.
Overland flow rarely occurs on undisturbed forest
soils in the west, but streams rapidly respond to precipitation on
areas having steep slopes and highly permeable surface soils.
portance
been
being
ing
Field
rapid, shallow subsurface flow to streamflow response has
repeatedly
Harr
Harr
shed
of
documented
(1977)
working
by
a
number
in
the
of
Oregon
authors,
the
most
recently
Cascades.
and others have reported that only a small part of a water-
produces
to
The im-
changes
studies
storm
in
have
runoff.
rainfall
This area expands and contracts accordintensity
demonstrated
and
the
and saturated flow and streamflow.
soil
water
interaction
conductivity.
between
unsaturated
In other studies, subsurface water
has been shown to move rapidly through piping channels and other noncapillary
As
biologically
discussed
created
channels
in
otherwise
unsaturated
soil.
earlier, the study tree plot (L1) and the uncut con-
trol plots (C1 and C2) were selected for study only after it was determined that no water table could be found within 50 feet of the
surface at any time of the year.
criteria was to eliminate
The
reason
for
this
selection
the probability of capillary recharge from
a water table to the surface 20 feet of soil where soil moisture depletion was being measured.
Any such capillary recharge would obscure
102
actual
soil
moisture
original
21
plots
appeared
in
the
year.
the
In
did
the
study
the
this
wells
plots, there
of
by
not meet
observation
three
duration
depletion
criteria
during
was
and
a
and a
at
least
persistent
the
Several
vegetation.
water
free
a
the
water
portion
water
levels
of
table
of
the
table
throughout
seasonally
fluctuated
between a depth from about 10 to 40 feet below the soil surface.
In August 1965, Leupold-Stevens FW-1 water level recorders were
installed
on
the
water
Fluctuations
in
tored
August
from
the
table observation
depth
1965
of
the
through
wells
water
March
table
1970
from
to
remove
that
well
was
Recession.
at
Challenge
the
carcass
the
rendered
useless.
general
pattern
The
was
from
similiar
from
year
5,
of
35-39).
failed
the
water
year.
7,
and
continuously
In
in plot 16.
well
to
plots
was
(Fig.
a wood rat drowned in the observation well
efforts
in
16.
moni-
April
1969
Repeated
and
subsequent
table
data
fluctuations
Water table levels began
to fall at the end of the winter rainy period and continued to drop
through
the
Each
of
The
shape
rainless
the
of
summer
until
three
wells
these
recession
the
produced
beginning
similar
curves
was
of
heavy
recession
slightly
curves
concave
fall
rains.
(Fig.
with
35-39).
the
water
levels dropping an average of 0.082 feet per day in June to 0.067 feet
per
day
in
September.
occurred
in
late
The
November
maximum
or
depth
attained
each
year
total
minimum
soil
moisture
depth
early
the
December.
corresponded
found
to
closely
in
the
water
table
usually
The maximum water table
to
the
pattern
well-drained
uncut
of
the
control
plots at the end of each summer (Fig. 40).
The
tive
minimum
wetness
or
depth to
dryness
of
the
the
water
table
winter.
was
indicative
of the
For example, 1965-66 and
rela-
109
1967-68 were dry years with the October to April rainfall being 41.84
inches and 47.72 inches, respectively.
The
water
table
were
during
during
the
wetter
April
rainfall
correspondingly
greater
years, 1966-67
was
70.51
inches
minimum
depths
these
and
1968-69,
when
and
78.51
inches,
to
dry
the
the
years
than
October
to
respectively.
There
was also a carry over to the level of ground water at the end of the
summer.
level
That
which
is, a
persisted
wet
winter
produced
to
result
in
a
at the end of the following summer.
a
low
minimum
water
table
level
and,
a
high
minimum
relatively
high
Conversely,
a
thus,
table level at the end of the summer.
a
water
water
dry
deeper
table
table
winter
level
produced
maximum water
A dry fall and winter tended to
be followed by a dry spring and, conversely, a wet fall and winter was
followed
by
a
wet
spring.
For
example, March
to
July
rainfall
was
7.33,
9.56, 9.11, and 17.80 inches for 1966, 1968, 1969, and 1967 respectively. This
order
is
closely
correlated
with
the
order
of
Thus, water
table depth found at the end of the summer (Fig. 40).
levels
ally
were
began
The
not
only
later
in
high
higher
the
in
the
wet
years,
but
maximum water
the
recession
gener-
spring.
correlation
between
minimum
soil
moisture
in
the
uncut
control plots in the late fall and the maximum depth to the water
table in plots 5, 7, and 16 may be related to direct
in
capillary
fringe
above
the
water
table
by
trees
use of the water
as
suggested
by
Lewis and Burgy (1962) for oak trees in their Placer and Hopland
sheds.
moisture
However,
an equally
depletion
and
plausible explanation is that both soil
groundwater
recession
are
responding
to
different
processes,
but which began later in wet years than in dry years.
sequently,
the
time
water-
available
for
evapotranspiration
from
the
Con-
uncut
plot
110
as well as gravitational recession of the water table was shorter in
wet years than in dry years.
that
the
vegetation
water
recession
of
or
the
table
capillary
that
was
the
does
not
groundwater
table
was
moisture
depletion
soil
observed
recharge
Thus, it
from
within
some
a
depth
deeper
of
50
water
necessarily
directly
within
feet
follow
influenced
plots
was
by
where
no
influenced
by
table.
The processes of groundwater recession were not investigated in
detail.
However,
additional
and
the
recession
curves
are
continuous information
processes on the soil wat er
of
useful
the
in
that
influence
regime, particularly
in
they
of
the
provide
climatic
winter
and
late spring when the interaction of rainfall on soil moisture content
is
complex.
Rise.
-
The response of the water table levels in the three ob-
servation
to
wells
that
at
reported
Challenge
by
Harr
mountainous
terrain.
110
whereas
percent
to
(1977)
rainfall
and
was
others
greatly
working
in
delayed
relative
forested
and
Harr was working on slopes ranging from 50 to
the
Challenge
plots
were
on
gentle
10
to
20
'
percent
slopes.
Harr was working in shallow clay loam soils about 3 feet deep
underlain
by
6
to
20
feet
of
saprolitic
subsoil
whereas
the
Challenge
plots were in finer textured silty clays to silty clay loams, which
were
uniformly
found
only
deeply
weathered
from
temporary
saturated
zones
50
to
whereas
persistent and perennial water table.
pattern
of
precipitation
and
the
dissimilar.
feet
at
Thus,
vegetative
cades and at Challenge were similar, the
certainly
100
cover
in
Challenge
although
in
groundwater
the
depth.
Harr
there
the
was
amount
Oregon
patterns
Cas-
were
a
and
111
At Challenge no rapid subsurface saturated responses to precipitation
could
be
detected.
Where Harr found a piezometric response
within hours after rainfall, several
quired
of
for
the
and
the
piezometers
Challenge
Burgy
However,
(1962) on
the
are
weekly
Lewis
by
sponse
it
is
of
and
investigating
several
rising.
The
that
observed
by
watersheds
level
were
begin
response
water
weeks
in
in
the
depths
reresponse
Lewis
California.
Placer
were
County
measured
only
Burgy.
and
if
the
of
County
table
because
to
to
typical
Placer
earlier, the
forested
difficult
Challenge
more
water
obscured
mentioned
in
were
their
details
watersheds
As
wells
at
days
processes
mountainous
not
terrain
dangerous
detailed
affecting
to
processes
are
groundwater
extremely
generalize
without
re-
varied
and
thoroughly
involved.
Even in the relatively uniform soils at Challenge, the groundwater
in
response
terms
35-39).
of
to
the
rainfall
between
hydrograph
shape
Precipitation
patterns
plots
and
5,
timing
during the
7,
and
of
first
16
the
were
varied
initial
storms
both
rise (Fig.
of
fall
1965
and 1966 were similar in that 13 to 16 inches of rain fell within about a 20-day period.
The water levels in plot 7 began to rise within
8 to 12 days after the initial rainfall (Fig. 41, 42).
In plot 16,
water levels began rising within 13 to 18 days and in plot 5 within
23 to 24 days.
est
period
in
The
which
shallowest
to
water
table,
to
rainfall.
respond
1968, and 1969 were spread over a longer
the
hydrographs
Once
out
the
the
were
water
more
extended
tables
began
winter, generally
showing
in
to
only
plot
5,
required
the
long-
The fall rains of 1967,
period
and
the
rising
phase
of
time.
rise, they
one
peak
continued
in
early
rising
spring,
through-
114
followed
by
a
Occasionally,
continued
recession
of
water
and
deviated
and
this
mid-January
from
showed
the
two
winter
through
the
summer.
a heavy burst of rainfall would cause a very rapid rise
in water level in all three wells, such
35)
level
1967
general
or
was
(Fig.
36).
pattern
possibly
three
characterized
by
of
as in
In
a
1966-67,
single
distinct
three
early
the
peak
peaks.
definite
January
1966
hydrograph
during
the
Precipitation
heavy
rainfall
(Fig.
shape
winter
during
periods,
early November through early December, mid- to late January, and midMarch to mid-April.
duration
study
the
were
The two intervening periods of about 1 1/2 months
essentially
frequency
of
rainless.
rainfall
tributed throughout the winter.
During
events
were
the other
much
more
years
of the
uniformly
dis-
CHAPTER V
SUMMARY
AND
CONCLUSIONS
The soil moisture regime of a 15-foot profile was measured for a
5-year period under a second growth mixed conifer forest which had not
been cut for about 80 years and under an adjacent stand in which 88
percent
of
the
leaving
one
stand
basal
dominant
area
residual
had
sugar
been
pine
removed
about
3 years
30
inches
earlier,
in
diameter
surrounded by much smaller understory vegetation of fir, pine, and tanoak.
Within
distances
of
this
plot, neutron
meter
access
tubes
were
tubes
at
2, 5, 10, 20, 40 and 60 feet from the sugar pine in con-
centric circles of increasing distance from the tree.
access
installed
were
installed
in
the
individual
tree
A total of 23
plot.
In
the
uncut
stand, three access tubes were located at random within each of two 50by 50-foot
blocks.
The criteria for plot selection was (1) no water '
table present within the plots to a depth of 50 feet at any time during
the
year, (2)
a
uniform
pattern
of
dication of lateral subsurface flow,
soil
moisture
(3)
recharge
well-drained
with
sites
no
with
inno
surface ponding or water runoff concentration, (4) no unexplained anomalies
in
soil
moisture
data
during
the
depletion
or
recharge
measure-
ments, and (5) uniform soil with all access tubes at least 15 feet in
depth.
These
criteria
tween access tubes and
data
between
access
were
established
to
reduce
the
variability
be-
to make comparison of soil moisture depletion
tubes
within
the
plot
and
between
plots
possible.
After 2 years of soil moisture measurement, all of the vegetation
surrounding the sugar pine was cut to a distance of 120 feet from the
116
tree
an
leaving
a
additional
bare plot
2
years
of
leaving
the
plot
bare.
for
additional
year.
1
By
ly
each
recharged
content
soil
in
the
years
an
soil
moisture.
spring, there
after
when the
was
measurement, the
a
in
tree.
the
was a
of
was
with
a
dramatic
logged
Surface
soil
moisture
soil
evaporation
soil.
content
was
evident
uniform
was
moisture
cut
pattern
each of
the
when
occurred
of
2
there
at
a
and extended to a distance
Beyond
about
fairly
the
complete-
moisture
the
years
depletion
remained
within
2
and
soil
in
for
and the
approximately 20 feet from the tree.
tree, the
pine
uniformly
change
depth between 8 and 13 feet beneath the tree
of
sugar
evapotranspiration
partially
Most
plots
Beginning
summer
plot was
isolated
isolated
After
Soil moisture was measured in the bare plot
spring, the
with
moisture
with one isolated tree in the center.
30
uniform
surface
feet
from
with
several
the
depth.
feet
of
After the isolated tree was removed, leaving the plot bare, the
zone of depletion 8 to 13 feet under the tree disappeared and the soil
moisture
content
eccentric
remained
pattern
Within
the
of
rather
low
surface
5
uniform
soil
moisture
feet
of
below
a
depth
adjacent
soil, there
to
was
of
the
no
2
feet.
tree
The
disappeared.
dramatic
change
in
soil moisture content at the end of the summer after isolating the tree,
or
after
pattern
other
of
than
removing
the
soil
moisture
a
to the tree.
vague
isolated
tree.
content
increase
in
There
related
moisture
to
was
also
distance
content
in
no
from
the
definitive
the
tree,
regions closer
At a depth of 5 to 10 feet, there was a definite correl-
ation between soil moisture content at the end of the summer and distance
soil
from
the tree.
moisture
content
At a distance of 40 to 60 feet from the tree,
remained
fairly
uniform
at
the
end
of
each summer
117
throughout
the
duration
of
the
study
with
no
measurable
impact
of
iso-
lating or later removing the isolated tree. However, for the areas
within 10 feet of the tree, there
was
a
definite
increase
in
soil
mois-
ture content at the end of the summer in which the tree was isolated
and
another
increase
after
the
isolated
tree
was
cut.
At a depth of
10 to 15 feet the soil moisture response to tree cutting was similar
to that at a depth of 5 to 10 feet.
There
is
a
linear
relationship
between
distance
from
the
study
tree and relative soil moisture content at the end of summer.
tive soil moisture" was
obtained
by
adjusting
the
measured
"Rela-
total
mois-
ture content in the plot by the total soil moisture in the area 40 to
60
feet
from
the
tree
and
then
adjusting
to
equalize
throughout the plot after the isolated tree was cut.
soil
moisture
As distance from
the tree increases, the relative soil moisture content decreases, that
is, the soil moisture "savings" obtained by removing the tree decreased
as
distance
each
from
tree
increased.
There
was
a
different
curve
for
During dry years the slope of the curve was steeper than
year.
during
the
wet
years,
that
is, the
soil
moisture
content
closer
to
the
tree was lower in dry years than in wet years relative to the soil
moisture
content
explained
fit
to
in
the
area
outside
the
influence
of
the
tree.
variance, r 2, for each curve is in excess of 0.99.
the
data
was
not
significantly
better
than
the
linear
The
A sigmoid
fit.
The
influence of the tree extended to a distance of 38 to 42 feet from the
base
of the
tree.
In the 2 years prior to isolating the tree, the
vegetation within 40 feet of the tree depleted about 3200 cubic feet
more soil moisture each summer than the area 40 to 60 feet fr o m the
tree.
After
the
tree
was isolated,
the
sugar
pine
depleted
about 2200
and 2600 cubic feet more water than the area 40 to 60 feet from the
tree
in
On
the
a
first
number
uncut
that
of
the
the
of
control
is, after
and
plots
a
front
depletion
after
substantial
wetting
year
after
occasions, soil
can
All of the tubes, with
ture
second
end
amount
of
the
one
respectively.
measurements
of
the
rain
clearly
only
below
moisture
the
be
treatment,
summer
had
observed
were
depletion
fallen.
on
made
in
season,
The
progress
successive
measurements.
exception, showed continued soil mois-
wetting
front.
The
most
plausible
explanation
for the continued depletion is that it is the result of continued evapotranspiration
by
the
trees.
An
alternative
hypothesis
is
that
this
de-
pletion is due to continued drainage rather than evapotranspiration.
The
no
probability
of
measurement
of
continued
soil
drainage
water
seems
potential
to
was
be
made,
rather
small.
can
neither
we
Since
prove
nor disprove the drainage hypothesis or the evapotranspiration hypothesis.
However,
front
has
depletion of soil moisture by vegetation below a wetting
been
clearly
shown
in
figures
published
by
others.
One of the criteria for selecting s o i l moisture depletion plots
was that no water table was to be found within a depth of 50 feet.
ever, in
searching
groundwater
40
feet.
table
There
for
such
routinely
was
a
plots
some
fluctuated
definite
areas
between
seasonality
began
ing
then
a
maximum
rising
a
end
after
level
sometime
beginning
was
the
shortly
minimum
pattern
at
the
to
gradually
same
for
of
the
in
fall
all
the
a
in
depth
the
dry
summer
beginning
of
the
late
winter
as
the
summer
three
found
wells
where
of
depth
Groundwater
table under this precipitation regime.
reached
were
or
and
water
generally
season
and
precipitation,
reach-
early
and
spring
progresses.
measured.
10
the
depths
depletion
fall.
the
about
of
How-
In
some
This
general
years,
there
119
was
a
single
broad-crested
peak
occurring
in
the
winter.
In other
years, several distinct peaks were observed during the winter, generally
in
large
as
the
the
out
to
storms, there
rise
in
response
was
The
period.
water
table
the
recession
large
a
level
first
During the interval between these
typical
of
was
summer
after
storms.
a
series
spring, there
the
weeks
next
large
storms
general
significant
by
another
When
rainfall
ceased
recession
curve
through-
arrived.
and
responded
curve,
gentle
within
rainfall
days
in
followed
and
the
in
some
fall.
cases
Even in the
relatively uniform soils at Challenge, the groundwater response to
rainfall within the three plots varied, both
shape
and
timing
first
storms
within
8
in
days
24
of
of
the
initial
rise
in
following
terms
of
hydrograph
rainfall.
During
the
fall, the water level in one of the wells began to rise
days, in another well within 15 days, and in a third well withafter
the
initial
rainfall.
Though
the
length
of
time
before
water table levels began to rise were rather lengthy relative to that
which some researchers have reported in mountainous terrain, this time
was much shorter than that required for the surface 15 feet of soil to
be recharged by precipitation.
in
the
summer, no
detected.
Such
pletion
direct
or
This
and
pattern
ing
depth
of
research
more
basic
diurnal.
of
would
the
provided
some
of
soil
moisture
storage
isolated
that
a
study
and
detailed
water
sugar
designed
to
and
This
table
by
into
recession
depth
could
of
direct
period
be
de-
vegetation.
the
depletion
quantity,
throughout
timing,
the
root-
It seems to be a corollary
pine.
answer
table
indicative
table
insight
mature
question.
water
been
water
has
an
in
have
study
of
the
fluctuations
fluctuations
access
During
one
study
question
was
leads
undertaken
to
a
because
of
120
an
attempt
to
measure
the
tially logged forests.
evident
made
that
an
the
of
an
around
trees
trees
the
and
more
the
This
in
soil
moisture
before
influence
tree
moisture
between
understanding
complex
isolated
soil
differences
step, then, was
individual
in
depletion
in
par-
In the first year of that study, it became
variance
analysis
evaluate
changes
it
study
soil
moisture
depletion
patterns
a
to
possible
has
plots
A
was
the
selective
the
tenuous.
the
of
within
plots
of
question
of
depletion
design
interaction
logging
raised
to
on
some
study
of
soil
even
preliminary
individual
moisture
more
depletion.
basic
questions
which reflect the inadequacy of our understanding of soil-vegetationwater
interrelationships.
shown
of
that
roots
these
roots
function.
This
are
Root distribution and biomass studies have
concentrated
have
a
study
in
structural
has
shown
the
upper
function,
the
most
layers
others
dynamic
of
have
how
the
pattern
of
soil
moisture
an
depletion
moisture by the tree occurs at a depth of 8 to 13 feet.
stand
soil.
depletion
varies
Many
absorption
of
soil
We need to
by
tree
size
and species.
Unfortunately,
ed.
Interest
within
the
in
past
many of these basic questions are not being addressforest
decade
soil
in
water
favor
research
of
"more
has
waned
pressing"
considerably
problems
related
We have not progressed as far as we might like to be-
er q u a l i t y
lie ve since the early studies of Charmow
and Wyssotzky in the
1800's.
future
soil
Perhaps
moisture
to
in
regime
another
might
generation
also
be
a
forced
to
student
the
same
of
the
late
forest
realization.
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APPENDIX