Agricultural
Water
Management
110 (2012) 41–
54
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Agricultural
Water
Management
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Temporal
stability
of
shallow
soil
water
content
for
three
adjacent
transects
on
a
hillslope
Lei
Gao
a,c
,
Mingan
Shao
b,∗
a
State
Key
Laboratory
of
Soil
Erosion
and
Dryland
Farming
on
the
Loess
Plateau,
Institute
of
Soil
and
Water
Conservation,
Chinese
Academy
of
Sciences
and
Ministry
of
Water
Resources,
Yangling
712100,
Shaanxi,
China
b
Key
Laboratory
of
Ecosystem
Network
Observation
and
Modeling,
Institute
of
Geographic
Sciences
and
Natural
Resources
Research,
Chinese
Academy
of
Sciences,
Beijing
100101,
China
c
Graduate
University
of
Chinese
Academy
of
Sciences,
Beijing
100049,
China
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
28
December
2011
Accepted
23
March
2012
Available online 21 April 2012
Key
words:
Soil
water
content
Temporal
stability
Representative
locations
Soil
properties
a
b
s
t
r
a
c
t
Identifying
representative
locations
that
estimate
mean
soil
water
content
(SWC)
for
an
area
of
interest
is
one
of
the
most
important
applications
of
the
concept
of
temporal
stability
but
typically
requires
extensive
sampling
on
multiple
occasions.
This
study
aimed
to
examine
the
feasibility
of
identifying
temporally
stable
locations
by
using
other
properties
(mainly
soil)
that
were
themselves
relatively
temporally
stable,
thus
reducing
the
cost
of
sampling.
From
July
2008
to
October
2010,
SWCs
at
four
soil
depths
(0.1,
0.2,
0.4
and
0.6
m)
were
measured
using
a
neutron
probe
on
20
occasions,
along
three
transects
(∼30
locations
for
each
transect)
on
a
hillslope
of
the
Loess
Plateau,
China.
Summary
variables
were
determined
at
corresponding
locations.
The
results
showed
good
temporal
patterns,
with
mean
Spearman
correlation
coefficients
ranging
from
0.63
to
0.83
for
the
three
transects
at
four
soil
depths.
Identified
representative
locations
for
the
three
transects
well-represented
the
mean
SWC,
with
a
root
mean
square
error
of
less
than
2%
and
a
mean
error
of
less
than
1%.
Elevation
and
clay
content
of
soil
were
the
main
factors
affecting
the
spatial
and
temporal
distribution
of
soil
water
at
the
hillslope
scale.
However,
the
characteristics
of
temporal
stability
differed
in
part
among
the
three
transects,
both
in
temporal
persistence
and
in
the
number
of
representative
locations.
Multiple
linear
regression
equations,
determined
between
the
mean
relative
difference
and
the
measured
variables
based
on
the
datasets
of
transects
1
and
2,
did
not
accurately
predict
temporally
stable
locations
for
transect
3.
The
a
priori
selection
of
representative
locations
based
solely
on
properties
of
soil
and
elevation
was
determined
to
be
infeasible
at
the
present
time.
© 2012 Elsevier B.V. All rights reserved.
1.
Introduction
Soil
water
is
the
principal
limiting
factor
in
semi-arid
ecosys-
tems
for
agricultural
production
(Tallon
and
Si,
2004)
and
for
restoration
of
vegetation
(Hu
et
al.,
2009).
For
example,
it
deter-
mines
seed
germination
rates
(Bochet
et
al.,
2007)
and
the
carrying
capacity
for
vegetation
(Xia
and
Shao,
2008).
Consequently,
knowl-
edge
of
the
status
of
soil
moisture,
especially
in
the
root
zone,
is
critical
for
the
management
of
water
resources,
restoration
of
vegetation,
and
farm-specific
development.
The
amount
of
water
in
the
soil
is
a
result
of
interactions
among
a
series
of
variables,
such
as
topography,
soil
properties,
vegetation,
water-routing
processes,
depth
of
water
table
and
meteorological
conditions
(Western
and
Blöschl,
1999;
Gómez-Plaza
et
al.,
2001).
Soil
water
content
(SWC)
has
been
recognized
as
variable
in
space,
∗
Corresponding
author.
Tel.:
+86
29
87018861;
fax:
+86
29
87012334.
E-mail
address:
shaoma@igsnrr.ac.cn
(M.
Shao).
mainly
due
to
soil
variability,
and
in
time,
due
to
climate
(Hu
et
al.,
2009).
The
variability
of
SWC
necessitates
the
collection
of
numer-
ous
samples
to
acquire
a
complete
picture
of
its
spatio-temporal
pattern.
Information
about
mean
SWC
values
and
variances
are
generally
sufficient
for
most
practical
applications
(Tallon
and
Si,
2004).
Traditional
sampling
methods
assume
that
spatial
patterns
of
variability
in
soil
water
are
random.
However,
factors
controlling
soil
water
exhibit
non-random
patterns
that
may
persist
over
time.
Vachaud
et
al.
(1985)
first
introduced
the
concept
of
tempo-
ral
stability
to
describe
the
temporal
persistence
of
the
spatial
pattern.
They
further
assumed
that
specific
locations
could
rep-
resent
the
mean
values
of
a
study
site
over
a
period
of
time.
Other
scientists
later
supported
this
assumption
(Grayson
and
Western,
1998;
Gómez-Plaza
et
al.,
2000;
Starr,
2005;
Brocca
et
al.,
2009;
de
Souza
et
al.,
2011).
Although
the
concept
of
temporal
stabil-
ity
can
greatly
help
to
reduce
the
number
of
samples
needed
to
characterize
SWC
in
a
field,
the
process
of
identifying
time-stable
locations
is
still
time-consuming.
To
capture
possible
changes
in
the
spatial
pattern
due
to
seasonality
and
to
remove
the
influence
0378-3774/$
–
see
front
matter ©
2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.agwat.2012.03.012