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Availability of Soil Water to Plants as Affected by Soil Moisture Content
and Meteorological Conditions
~
O. T. Denmead and R. H. Shaw
2
SYNOPSIS. Actual transpiration decreased with de-
creasing soil moisture content and increasing potential
transpiration. Average soil suction in the root zone when
the actual transpiration rate fell below the potential
rate varied from 12 bars when the potential transpira-
tion rate was 1.4 mm. per day to 0.3 bar when the
potential rate was 6 to 7 mm. per day.
M
UCH attention has been given to the problem of pre-
dicting rates of water use from crops under conditions
where soil water supply is not limiting. For a homogeneous,
actively growing crop it is usual to call this water use the
potential transpiration rate. The potential transpiration rate
is primarily determined by weather factors. Less attention
has been given to the question: What happens when soil
water supply limits transpiration, i.e., at what soil moisture
content does the actual transpiration rate fall below the
potential rate, and can this be predicted for any given soil-
plant-weather combination ? This paper presents the results
of experiments in which this question was studied.
GENERAL CONSIDERATIONS
The problem has been considered from a dynamic view-
fpoint by Philip (8) and Gardner (2). The reader is
erred to these two papers for a detailed theoretical treat-
ment of transpiration as a dynamic process. Water moves
through the soil to the plant root and from the root to the
transpiring leaves along pressure gradients, gradients of
suction (negative pressure) in the soil, and gradients
diffusion pressure deficit, DPD, in" the plant. By analogy
with the flow of heat into an infinitely long cylinder,
Gardner has obtained the solution of the differential equa-
tion describing the flow of water through the soil to the
root in the course of transpiration. His solution of the flow
equation reveals that the suction gradient between root and
soil necessary to maintain a given rate of water uptake by
the root, i.e., a given transpiration rate, is proportional to
the rate of water uptake or the potential transpiration rate
and inversely proportional to the capillary conductivity of
the soil.
The capillary conductivities of soils decrease rapidly with
increasing soil suction. Consequently, as the soil dries, large
suction gradients develop between the root and the soil
around it. In the case of passive absorption, water move-
ment through the plant arises from a gradient in DPD
between the transpiring leaves and the roots. This DPD
gradient is assumed to be proportional to the transpiration
rate (13). Thus, to maintain transpiration in a drying soil
1 Journal Paper No. _1-4017 of the Iowa Agricultural and Home
Economics Experiment Station, Ames, Iowa. Project 1276. Part of
a thesis submitted by the senior author in partial fulfillment of
requirements for the Ph.D. degree. Presented in part before Div.
VIII, Annual Meeting Am. Soc. Agron., Cincinnati, Ohio, Nov.
16, 1959. Received Dec. 20, 1961.
~ Research Associate and Professor in Agricultural Climatology,
Agronomy Department, Iowa State University, Ames, Iowa. Part
of the work reported in this paper was performed by the senior
author as a C.S.I.R.O. (Australia) overseas post-graduate student.
He wishes to express his gratitude to ’C.S.I.R.O. for the assistance
afforded him.
385
where the capillary conductivity is decreasing and the suc-
tion at the plant root is increasing correspondingly, the
DPD in the leaves must continually rise so that the neces-
sary DPD gradient between leaf and root is still present.
The rise in DPD in the leaves is accompanied by a decrease
in turgor pressure resulting in closing of the stomata and
dehydration of the leaves. Consequently, the permeability
of the plant to water flow decreases and the transpiration
rate must decline.
Likewise, an increase in the potential transpiration rate
will hasten the rise in the DPD of the leaves leading to a
more rapid fall in turgor and permeability of the plant
with decreasing soil moisture, supply.
Thus, we expect transplrat~on rates to decline with
decreasing soil moisture content and we expect that this
decline will be evident at higher and higher soil moisture
contents as the potential transpiration rate increases. The
particular soil moisture content at which the decline in
transpiration occurs will also depend on the soil properties.
In soils in which most of the water is held at low suction,
the decline should not be evident until most of the "avail-
able" soil water has been extracted. In soils in which suc-
tion increases rapidly as soil moisture content decreases, the
decline in transpiration should be noticeable at compara-
tively high soil moisture contents. The reader is referred to
Gardner (2) for an illustration of the differences betweett
soil types.
Since the decrease in permeability of the pIant and the
consequent decrease in transpiration result from turgot-
induced changes such as closing of the stomata and dehy-
dration of the leaves, one expects that the soil moisture
content at which transpiration rate decreases should be
coincident with the soil moisture content at which the plant
wilts. That is, the wilting point will also be expected to
vary with soil moisture properties and with the potential
transpiration rate.
The foregoing discussion has been concerned with some
of the dynamic aspects of water availability to plants. The
remainder of the paper presents observations made by the
authors in experiments to verify, at least qualitatively, these
theoretical predictions concerning the effects of variations
in soil water supply and the potential transpiration rate or,
actual transpiration and the wilting point.
EXPERIMENTAL PROCEDURE
Transpiration rat~s and growth rates of corn plants were
examined in detail during a 5-week period commencing just prior
to tasseling. In order to impose soil moisture treatments and
achieve reasonable uniformity of soil moisture throughout the
root zone, it was necessary to restrict the volume of soil available
to the roots. This was accomplished by raising the plants in con-
tainers filled with soil. The 20-gallon containers were 18 inches
in diameter and 24 inches in depth. They were set in the field witl~
their tops level with the ground surface at a spacing of 40 inches
from center to center. A 4-plant hill of corn was raised in ead~
container. Field-grown corn, also in 40-inch hills, was raised o~
all sides. There were 136 containers.
The soil with which the experiment was performed is a Colo
silty clay loam. Field capacity for this soil is 36% by volume and
the soil moisture content corresponding to a suction of 15 bars is
22% by volume. The moisture characteristics are shown in Figure
Published September, 1962
386
AGRONOMY JOURNAL
1. Reference to this figure will disclose that of the water held in
the soil between field capacity and the 15-bar percentage, more
than half is held at suction values greater than 1 bar and almost
40% is held at suction values greater than 2 bars.
’~rhen the containers were filled, an access pipe for the probe
of a neutron moisture meter was inserted in each container. An
outlet was provided in the bottom of each container to allow for
drainage of excess water.
After the plants had emerged, the soil surface was covered with
black plastic film to prevent surface evaporation. Losses in soil
moisture could then be attributed to transpiration. As the plants
grew, watering was accomplished by pumping known amounts of
water from a large storage tank at the experimental site into each
container. Until the treatments involving variation in the soil
moisture regime were imposed, the soil in all containers was
maintained as close as possible to field capacity by application of
water at appropriate intervals.
During the period in which transpiration rates were measured,
soil moisture readings were made daily between the hours of 1600
and 1800. The size of the containers was such that the sphere of
measurement of the neutron meter was just contained within the
walls of the container when the soil was at its driest.
The moisture contents reported in later sections are averages for
the effective root depth of the plants in the containers, which was
21 inches. The roots permeated the soil in the containers thor-
oughly except for a small zone of about 1 inch at the bottom.
Since the root zone was restricted, the rates of water use could
be determined with much more precision than would have been
possible in the field where plants extract water at varying rates
from different positions of the profile. The water in the containers
was depleted from the various depths at a uniform rate; gradients
in soil moisture content from top to bottom were small. In view
of these facts it is felt that the average soil moisture contents
quoted for the root zone of the plants are meaningful and provide
a basis for experimental interpretation of the analyses of Philip
and Gardner referred to in the introductory section.
Soil moisture treatments consisted of the depletion of soil
moisture content at regular intervals during some 5 weeks of the
growing season to values corresponding to soil suctions of 2.5, 5,
and 15 bars. Depletion of soil moisture was accomplished by with-
holding irrigation. Control treatments in which soil suction was
10"
¯
.~.~.~..~b or percent age I field ca~
0 20 30 40
SOIL MOISTURE CONTENT,% by volumt
Figure 1--Relation between soil suction and volumetric soil
moisture content for ColD silty clay loam.
not allowed to exceed 0.5 bar were also maintained. There were
18 treatments comprising 7 controls and 11 variable soil moisture
treatments. The basic design was replicated eight times. Full de-
tails of the treatments imposed are given by Denmead (1)3 The
treatments were so arranged that on each day during the 5..week
period there were a number of containers at different soil moisture
contents ranging from field capacity to the 15-bar percentage. In
this way, the combined effects of daily variations in weather’ con-
ditions and variations in soil moisture supply on transpiration rate
could be studied.
RESULTS AND DISCUSSION
Actual transpiration rate vs. potential~For convenience,
we can regard the transpiration rate at field capacity as
representing the potential transpiration rate. The actual
transpiration rates at different soil moisture contents on 3
of the days in the experiment are shown in Figure 2. The
days shown represent high, moderate, and low potential
*Denmead, O. T. 1961. Availability of soil water to plants.
Unpub. Ph.D. thesis, Iowa State University, Ames, Iowa.
¯ JULY 30-CLEAR,DRY . ¯ -
, AUG, S-HEAVILY OVERCAST, HUMID
~. AUG.~3-PARTLY CLOUDY, HUMID
2OIs’BAR~ ~ ¯ VOLUMETRIC SOIL MOISTURE CONTENT, PER CENT CAPACITY
Figure 2--Actual transpiration rate as a function of soil mois-
ture content.
22 24 26 28 :50 32 54 36
0= VOLUMETRIC SOIL.MOISTURE CONTENT, PER CENT
Figure 3--Relative transpiration rate as a function of soil mois-
ture content for different potential transpiration conditions.
The curves represent days on which the transpiration rates
at field capacity had the value shown in the body o~f the
figure. The numbers in parentheses refer to the number of
days of observation represented by each curve.
DENMEAD & SHAW: MOISTURE CONTENT AND METEOROLOGICAL CONDITIONS VS. SOIL WATER AVAILABILITY 387
transpiration rates. The transpiration rate at field capacity
(the potential transpiration rate) was determined primarily
by meteorological conditions as shown in a later section.
On all days the actual transpiration rate decreased as soil
moisture content decreased. The decline in transpiration
rate occurred at higher soil moisture contents as the poten-
tial rate increased.
Among the many days examined, days similar to each
other in meteorological conditions and the potential tran-
spiration rate were encountered. These days were grouped
according to the value of the potential transpiration rate;
then the mean relative transpiration rate (actual/potential)
was calculated for each group for selected values of soil
moisture content. The relative transpiration rate is shown
as a function of soil moisture content for a range of poten-
tial transpiration rates in Figure 3. The same trends as were
observed in Figure 2 are again evident. The data in these
figures illustrate the importance of knowing the particular
meteorological conditions (which in turn determine the
potential transpiration rate) under which results have been
obtained when considering reports on the availability of
soil water to plants. Under conditions leading to a high
potential transpiration rate, the actual transpiration rate
may be considerably less than the potential rate even though
soil moisture supply might be considered adequate. Under
conditions leading to a low potential transpiration rate, the
actual transpiration rate will equal the potential rate down
to very low soil moisture contents.
Figure 4 depicts relative transpiration rate as a function
of the average soil suction in the root zone for a range of
potential transpiration rates. For the most extreme condi-
tions encountered in the experiment, when the potential
transpiration rate was 6.4 ram. per day, the actual transpira-
tion rate fell significantly below the potential rate when the
average soil suction was only about 0.3 bar. For moderate
potential transpiration rates of 3 to 4 mm. per day, the
potential transpiration rate could be maintained until the
average soil suction was about 2 bars. When the potential
transpiration rate was only 1.4 mm. per day, this rate was
maintained until the average soil suction was as much as
12 bars.
Comparison with reported observations--In Figure 5,
four proposals for the variation in relative transpiration
rate with variation in soil moisture content are shown.
Veihmeyer and Hendrickson’s (14)thesis represented
curve A is essentially for equal availability of soil water
almost to the 15-bar percentage. The data presented in
Figure 3 for low potential transpiration rates and those
presented by Gardner (2) for a sandy soil in which most
of the soil water is held at low suction indicate that such
a thesis is indeed tenable.
Pierce’s (9)proposal, curve B, is based on records
obtained from a weighing lysimeter over ~everal weeks.
During this time, environmental conditions would be ex-
pected to vary widely. His curve agrees well with those
obtained under the "usual" weather conditions of a mod-
erate potential transpiration rate pertaining jn the experi-
ments described here.
Thornthwaite and Mathers’ (12) proposal for a linear
relation between relative transpiration rate and "available"
soil water, curve C, is based on observations made at
O’Neill, Nebraska, in the Great Plains study of 1953 (4).
The soil was a sandy loam. The observations were made
under very dry atmospheric conditions with high radiation
intensities. It is seen that curve C, or, better, curve D which
SOIL SUCTION, bo~
Figure 4--Relative transpiration rate as a function of soil suc-
tion for different potential transpiration conditions. The
curves represent days on which the transpiration rates at
field capacity had the values shown in the body of the figure.
B
I
1.5 aAR e = SOIL MOISTURE CONTENT
PERCENTAGE
FIELD~
CAPACITY
Figure 5--Various proposals for the relationship between rela-
tive transpiration rate and soil moisture content. See text
for explanation of curves.
a ~
I I I
I
~
:3 4 5 (~ .~ I
TFC
=
TRANSPIRATION RATE AT FIELD CAPACITY~mm.P4hr=2
=
Figure 6--Variation in the estimated turgot loss point with
variation in potential transpiration rate.
388 AGRONOMY JOURNAL
is redrawn from the original O’Neill data, agrees well with
a curve obtained for a high potential transpiration rate in
the present experiment.
It should be mentioned here that the decline in relative
transpiration rate with increasing soil suction has been
observed by many workers investigating rates of water use
by crops. Reports of such observations include those of
Makkink and Van Heemst (6), Slatyer (11), Hagan et
(3), Lemon et al. (5), Scholte Ubing (10) and Bahrani
and Taylor (1).
Wilting in relation to soil moisture content and meteo-
rological cond#ions--It was pointed out in the introductory
section of this paper that the soil moisture content at which
the transpiration rate fell below the potential rate should
be coincident with the wilting point. This soil moisture
content, referred to here as the turgor loss point, was esti-
mated for each of the 25 days of observation from the
graphs of actual transpiration rate v. soil moisture content
which were similar to those shown in Figure 2. For ex-
ample, the estimated turgot loss point was 34.2% on July
30, 28.2% on August 13, and 22.6% on August 5. The
relationship between these estimated turgor loss points and
the potential transpiration rate is shown in Figure 6. The
turgot loss point increased rapidly from 23% soil moisture
when the potential transpiration rate was 1.4 mm. per day
to 34% when the potential transpiration rate exceeded 6
mm. per day.
The turgor loss points estimated from the transpiration
curves agreed well with visual observations of the incidence
of wilting taking during the experiment. Observations on
many days confirmed the fact that plants growing at soil
moisture contents greater than the estimated turgor loss
point appeared to be maintaining full turgor while plants
growing at lower soil moisture contents showed greyish
discoloration with some curling, particularly in the top
leaves. It was also observed that, whereas on days when
the potential transpiration rate was high, plants growing
at soil moisture contents less than the estimated turgot loss
point were wilting, the same plants on a succeeding day
with a lower potential transpiration rate would show no
signs of wilting as long as the soil moisture content was
greater than the estimated turgor loss point.
Measurements of dry matter accumulation were made in
one section of the experiment on plants which had been
140
I~C
DAYS BELOW ESTIMATED TURGOR LOSS POINT
Figure 7--Reduction in dry weight of plants subjected to
various periods of soil moisture stress as a function of the
number of days in the stress period on which soil moisture
content was less than the estimated turgot loss point.
subjected to various periods and intensities of soil moisture
stress. The total number of days on which soil moisture
content was less than the estimated turgor loss poirtt was
determined for each treatment along with the reduct:ion in
dry weight below that of the control plants. In Figure 7
the relationship between the period of soil moisture stress
and the reduction in dry weight is depicted.
The fitted regression line in Figure 7 is linear and passes
close to the origin; the intercept is not significantly different
from zero. The slope of the regression line, 14.5 g. per
hill per day is close to the mean growth rate of the control
plants, viz., 13.9 g. per hill per day. This evidence suggests
that once the soil moisture content is less than the turgor
9o
8~
+.Eeo
75
20 25 :50
5 Ib
JULY AUGUST
t8
6
IS
Figure 8--Net assimilation rate, NAR, reduction in rate of dry
matter production, AW, average net radiation, Rn, average
transpiration rate, T, and average daily maximum tempera-
ture, tmax for different periods in the experiment.
I: I L.INE/
~.~.6
o
"" ¯
; ¯ oJ~, o.r3x
~
r- o..~’"
~o! , ,
Eo= EVAP~ATION OF AN O~N WAT~ ~RFA~
COMPUTED BY PENMAN’S METHOD, mm. 24hrs.
c~padt~ and th~ estimated evapo~ado~ o~ a~ open wate~ su~-
fac~ computed b~ the m~[hod of Penman
DENMEAD
&
SHAW:
MOISTURE
CONTENT
AND
METEOROLOGICAL
CONDITIONS
VS.
SOIL
WATER
AVAILABILITY
389
loss
point,
the
plant
virtually
ceases
to
assimilate
carbon
dioxide.
To
compare
the
slope
of the
regression line
in
Figure
7
with the mean growth rate of the control plants is to over-
simplify
the situation, since the control plants were not
gaining
weight
at a
constant rate throughout
the
experi-
ment.
To
show
the
variation
in the
growth rate
of the
con-
trol
plants
and the
coincident variation
in the
effect
of
moisture
stress
on dry
matter accumulation, Figure
8 is
pre-
sented.
In this
figure
the net assimilation rate, NAR, of the
control plants
is
shown
as a
function
of
time
for the
dura-
tion
of the
experiment, along with
the
average reduction
in
dry
weight
per
day,
Aw,
measured over periods
in
which
the
soil moisture content
was
less than
the
turgor loss
point.
NAR is a
measure
of the
rate
of dry
matter production
of
a
plant.
It is the
rate
of
increase
of dry
weight
per
unit
leaf area.
Thus,
NAR
=4(
W
2-
W
J)
A
tj - tj
when
A is the
mean leaf area
of the
plant between times
tjj
and t
±
and W
2
and W
1
are the dry
weights
of the
plant
at
times
t, and t
r
NAR is
here expressed
in
units
of g. dry
matter
per
dm.
2
leaf area
per day X
10
2
.
Aw is
calculated
on
the
same basis, i.e.,
when A is the mean leaf area of the treated plants during
time
t
a
to t
1;
which
is the
number
of
days
for
which soil
moisture
content
was
less than
the
estimated turgor loss
point,
W
2
is the
weight
of the
control plants
at
time
t
2
,
while
W
2t
is the
weight
of the
treated plants
at
time
t
2
.
Neglecting
respiratory losses,
if
assimilation were
to
cease
and
soil
moisture
content
was
less
than
the
turgor
loss
point,
then
W
2t
should equal
W
1
and the AW
measured
over
a
period
of
soil moisture stress should equal
the NAR
over that period.
The
quantitative agreement between
NAR and AW is
good. From Figures
7 and 8 it is
evident that
the
turgor
loss point
is a
significant
soil moisture constant
in
soil-
plant-water
relationships, apparently representing
the
lower
limit
of
availability
of
soil water
for dry
matter accumu-
lation.
It is
also evident from Figure
8
that
a
quantitative evalu-
ation
of the
effects
of
soil moisture stress
on
plant produc-
tion requires some
knowledge
of the
variation
in
NAR.
NAR
itself
is
influenced
by
weather conditions
and for
this
reason
the
average
net
radiation,
Rn, and the
maximum
temperature,
t
max
,
have also been
plotted
in the
figure.
Slow
growth
in the
first
week
of
August
was
associated
with
a
period
of
cool, cloudy
weather.
The
dependence
of
NAR on the
radiation
intensity
is
evident.
Transpiration
rate
in
relation
to
meteorological condi-
tions—The
dependence
of the
potential transpiration rate
on
weather conditions
has
been mentioned several times
in
previous
paragraphs.
For
completeness,
the
average tran-
spiration rate
at
field
capacity,
T, is
show.n
as a
function
of
time
in
Figure
8
where
the
average
net
radiation
is
also
shown.
The
relationship between
the
daily measurements
of
transpiration rate
at
field
capacity
and the
evaporation
of
an
open
water surface
computed
from observations of
net
radiation,
temperature,
humidity,
and
wind
velocity
at
the
experimental site
by
Penman's
(7)
method
is
shown
in
Figure
9.
SUMMARY
Dynamic
aspects
of the
availability
of
soil
water
to
plants
are
discussed
briefly.
It is
pointed
out
that
as the
soil
dries,
the
actual
transpiration
rate
should
fall
below
the
potential rate
and
that this decline
in
relative transpira-
tion rate should occur at
higher
and
higher
soil moisture
contents
as the
potential transpiration rate increases. Since
the
decline
in
relative transpiration rate results from
a
loss
of
turgor
in the
plant,
the
soil moisture content
at
which
plants wilt should also increase
as the
potential transpira-
tion
rate increases.
Transpiration rates
from
corn plants grown
in
containers
in
the
field
were determined under varying conditions
of
soil water supply
and
varying potential transpiration rates.
For
moderate potential transpiration rates
(3 to 4 mm. per
day),
the
actual transpiration rate
fell
below
the
potential
rate when
the
average soil suction
in the
root zone
was
about
2
bars.
When
the
potential transpiration rates were
as
high
as 6 to 7 mm. per
day, this decline
in
relative
transpiration
rate occurred
at
about
0.3
bar.
When
the
potential
transpiration rate
was
only
1.4 mm. per
day,
the
relative
transpiration
did not
decline until about
12
bars.
The
soil
moisture content
at
which
the
decline
in
rela-
tive
transpiration rate occurred, referred
to as the
turgor
loss
point, varied from
a
volumetric soil moisture content
of
23%
when
the
potential rate
was 1.4 mm. per day to
34%
when
the
potential transpiration rate exceeded
6 mm.
per
day. Measurements
of dry
matter production suggested
that once
the
soil moisture content
was
less than
the
turgor
loss point,
the
plants virtually ceased
to
assimilate.
390
AGRONOMY
JOURNAL
- CitationsCitations335
- ReferencesReferences0
- Los bosques de pino carrasco españoles, ubicados actualmente en zonas donde el déficit hídrico es ya recurrente, empeorarán sus condiciones de vida debido al incremento de aridez promovido por el cambio climático. Los árboles absorben el agua del suelo mediante diferencias de presiones hidrostáticas entre éste y la atmósfera (Denmead yShaw 1962). Dicha absorción está condicionada por la apertura estomática y la capacidad de conducción de agua desde las raíces hasta las hojas a través de las traqueidas en coníferas y los vasos conductores en las angiospermas.
[Show abstract] [Hide abstract] ABSTRACT: Most of Spanish forests are currently growing under water deficit conditions. Climate change projections indicate an increase of water deficit stress upon Aleppo pine (Pinus halepensis Mill.) forests. Thus, soil water holding capacity and its availability for trees are important traits to consider when modeling forest responses to climate change, and the effective soil volume for trees is closely related to their maximum rooting depth. In this paper, we evaluate the evolution over the 21th century of P. halepensis spanish forests under climate change conditions and at two maximum rooting depths. We conclude that, under moderate climate change conditions, the fertilizing effect of an increasing atmospheric CO2 concentration would result on higher productivity, increased carbon sink capacity and higher resilience of P. halepensis forests. Conversely, under severe climate change scenario conditions, the CO2 fertilizing effect would be counterbalanced by the negative effects of increasing aridity. We show that the positive effect of an increased soil depth decreases under major climatic constraints (higher temperature and lower precipitation). However, we also observed that it also decreases under favorable conditions (mild temperatures 15 ºC > T > 19 ºC, and precipitation > 520 mm·year-1). We explain that fact because, as precipitation becomes less restrictive, photosynthesis is more linked to seasonal precipitation patterns than to soil water storage capacity.- At certain soil moisture contents (<0.28 in this case –Figure 4), higher atmospheric evaporative demand would give lower transpiration fluxes: a seemingly counterintuitive atmospheric flux response accounting for emerging land-surface feedback in a changing climate[Reynolds-Henne et al., 2010;Teskey et al., 2015;Medina and Gilbert, 2016;Sulman et al., 2016;Schauberger et al., 2017]. This implies a varying soil-plant resistance under different atmospheric and soil moisture conditions[Denmead and Shaw, 1962;Newman, 1969;Mallick et al., 2013]which is successfully accounted for by the proposed parametric transpiration model (equation (17) andFigure 4). For instance, at the intersection points where the lines cross over (see the inset inFigure 4), plants have the same transpiration rate at the same soil moisture content; however, the atmospheric demand (or the potential transpiration rates) for the two sets of data are different.
Near-surface turbulence as a missing link in modeling evapotranspiration-soil moisture relationships
[Show abstract] [Hide abstract] ABSTRACT: Despite many efforts to develop evapotranspiration (ET) models with improved parametrizations of resistance terms for water vapor transfer into the atmosphere, estimates of ET and its partitioning remain prone to bias. Much of this bias could arise from inadequate representations of physical interactions near non-uniform surfaces from which localized heat and water vapor fluxes emanate. This study aims to provide a mechanistic bridge from land-surface characteristics to vertical transport predictions, and proposes a new physically based ET model that builds on a recently developed bluff-rough bare soil evaporation model incorporating coupled soil moisture-atmospheric controls. The newly developed ET model explicitly accounts for (1) near-surface turbulent interactions affecting soil drying and (2) soil-moisture-dependent stomatal responses to atmospheric evaporative demand that influence leaf (and canopy) transpiration. Model estimates of ET and its partitioning were in good agreement with available field-scale data, and highlight hidden processes not accounted for by commonly used ET schemes. One such process, nonlinear vegetation-induced turbulence (as a function of vegetation stature and cover fraction) significantly influences ET-soil moisture relationships. Our results are particularly important for water resources and land use planning of semiarid sparsely vegetated ecosystems where soil surface interactions are known to play a critical role in land-climate interactions. This study potentially facilitates a mathematically tractable description of the strength (i.e., the slope) of the ET-soil moisture relationship, which is a core component of models that seek to predict land-atmosphere coupling and its feedback to the climate system in a changing climate.- The relative contribution of transpiration to evapotranspiration has been found to be dependent on soil moisture availability [Denmead and Shaw, 1962;Shuttleworth and Wallace, 1985], climatic factors such as vapor pressure deficit[Granier et al., 1996], and turbulent characteristics[Tuzet et al., 1997]. Recently, the fraction of T has been directly linked to vegetation morphological attributesZhou et al., 2016], in particular, leaf area index (LAI, Wang et al.[2014]).
[Show abstract] [Hide abstract] ABSTRACT: Even though knowing the contributions of transpiration (T), soil and open water evaporation (E), and interception (I) to terrestrial evapotranspiration (ET = T + E + I) is crucial for understanding the hydrological cycle and its connection to ecological processes, the fraction of T is unattainable by traditional measurement techniques over large scales. Previously reported global mean T/(E + T + I) from multiple independent sources, including satellite-based estimations, reanalysis, land surface models, and isotopic measurements, varies substantially from 24% to 90%. Here we develop a new ET partitioning algorithm, which combines global evapotranspiration estimates and relationships between leaf area index (LAI) and T/(E + T) for different vegetation types, to upscale a wide range of published site-scale measurements. We show that transpiration accounts for about 57.2% (with standard deviation ± 6.8%) of global terrestrial ET. Our approach bridges the scale gap between site measurements and global model simulations, and can be simply implemented into current global climate models to improve biological CO2 flux simulations.- where í µí± í µí¼ and í µí± í µí¼ take different values for different soil types (Denmead and Shaw, 1962). The over simplification represented by the monthly time-step was recognised as inherently unsatisfactory, resulting in the distortion of recharge and depletion patterns.
[Show abstract] [Hide abstract] ABSTRACT: IN the early to mid-1990s, Australia governments adopted significant forestry and water policy agendas. The forest policy stimulated plantation expansion, and articulated benefits to water resource degradation which became a focus of the water agenda. The prospect of changing water availability during a severe drought resulted in the National Water Initiative (NWI), which sought to protect the integrity of water entitlements from plantation expansion. State government agendas provided additional complexity, notably in South Australia where forest water use became subject to regional regulation in 2004, reminiscent of South African experiences in 1972. Inconsistencies between assessments used to support/contest the sustainability of plantation developments resulted in the amendment of South Australian planning frameworks, to ensure competing policy issues were addressed in a balanced manner. Mixed progress in implementing the interception clauses of the NWI have been relegated to unfinished business without critically evaluating its capacity to deliver the required policy and scientific outcomes. Here, national and jurisdictional forest water policies are analysed and weaknesses identified in the lack of a cohesive national policy agenda arising from jurisdictional independence under the Australian Constitution. Inefficiencies in implementation are traced to: competing agendas; the complexities of their inter/intra-jurisdictional administration; a lack of regard to relevant international precedents and a tendency for Australian water reform to be initiated as short-term responses to predictable disasters rather than long-term planning. Reforms under the NWI are found to have had little direct effect in progressing jurisdictional forest water management agendas. Australian empirical hydrological assessment approaches used to support forest water decision-making are examined in the context of international systems and learnings from South Africa. An approach for evaluating and transparently integrating Australia’s limited forest hydrology datasets with modelled information is developed to improve confidence in decision-making. A similar group of empirically-based models are subjected to a comprehensive Bayesian evaluation with South African and American approaches to identify an option with the greatest integrity to underpin current Australian forest water management. Systemic limitations in a widely used approach developed by a leading Australian research organisation are confirmed and revealed as being compounded by weak model structure, highlighting the challenges faced by water management agencies in securing research to support defensible decision-making. Challenges associated with agency capacity limitations and the inevitability of using more complex modelling in supporting future Australian forest water management are addressed by noting South African learnings which identify the importance of growth in plantation water use; and exploring the feasibility of using simpler elements of a sophisticated Bayesian assessment to establish confidence in a plant growth model. Complexity introduced into the 3-PG plant process model is shown to improve the model’s ability to extract information from data providing greater confidence in its potential for future development as a water management tool than more exhaustive, integrated assessments focused on marginal improvements in performance. Results are discussed in the context of water management and their implications for future research and policy developments.- In addition, under tropical meteorological conditions, due to the high atmospheric evaporative demand, the actual transpiration rate may be less than the potential transpiration rate even though soil moisture supply might be considered sufficient. This can lead to loss of turgidity, decreased carbon uptake, cessation of growth and lower productivity of crops (Denmead & Shaw, 1960). To calculate the total number of days when soil moisture contents is sufficiently low to cause crop water stress Benjamin, Nielsen, and Vigi (2003) suggested a soil physical indicator symbolized by the expression 'water stress day'.
[Show abstract] [Hide abstract] ABSTRACT: One major challenge to developing sustainable family farms in tropical regions is increasing nitrogen use efficiency. The aim of this study was to evaluate the combined effects of leguminous residues of low-and high-quality on nitrogen uptake, as well as on content of protein of a Quality Protein Maize (QPM) and of a hybrid maize in a tropical sandy loam soil. The experimental design consisted of randomized blocks with four replicates in a 6 × 2 factorial and six treatments: Gliricidia + Clitoria (GC); Gliricidia + Acacia (GA); Leucaena + Gliricidia (LG); Leucaena + Clitoria (LC); Leucaena + Acacia (LA) and a control without legumes (C). A sub-plot was constructed, sowing in each plot two maize cultivars, opened pollination QPM BR 473 and hybrid Ag 7088. We conclude that the combined use of leguminous residues applied on the soil surface might increase the uptake of nitrogen, the protein contents of maize and the grain yield. In bare soil prone to cohesion, the use of synthetic N is not feasible for both maize yield and for protein yield compared with use of covered soil. The results also showed that the effects of leguminous residue quality on N uptake may differ from year to year due to variation in water stress days.- For the case of the Feddes et al. (1978) transpiration reduction function, threshold values are available in the literature (Taylor and Ashcroft, 1972; Doorenbos and Kassam, 1986) for some crops and some levels of transpiration demand. Nevertheless, experimental (Denmead and Shaw, 1962; Zur et al., 1982) and theoretical (Gardner, 1960; De Jong Van Lier et al., 2006) studies indicate that these parameters should not depend only on crop type and atmospheric demand, but are also determined by root system parameters and soil hydraulic properties. Furthermore, threshold values are hardly ever validated, and they cannot be used for other models (like the Jarvis (1989) model) due to conceptual differences.
[Show abstract] [Hide abstract] ABSTRACT: Detailed physical models describing root water uptake (RWU) are an important tool for the prediction of RWU and crop transpiration, but the hydraulic parameters involved are hardly ever available, making them less attractive for many studies. Empirical models are more readily used because of their simplicity and the associated lower data requirements. The purpose of this study is to evaluate the capability of some empirical models to mimic the RWU distribution under varying environmental conditions predicted from numerical simulations with a detailed physical model. A review of some empirical models used as sub-models in ecohydrological models is presented, and alternative empirical RWU models are proposed. All these empirical models are analogous to the standard Feddes model, but differ in how RWU is partitioned over depth or how the transpiration reduction function is defined. The parameters of the empirical models are determined by inverse modelling of simulated depth-dependent RWU. The performance of the empirical models and their optimized empirical parameters depends on the scenario. The standard empirical Feddes model only performs well in scenarios with low root length density R, i.e. for scenarios with low RWU compensation. For medium and high R, the Feddes RWU model cannot mimic properly the root uptake dynamics as predicted by the physical model. The Jarvis RWU model in combination with the Feddes reduction function (JMf) only provides good predictions for low and medium R scenarios. For high R, it cannot mimic the uptake patterns predicted by the physical model. Incorporating a newly proposed reduction function into the Jarvis model improved RWU predictions. Regarding the ability of the models to predict plant transpiration, all models accounting for compensation show good performance. The Akaike information criterion (AIC) indicates that the Jarvis (2010) model (JMII), with no empirical parameters to be estimated, is the best model. The proposed models are better in predicting RWU patterns similar to the physical model. The statistical indices point to them as the best alternatives for mimicking RWU predictions of the physical model.
Project
Mame Balla Ndiaye- Roger Bayala
Bassirou Sine
The project is interested in the difference in soil drying dynamics of the peanut basin during the dry season. Indeed, after the rainy season, the humidity throughout the root zone is not zero. The…" [more]
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