Field Crops Research, 7 (1983) 299--312.
299
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
CROP EVAPOTRANSPIRATION -- A TECHNIQUE FOR CALCULATION
OF ITS COMPONENTS BY FIELD MEASUREMENTS
P.J.M. COOPER, J.D.H. KEATINGE and G. HUGHES*
International Center for Agricultural Research in the Dry Areas (ICARDA),
P.O. Box 5466, Aleppo (Syria)
(Accepted 27 July 1983)
ABSTRACT
Cooper, P.J.M., Keatinge, J.D.H. and Hughes, G., 1983. Crop evapotranspiration -- a technique for calculation of its components by field measurements. Field Crops Res., 7:
299--312.
A simple technique is described whereby standard field measurements of crop evapotranspiration (ET) , evaporation from an uncropped soil (Es) , green area index (G), the
crop extinction coefficient (K) and above ground dry matter production (TDM) are manipulated to compute the seasonal variation in crop transpiration (T), evaporation from the
soil beneath the crop (E s C) and transpiration efficiency (TE). The technique is illustrated
using data from a trial in which wheat (c.v. Mexipak) and barley (c.v. Beecher) were
grown with and without nitrogen and phosphate fertilizer at two locations in Northern
Syria, namely Tel Hadya and Breda.
T values were very low during the cool winter months, reflecting poor interception of
radiant energy, and E s c accounted for nearly 100% of E T during this period. In early
spring, as G increased, T values also increased and reached maximum values around anthesis of 3.7 and 3.4 ram/day for fertilized and unfertilized wheat at Tel Hadya, and 2.0
and 1.7 ram/day for fertilized and unfertilized barley at Breda. E s c values dropped correspondingly. In the post anthesis period, due to greater soil moisture depletion during
the pre-anthesis period, fertilized crops had lower T values and this was reflected in both
plant water status and grain weight measurements. During this period, as moisture stress
increased and leaf senescence occurred, T values dropped sharply and E s c values rose.
On a seasonal basis, T accounted for 68 and 61% of E T in the fertilized and unfertilized
wheat and 50 and 38% of E T for the fertilized and unfertilized barley.
The pattern of seasonal variation in TE was influenced, in the preanthesis period, by
the absence of root dry matter measurements, but reached values between 65 and 75 kg/
ha/ram around anthesis when root growth ceased. TE values were inversely related to
vapour pressure deficit (represented by E0) in the post anthesis period and corresponded
well to predicted values. On a seasonal basis, TE values were very similar across crop,
treatment and location ranging between 42.8 and 45.7 kg/ha/mm.
*Permanent address: Crop Division, Edinburgh School of Agriculture, West Mains Road,
Edinburgh EM9 3JG (Great Britain).
0378-4290/83/$03.00
© 1983 Elsevier Science Publishers B.V.
300
INTRODUCTION
Rainfed farming systems in Mediterranean t y p e environments of West Asia
and North Africa face t w o major constraints to improved and stable agricultural production, namely erratic and often chronically low rainfall, and widespread deficiencies in major plant nutrients, particularly phosphorus and
nitrogen. Recent work at the International Center for Agricultural Research
in the Dry Areas (ICARDA) has concentrated on investigating ways of improving the efficiency o f use of limited soil water and nutrient resources. Investigations combining fertilizer trials with crop growth and evapotranspiration studies have indicated that there is a large potential for improving the
water use efficiency of cereal production in areas receiving less than 500 mm
o f seasonal precipitation (Cooper et al., 1981).
To obtain a greater understanding of the effect of crop management on
crop water use, it was felt necessary to design an analytical technique based
on field measurements whereby evapotranspiration (ET) could be split into
its two components, namely crop transpiration (T) and evaporation from the
bare soil under the crop ( E s c ) . Ritchie (1981) states that... " I t is possible
to separate soil and plant evaporation logically when we k n o w the fraction
of the energy intercepted b y the plant canopy and the critical soil parameters." Attempts have been made to produce models which predict these
parameters (Ritchie, 1972; Tanner and Jury, 1976). Such models have relied
largely on empirical relationships which describe the temporal nature of
moisture loss from an uncropped soil and the effect of crop canopy developm e n t on radiant energy interception and the ratio T/Eo, where E0 is the potential evaporative demand. This predictive model approach, with minor modifications has proved successful (Kanemasu et al., 1976; A1-Khafaf et al.,
1978).
The technique described in this paper is based on the logic described by
Ritchie (1981), b u t is n o t a predictive model. Instead it describes h o w standard field measurements of ET, E S (evaporation from an uncropped soil),
crop c a n o p y size and its relationship with interception of radiant energy can
be logically and simply used to determine T and ESC. The technique is illustrated utilizing data collected from fertilizer trials conducted with wheat
and barley at t w o contrasting locations in Northern Syria. The implications
of fertilizer addition on the seasonal variation in T and ESC and the transpiration efficiency (TE) of the crop are discussed.
MATERIALS AND METHODS
Description o f trial, soil and crop measurements
The trial was c o n d u c t e d at two locations in Northern Syria, Tel Hadva
(long term seasonal precipitation 342 mm, 3 5 ° 5 5 ' N 36°55'E) and Breda
(275 mm, 3 5 ° 5 5 ' N 37°10'E). The soil at Tel Hadya is described as a Vertic
301
(calcic) Luvisoland that at Breda as Calcic Xerosol. Full profile descriptions
are given in Cooper et al. (1981). Two contrasting fertilizer treatments (see
Table I) were selected from part of a larger agronomy trial on wheat (c.v.
Mexipak) at Tel H a d y a and barley (c.v. Beecher) at Breda. Treatments were
replicated four times and were drill planted in early December at a seed rate
o f 90 kg/ha and a between r o w spacing of 17.5 cm. Germination occurred
on 1 0 / 1 2 / 8 0 at b o t h locations. Weeds were controlled with a single spraying
at B r o m o n y l at stem elongation.
TABLE I
E f f e c t o f fertilizer a d d i t i o n o n c r o p p r o d u c t i v i t y o f w h e a t (c.v. M e x i p a k ) a n d b a r l e y (c.v.
B e e c h e r ) a t t w o l o c a t i o n s in N o r t h e r n Syria, 1 9 8 0 / 8 1
Site
Crop
Fertilizer a
(kg/ha)
PzO~
N
TDM at
harvest
(kg/ha)
1 0 0 0 grain Seed yield
weight
(kg/ha)
(g)
Tel H a d y a
Wheat
Wheat
LSD (5%)
0
60
--
0
60
--
9460
10830
--
34.8
31.5
1.7
3560
3770
280
Breda
Barley
Barley
LSD (5%)
0
60
--
0
60
--
3550
4940
--
38.1
32.5
1.1
1720
2130
80
aAll P2Os and 20 kg/ha N drilledwith seed. Remaining N top dressed at stem elongation.
Moisture accession tubes (eight per treatment) were installed at each location to a depth of 180 cm prior to planting. These were monitored regularly
through the season at 15 cm depth intervals using a Mk II Wallingford
Neutron Probe. Moisture changes in the 0--15 cm horizon were determined
gravimetrically with a volumetric soil sampler. Precipitation (P) and Class A
Pan E0 were recorded on a daily basis and ET for given periods was computed from the equation:
Ew =P-
AM-R-D
(1)
where AM is the change in total moisture in the 0--180 cm profile, R is surface run o f f and D is drainage below 180 cm depth. In these trials, R and D
did not occur and thus were ignored in computing ET. Accession tubes (four
per location) were similarly installed and monitored in uncropped bare soil
plots adjacent to the trial area, and ES c o m p u t e d according to equation (1).
Destructive crop samples were taken weekly (8 single meter rows per
treatment) from which total above ground dry matter (TDM) and green area
index (G) were recorded. On alternate sampling dates the interception of
radiant energy by the crop canopy was recorded by use of 1 m t u b e solarimeters (three per treatment). From these data, an estimate could be made of
302
the extinction coefficient K in the relationship between intercepted radiation and G formulated as:
= 1
-
-
(2)
e -KG
where a is the proportion of incident radiant energy intercepted by the crop
canopy of green area index G. Monteith (1965) noted that for small grain
cereal crops, K has a value between 0.3 and 0.5. A value o f K = 0.37 was calculated from the present data pooled over crops, sites replicates and times.
At harvest, crop productivity was assessed from a larger sample of 96 single
meter rows per treatment (see Table I).
DATA PROCESSING
Stage 1 -- Curve fitting
Polynomial curves describing the changes over time (days from germination) of accumulated ET, accumulated ES, dry matter and green area were
//I
300
(A)
rr-
//
v
(B)
200
/
V
E
,<
100
/
40
8b
'
1~o
'
1~o
"
40
"
8b
" 1~o '
160
Days post germination
Fig. 1. A c c u m u l a t e d E s ( x - - x )
a n d E T o f ( A ) w h e a t at T e l H a d y a a n d ( B ) b a r l e y at
Breda, with (e
, ) a n d w i t h o u t (A
- ) a d d e d fertilizer.
303
T
" ~ e 4. . .
°° i ~"0
•
'
'
o
'
'
".t,
v
o
(etl/I)
uo}l:mpold
JO|lelll '(J(TI
i - ,==<
",,e
"¢'--
¢
E ~ a:
-<
(u'q/I) uo!~,mp(nd Jaue.m ,{~([
304
10,0
(A)
1.0
•
•
j/ •
,' A
0.1
/
iA
50
100
150
Days post germination
5.0
0a
• ~-
1.C
•
;j//
/
•
" s~)
•
"
"
16o
.
.
.
.
150
Days post germination
Fig. 3. Change in green area index o f (A) wheat at Tel Hadya and (B) barley at Breda with
(e
, ) and without ( A - - A ) added fertilizer.
305
fitted by multiple regression analysis (see Figs 1, 2a, b and 3a, b). In the
cases of the dry matter and green area data, a logarithmic transformation was
applied in order to obtain homogeneity of variance. In the cases of ET and
E S data, where such a transformation was unnecessary, the fitted curve was
constrained to pass through the origin. In fitting polynomial curves to plant
growth data, we have followed the guidelines of Hunt (1978}. In some cases
(e.g., Figs. 2a, b) we have used more than one curve to describe the data
since it appeared that this approach could better emulate the general trend
than a single high order polynomial regression line.
Stage 2 -- Computation o f evaporation from the soil under the crop (Esc)
Ritchie (1972} and others recognize two phases of moisture loss from a
bare soil surface. Phase one is the constant rate phase during which the soil
is sufficiently wet for the water to be transported to the surface at a rate
equal to the evaporative demand. During this phase, the rate of evaporation
is determined by the supply of energy reaching the soil surface. In phase
two, the falling rate phase, the soil surface water c o n t e n t has decreased to a
value such that the rate of evaporation becomes more dependent on soil
hydraulic properties, and is m u c h less dependent on the evaporative demand.
Recognition of these two phases is of particular importance when considering conditions where the soil surface is infrequently wetted, as under irrigation.
However, in rainfed systems, where the soil is frequently re-wetted at variable intervals, no such clear distinction exists. Experience has shown that
under such conditions, moisture loss from a bare soil can be roughly estimated by the simple expression:
1
ES = E0 - t
(3)
where t is the n u m b e r of days since it last rained. During the cool wet
months of December, January and February, when E0 values are low and
rainfall is frequent, the ES/Eo ratio approaches unity; and during such
periods, E s is well described by Ritchie's phase one model. During the
months of March, April and May, the soil surface continues to be wetted,
albeit less frequently, but E0 values rise sharply and the Es/Eo ratio decreases. In spite o f this, the actual rate of E s rises during this period as predicted by expression (3) (see Fig. 1). It is only in late May/early June, after
crop maturity when precipitation ceases and E0 values become very high that
Es clearly behaves according to the phase two model, and expression (3) becomes m u c h less reliable.
We therefore assume that during the crop growth cycle E s will be largely
determined by E0 values and frequency of rainfall. Since radiant energy is
the principal c o m p o n e n t of E0 and frequency of wetting is identical to both
306
bare and cropped plots, we further assume that E s c / E s ratio will depend
on the proportion of radiant energy intercepted by the crop and ESC can be
c o m p u t e d from the equation:
ESC =E s (1 - ~)
(4)
Thus from the curves fitted to accumulated Es vs. time and G vs. time,
daily values of E s (mm/day) and G are c o m p u t e d for the life of the crop.
Using the daffy value of G and the estimated value for K, a is calculated
on a daffy basis from equation (2). Utilizing this value of a, the daily rate
o f ESC is c o m p u t e d according to equation (4).
Stage 3 -- Computation o f transpiration (T)
Daffy rates of ET are c o m p u t e d from the curves fitted to accumulated
ET vs. time (see Fig. 1), and daily values of T are obtained from the expression:
T = E w - Esc
(5)
It can be seen from Fig. 1 that during early growth (up to day 60) when
leaf areas, and hence T, are very small there is no measurable difference between accumulated ET and ES at a given location. This has been observed
for a variety of crops in other locations (Cooper et al., 1981). However,
curves fitted to accumulated E T and E s spanning the whole growing season
do n o t coincide during this early growth period, and thus calculation of T
T A B L E II
C o m p u t a t i o n o f T d u r i n g t h e early g r o w t h p h a s e f o r w h e a t w i t h fertilizer a t Tel H a d y a
Period a
ET b
(days)
( m m per 5-day
ES b
Es c b
1-- 5
5--10
10--15
9.3
7.5
6.1
6.0
6.1
6.1
6.0
6.1
6.1
15--20
5.0
6.1
6.1
20--25
4.4
6.0
6.0
25--30
4.0
6.0
6.0
30--35
35--40
40--45
45--50
50--55
55--60
60--65
65--70
4.0
4.2
4.6
5.3
6.1
7.1
8.2
9.3
5.9
5.8
5.7
5.6
5.5
5.4
5.3
5.2
5.9
5.7
5.5
5.2
4.8
4.4
4.0
3.7
E T -- ESC
E s -- ESC
Computed T
period)
a D a y 1 is 1 0 / 1 2 / 8 0
b V a l u e s c o m p u t e d f r o m f i t t e d curves.
3.3
1.4
0
----
----
-I.I
--
--
-1.6
--
--
-2.0
--
--
-1.9
-1.5
-0.9
0.1
1.3
2.7
4.2
5.6
--
--
0.1
0.2
0.4
0.7
1.0
1.3
1.5
0.1
0.2
0.4
1.3
2.7
4.2
5.6
307
b y equation (5) can produce nonsensical results. We therefore assume that,
during the early growth period, the curve fitted to accumulated ES vs. time
adequately describes accumulated ET, and during this period T is calculated
from the expression:
T = ES - ESC
(6)
An example of this procedure is given in Table II. It can be seen that during
this period, accumulated T values are very small compared with the seasonal
total, and thus any error associated with this procedure will be negligible.
Accumulated ET, ESC and T data are presented in Fig. 4, and the seasonal
variation in daily rates of ESC and T are expressed as 5-day mean values in
Fig. 5a, b.
/
(A}
ET
300-
T
200"
E
E
?
~c
10040
v
E
<
sc
20(>
80
120
(C)
#
160
ET
{D}
/
ET
100-
40
do
1~0
Days post germination
Fig. 4. Accumulated ET, T and E s c of wheat at Tel Hadya (A) with and (B) without fertilizer, and of barley at Breda (C) with and (D) without fertilizer.
Stage 4 --Computation of transpiration efficiency
Daffy rates of increase in dry matter are c o m p u t e d from the fitted curves
in Fig. 2 and these are divided by the concurrent daily T values. The seasonal
variation in transpiration efficiency (TE) is thus obtained and is epxressed in
units of kgha -1 mm -1. These data are presented in Fig. 6. It must be emphas-
308
ized that these data express the transpiration efficiency of above ground dry
matter production only, and do not include dry matter production in the
r o o t system. This is discussed in more detail in the next section.
DISCUSSION
ET, ESC and T
Seasonal variation in dally rates of ESC, T and E0 are presented in Figs.
5a, b. During the first 50 days post germination, T values were very low for
b o t h wheat and barley, and ESC accounted for almost 100% of ET. This reflects the slow development of green area during the cool winter months and
the subsequent p o o r interception of radiant energy. As green area developed,
T became a more significant c o m p o n e n t of ET, and ESC values dropped. At
Tel Hadya, where the fertilized and unfertilized wheat crops reached maxim u m recorded G values of 6.7 and 4.7, almost complete interception of
radiant energy occurred, and maximum T values of 3.7 and 3.4 m m / d a y
were obtained whilst ESC values dropped to a minimum of 0.15 and 0.06
m m / d a y . At Breda however, where the fertilized and unfertilized barley crop
only reached G values of 3.8 and 1.8, the maximum T values obtained were
2.0 and 1.7 m m / d a y , respectively, and ESC values decreased to a lesser extent. At both locations, as leaf senescence occurred in the post anthesis
period, and moisture stress increased, T values dropped sharply and ESC increased as was suggested by Doyle and Fischer (1979). This rise in ESC during senescence was associated with atypically late rains close to crop maturity at both locations, namely 55 mm between day 140--155 at Tel Hadya and
25 mm between day 126--140 at Breda, which re-wetted the soil surface
under the crop. In more normal years such a pronounced increase in ESC
during this period would n o t be expected.
It is interesting to note that whereas fertilizer addition increased T values
prior to anthesis, the opposite effect was observed in the post anthesis period. This was due to greater depletion of soil moisture reserves by the fertilized crop during the vegetative growth period. This observation is supported
b y plant water status measurements which indicated that greater internal
moisture stress occurred in the fertilized crop during grain filling, water potentials being up to 1.5 MPa lower than in the unfertilized crop. It is also
reflected in the grain weights (Table I) which were significantly reduced in
the fertilized treatment at both locations, a well d o c u m e n t e d result of greater moisture stress during grain filling (e.g., Hochman, 1982).
Accumulated ET, ESC and T data are plotted in Fig. 4, and seasonal totals
are given in Table III.
Bearing in mind the inevitable loss of moisture from the soil prior to canopy closure, wheat at Tel H a d y a actively utilised a relatively high proportion
of the total ET as T, namely 68 and 61% for the fertilized and unfertilized
crop. However, corresponding values of 50% and 38% for the barley crop at
309
(A)
=
E
40
80
120
160
Days post germination
E0
(B}
E
E
~
~---$22~':
40
....
80
\?¢ Esc
120
Days post germination
Fig. 5. Seasonal variation in rates of E_, T and ESC for (A) wheat at Tel Hadya and (B)
barley at Breda with ( - - ) and without 1........ ) added fertilizer•
(A, Anthesis.)
310
TABLE III
ET, ESC , T, WUE and TE of wheat and barley at two locations in Northern Syria
TDM at
ET
ESC
T
WUE
TE
harvest
(kg/ha)
Tel Hadya
Wheat (+ fertilizer) 10830
Wheat (- fertilizer)
9460
(mm)
(kgha-1 mm-1)
372
357
120
137
252
220
29.1
26.5
43.0
43.0
216
220
108
137
108
83
22.9
16.1
45.7
42.8
Breda
Barley (+fertilizer)
Barley (- fertilizer)
4940
3550
Breda indicate t h a t there is great potential for improvement of water use efficiencies in dryland barley growing regions. This is reflected in the water use
efficiency (WUE) data presented in Table III. It should be emphasized that
these WUE's for barley were achieved by crops yielding 1.7--2.1 t/ha, whereas typical yields obtained by local farmers in Northern Syria range between
0.4 and 1.2 t/ha. By rough extrapolation, such crops could only be expected
to actively utilize between 15 and 25% of the seasonal E T as T.
Whereas there is, as would be expected, considerable variation in WUE
values across treatments and sites, it is encouraging to observe that the TE
values c o m p u t e d by this technique in Table III are very similar across treatment, crop and location. This is in agreement with the observation t h a t TE
values for C3 cereal cultivars such as wheat and barley do not differ greatly
within a given environment (Fischer, 1981).
It is of interest that the addition of fertilizer has little or no effect on the
TE of wheat and barley. This has been discussed in detail by Cooper (1983)
who showed that:
WUE =
TE
(7)
1 +Esc/T
It thus appears that the pronounced increase in WUE resulting from correct fertilizer use as reported here and elsewhere (see Cooper, 1983) is largely due to increased G and its effect on the E s c / T ratio rather than any specific effect on TE per se.
Seasonal variation in TE
Seasonal variation of TE's are presented in Fig. 6. TE values are inversely
related to vapour pressure deficit, which can be represented by E0 and from
an empirical relationship derived in Australia (Fischer, 1981), an almost
constant TE value of 70-80 kgha -1 mm -1 for shoot plus root dry m a t t e r production would be predicted for the cool winter m o n t h s in Syria when E0
311
.~=
=_
lOO
(B)
(A)
A
"-~,
50
J
•
80
A
120
160
80
120
160
Days post gcrmillalioH
Fig. 6. Seasonal variation in TE of (A) wheat at Tel Hadya and (B) barley at Breda with
( - - ) and w i t h o u t ( ...... ) added fertilizer.
(A, Anthesis.)
values range between 1 and 2 mm/day. It is clear from Fig. 6 that prior to
anthesis, a very different pattern is represented by our data. This would appear to be largely due to the fact that r o o t dry matter production, which occurs prior to anthesis (Gregory et al., 1978), is not included in our analyses.
This can be illustrated in more detail for the barley crop at Breda. If the
plateau TE value of 60 kgha -1 mm -I, achieved around anthesis, when r o o t
growth is ceasing, is assumed to have been constant throughout the preceding period, we can calculate root plus shoot dry matter production per 5-day
period from the c o m p u t e d T values (see Fig. 5b). If the TDM per 5-day
period (see Fig. 2b) is subtracted from these values, an estimate of root dry
matter production is obtained. Summation o f these 5-day estimates of root
dry matter production gives a total of 1560 and 1430 kg/ha of root dry matter at anthesis for fertilized and unfertilized barley. These values fall well
within the range of 300--1890 kg/ha for r o o t dry matter of barley at anthesis as cited by Gregory et al. (1978). We do not suggest that this procedure
can be used to predict root dry matter production with any great accuracy
at this stage, b u t include the above example to illustrate the likely cause for
the pre-anthesis pattern of TE variation.
Reference to Fig. 6 indicates that between day 125 and 135 there was a
sharp decline in TE for wheat at Tel Hadya. This coincided with a sudden
rise in E0 values from a b o u t 5 to 8.5 mm/day. It has already been observed
that TE is inversely related to E0, and the empirical relationship derived by
Fischer (1981) in Australia predicts a TE value of 30 kgha -1 mm -1 for an E0
value of 8.5 mm. This value corresponds closely to the values observed for
wheat in Fig. 6. A similar sharp increase in E0 also occurred at Breda between day 125 and 130, and y e t there was no corresponding decrease in
TE. However, the crop was close to maturity at this stage and leaf area, dry
312
matter production and transpiration levels were all very low. (Figs. 2b, 3b,
5).
Doyle and Fischer (1979) suggest from field studies on wheat in Australia,
that water limitations may increase TE, b u t that was not found to be the
case in this study. Examination of both extractable soil moisture levels and
plant water status indicated that the unfertilized wheat and barley crops suffered less water limitation in the post anthesis period, and y e t in both cases
the unfertilized crop had a higher TE value during this period.
CONCLUSION
The data presented describing the seasonal variation in T, ESC and TE correspond well with results reported by other workers in similar environments.
We believe that the apportioning of ET into its two components, as described in this paper, adds significantly to the interpretation of the results
obtained from crop water use studies at the cost of very little extra workload
in the field. Apart from the monitoring of ES at the trial site, no additional
measurements are required b e y o n d those normally undertaken in such
studies.
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