Directory UMM :Data Elmu:jurnal:A:Agricultural Water Management:Vol42.Issue2.Nov1999:
Agricultural Water Management 42 (1999) 205±218
The influence of early sowing of wheat and lupin crops
on evapotranspiration and evaporation from the soil
surface in a Mediterranean climate
J. Easthama,*, P.J. Gregorya,1, D.R. Williamsonb, G.D. Watsonb
a
CSIRO Division of Plant Industry, Private Bag, P.O., Wembley, WA 6014, Australia
b
CSIRO Land and Water, Private Bag, P.O., Wembley WA 6014, Australia
Accepted 21 January 1999
Abstract
The losses of water by evapotranspiration and evaporation from soil were investigated during two
seasons from wheat and lupin crops sown at two times. Evapotranspiration was measured using
ventilated chambers and microlysimeters were used within the chambers to measure evaporation
from the soil surface. These techniques allowed the partitioning of evapotranspiration into its two
components. In the early part of the season, evaporation from the soil surface was greatest beneath
late-sown crops. Larger canopies, associated with early sowing, reduced evaporation during the
energy-dependent first stage. The greater losses beneath late-sown crops were not sustained as
surface soil water contents declined, decreasing the influence of canopy area on evaporation. Early
sowing may increase evapotranspiration early in the season and thereby decrease the risk of
drainage losses contributing to groundwater recharge. However, the magnitude of the hydrological
advantages from early sowing is likely to vary each year according to seasonal climatic conditions.
# 1999 Elsevier Science B.V. All rights reserved.
Keywords: Evapotranspiration; Soil evaporation; Crop management; Groundwater recharge
1. Introduction
A large proportion of the grain produced in Australia is grown in areas with a
Mediterranean climate characterised by hot, dry summers and cool, wet winters. Under
* Corresponding author. Present address: The University of Western Australia, Faculty of Agriculture,
Nedlands, WA 6907, Australia. Tel.: +61-89-380-2491; fax: +61-89-380-1108
E-mail address: jeastham@cyllene.uwa.edu.au (J. Eastham)
1
Department of Soil Science, The University of Reading, Whiteknights, P.O. Box 233, Reading, Berkshire,
RG6 6DW, UK.
0378-3774/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 0 3 6 - 0
206
J. Eastham et al. / Agricultural Water Management 42 (1999) 205±218
these conditions, crop growth is limited by water and yields are often positively related to
rainfall during the growing season. Moreover, 30±60% of the seasonal evapotranspiration
may be lost as evaporation from the soil surface (Siddique et al., 1990). This large loss
occurs because, during early winter, crops have low leaf area indices and the soil surface
is frequently wetted by rainfall. Evapotranspiration during this period is dominated by
soil evaporation (Yunusa et al., 1993a) but if this water could be transpired, growth and
grain yields of crops may be increased. Plant characteristics such as early vigour (Turner
and Nicolas, 1998) and management practices such as early sowing, increased fertiliser
input and planting density which increase early growth, have been shown to increase crop
yields through improved water use efficiency (Anderson, 1992; Anderson et al., 1992;
Connor et al., 1992). Additional benefits of increasing the early growth and water use of
crops may be derived through reducing deep drainage losses contributing to salinity and
waterlogging in the Western Australian wheatbelt. Greenwood et al. (1992) proposed early
sowing of agricultural crops as a strategy for increasing evapotranspiration from catchments.
Several approaches have been used to separate the evaporation and transpiration
components of evapotranspiration (Wallace, 1991). Some have measured the total
evapotranspiration and transpiration components (Azam-Ali, 1983; Wallace et al., 1990)
and obtained evaporation by difference, while others have used physically based process
models to estimate the components (Ritchie, 1972; Lascano et al., 1987). While evaporation from bare soil can be estimated with relative ease, root water uptake and modification
of the microclimate by canopies complicate determination of evaporation from soil beneath
crops. Direct measurements of soil evaporation have been made using small lysimeters
placed between rows (Boast and Robertson, 1982; Allen, 1990; Villalobos and Fereres, 1990;
Yunusa et al., 1993a, b) but the method is time-consuming and care must be taken to
ensure that the soil surface is undisturbed and representative of the surrounding soil. The
technique proposed by Cooper et al. (1983) for estimating evaporation from soil assumes
that the principal factor controlling evaporation from the soil surface beneath a crop is
the proportion of radiant energy reaching the soil. Evaporation can be estimated as the
product of the fraction of radiation transmitted through the crop canopy and evaporation
from a bare plot. Such measurements are relatively easy to obtain over long periods.
The present experiments formed part of a broader study to investigate the potential for
agronomic management to increase water use and yields of wheat and lupin crops and to
decrease groundwater recharge in the wheatbelt of Western Australia. Gregory and
Eastham (1996) describe the growth, radiation interception and yields of the crops. This
paper reports an investigation of the effects of early sowing on evapotranspiration and
evaporation from soil beneath wheat and lupin crops. A comparison is made between
direct measurement and predictive methods of estimating evaporation.
2. Materials and methods
2.1. Experimental site
The experiments were carried out in 1990 and 1991 at the East Beverley Research
Annex, approximately 100 km east of Perth in Western Australia (328080 S; 1178100 E).
J. Eastham et al. / Agricultural Water Management 42 (1999) 205±218
207
The climate is Mediterranean with a mean annual rainfall of approximately 380 mm. The
soil is a yellow duplex soil with a sand layer approximately 0.35 m deep overlying
kaolinitic clay (Northcote, Dy 2.82; USDA, Typic Natrixeralf); see Gregory and Eastham
(1996) for details.
2.2. Crop growth
Wheat (cv. Kulin) was sown on 25 May and 14 June 1990 and both wheat (cv. Kulin)
and lupins (cv. Gungurru) were sown on 31 May and 28 June 1991. Crops sown in May
are referred to as the early-sown crops, and those in June as the late-sown crops. The
experiment was planted in a randomised block design with six replicates, each plot being
10 60 m2. In both years, crops were sampled coincident with evapotranspiration
measurements from six adjacent 1 m rows in each plot. The area of green leaf and stem
was measured using a planimeter. Gregory and Eastham (1996) give further agronomic
details of the experiment and climatic data for the site.
2.3. Evapotranspiration
Evapotranspiration (ET) was measured using ventilated chambers in three replicates of
each treatment for four 48 h periods in 1990, and in two replicates for five 24 h periods in
1991. In 1990, measurements commenced on July 24, August 21, September 18 and
October 3, which were 60, 88, 116 and 130 days after the first sowing (DAFS)
respectively. In 1991, measurements began on August 13 and 29, September 10 and 29
and October 8 which were 74, 90, 104 118 and 132 DAFS, respectively. The design and
operation of the chambers are described in Farrington et al. (1992). The chambers were
5.25 m long, 3.0 m wide and 1.7 m high, and were made from clear high-density
polyethylene 200±250 mm thick which transmitted 78% of the incoming solar radiation.
Air was blown from an axial electric fan located at one end of the chamber and passed
through a baffle before moving horizontally across the chamber and venting through an
orifice at the other end of the chamber. The size of the orifice was adjusted to maintain a
chamber pressure of 0.3 mbar. Measurements with a pitot tube in the inlet fan duct gave a
mean air velocity of 0.3 m sÿ1. Samples of air from the inlet and outlet of the chamber
were pumped through insulated and heated lines and a heated homogenising container to
an infra-red gas analyser operating in the differential mode. The rate of evapotranspiration was calculated from the difference in vapour pressure between the air entering and
leaving the chamber. Air was sampled and analysed for approximately 6 min from each
chamber before switching to the next chamber. Evapotranspiration was expressed as a
daily rate (mm dayÿ1) by calculating the mean rate of evapotranspiration measured over
each 24 or 48 h period of sampling.
2.4. Evaporation measured by microlysimeter
Evaporation of water from the soil surface within each chamber was measured using
three microlysimeters containing undisturbed samples of soil. Lysimeters were made
from PVC drainage pipe and were 0.064 m in diameter and 0.15 m deep. In 1990,
208
J. Eastham et al. / Agricultural Water Management 42 (1999) 205±218
lysimeters were filled by forcing them into the soil just before each measurement. In
1991, 0.012 m diameter holes were drilled in the lysimeter walls (removing 10% of the
area) allowing roots to penetrate and take up water from within the lysimeter; these
lysimeters were installed between rows soon after crop emergence. Immediately before
use, the lysimeters were excavated, sealed at the base and in 1991 the sides were sealed
with adhesive tape. Lysimeters were weighed and then placed in sleeves in the ground
within each chamber with their surface at the same level as the surrounding soil. They
were re-weighed after 24 h to determine the water loss by evaporation. In 1991, the water
content of the upper 0.1 m of soil in each lysimeter was subsequently determined
gravimetrically by oven-drying. Gravimetric water content was converted to volumetric
water content assuming a mean bulk density of 1.64 t mÿ3, determined by oven-drying
eight undisturbed soil cores of known volume.
In 1991, measurements of evaporation from fallow soil and cropped soil not enclosed
by chambers were made for all but the first period of measurement to allow evaluation of
the Cooper method of estimating evaporation. A microlysimeter was installed in each of
the six replicate plots of the cropped soil (i.e. 6 microlysimeters per treatment), and four
microlysimeters were used for fallow soil.
2.5. Evaporation estimated by the Cooper method
In 1991, estimates of evaporation were made using the method of Cooper et al. (1983).
Evaporation from fallow soil (Ef) and within the chambers was measured concurrently
using microlysimeters, and evaporation from cropped soil (Ec) was estimated using:
Ec Ef 1 ÿ i
where i is the proportion of radiation intercepted by the crop (Cooper et al., 1983). Radiation
interception was recorded continuously in 1991 using Delta-T tube solarimeters in three
replicate plots of each treatment. Solarimeters were placed close to the soil surface across
six rows of each crop and the integrated reading recorded weekly. One solarimeter was
placed above the crop canopies to measure the incident radiation so that the fraction of
incident radiation intercepted by the canopy could be calculated. Radiation interception
data for each treatment is shown in detail in Gregory and Eastham (1996).
2.6. Soil water content
Soil volumetric water contents were measured in each plot using a neutron water meter
at sowing and on the same days as ventilated chamber measurements. Readings were
taken at 0.1 m depth intervals from 0.1 to 0.7 m and at 0.2 m intervals from 0.9 to 1.7 m.
Separate calibrations for the 0.1, 0.2 and >0.3 m depths were used to convert counts to
volumetric water content.
2.7. Statistical analysis
Student t-tests were undertaken to determine the significance of differences
between the treatment means. Evapotranspiration data were grouped into periods
J. Eastham et al. / Agricultural Water Management 42 (1999) 205±218
209
of high and low soil water content and analysis of variance was undertaken to
determine the significance of differences between treatments in early and late season
evapotranspiration. All differences reported in the text are significant at the 5% level
or greater.
3. Results
3.1. Canopy development
In both 1990 and 1991, time of sowing significantly influenced the size of the
crop canopies on each day of measurement (Fig. 1). Earlier canopy development
resulted in a greater green area index (GAI) for early-sown wheat for the first part
of both seasons (Fig. 1). However, senescence of early-sown wheat occurred sooner,
so that later in the season its GAI was reduced to less than that of the late-sown
wheat.
Vegetative growth of wheat was greater in 1990 than 1991 with a maximum GAI for
early-sown wheat of 2.2 in 1990 and 1.8 in 1991; comparable values for late-sown wheat
were 3.0 and 1.0. Canopy development of wheat was more rapid than that of lupins in
1991 so that GAI was greater than for the corresponding lupin crop until 104 DAFS.
Senescence of wheat occurred before that of lupin and by the final measurement in 1991
both lupin crops had a greater GAI than wheat crops. The maximum GAIs for early- and
late-sown lupins were 1.5 and 1.2 respectively.
Fig. 1. Green area indices of early- and late-sown crops for each ventilated chamber measurement in (a) 1990
and (b) 1991. Bars indicate one standard error of the mean.
210
J. Eastham et al. / Agricultural Water Management 42 (1999) 205±218
Fig. 2. Mean volumetric water contents of the upper 1.2 m of soil beneath early- and late-sown crops
for each ventilated chamber measurement in (a) 1990 and (b) 1991. Bars indicate one standard error of
the mean.
3.2. Soil water content
Ventilated chamber measurements commenced when soil water was close to its
maximum for the season following recharge of the profile by winter rainfall. Mean
volumetric water contents to a depth of 1.2 m decreased progressively with time beneath
all crops from the first to the last measurement in both 1990 and 1991 as soil water was
depleted (Fig. 2). At any given time in both seasons, there was no significant difference in
mean soil water content beneath early- or late-sown crops, or beneath either wheat or
lupin crops in 1991.
Fig. 3 shows surface soil water contents at 0.1 m measured by neutron moderation in
1990 and gravimetrically in 1991. In both years, surface water contents decreased
progressively with time under each crop from the first to the last measurement. In 1990,
surface water contents were similar beneath early- and late-sown wheat for each of the
four measurements. In 1991, water contents were similar beneath all treatments at 74 and
90 DAFS. However, for the final three measurements the soil surface was significantly
drier under the early- compared with the late-sown wheat, suggesting more rapid
depletion of water.
3.3. Evapotranspiration
In 1990, the maximum rate of ET occurred for both early- and late-sown wheat at 88
DAFS (Fig. 4) and subsequently decreased with time as rainfall decreased and soil water
was depleted (Fig. 2). Similarly, the maximum rate of ET for all crops, except late-sown
J. Eastham et al. / Agricultural Water Management 42 (1999) 205±218
211
Fig. 3. Mean volumetric water contents of the surface soil beneath early- and late-sown crops for each ventilated
chamber measurement in (a) 1990 and (b) 1991. Bars indicate one standard error of the mean.
Fig. 4. Evapotranspiration from early- and late-sown crops in (a) 1990 and (b) 1991. Bars indicate one standard
error of the mean.
lupins, occurred at 90 DAFS in 1991, followed by subsequent decrease (immediately in
early-sown crops but after 118 DAFS in late-sown crops). ET was lowest for all crops on
the final day of measurement in each year.
212
J. Eastham et al. / Agricultural Water Management 42 (1999) 205±218
In 1991, when ET was measured for both wheat and lupin crops, there was generally no
significant difference in ET between either crop sown on the same date, despite
differences in their GAI caused by more rapid canopy development in wheat. At 90
DAFS in 1991, ET from both early-sown crops was greater than that from late-sown crops
by >1 mm dayÿ1 due to their larger canopies (Fig. 1), but for all other measurements in
both years there was no significant difference in ET despite differences in GAI. When
measurements from both seasons were grouped into high and low mean soil water
contents, analysis of variance indicated significantly greater ET from early crops when
soil water availability was high early in the season. There was no effect of time of sowing
on ET when soil water availability was low towards the end of each season. High and low
mean soil water contents were defined as water contents either greater than (60 and 88
DAFS for 1990, and 74, 90 and 104 DAFS for 1991) or less than was measured when the
early crop was sown.
3.4. Soil evaporation
In both years, E measured in ventilated chambers was highest for the first measurement
of the season when the surface soil was wettest (Fig. 5). Maximum E in 1990 was 1.0 mm
dayÿ1 from early-sown wheat and 1.4 mm dayÿ1 from late-sown wheat (Fig. 5(a)).
Maxima in 1991 were 0.8 mm dayÿ1 for both early-sown crops and 1.6 mm dayÿ1 for
late-sown crops. These differences between treatments were significant in both years. In
both seasons, E from each crop generally decreased with time as surface soil water
became depleted, and minimum values for E (0.1±0.4 mm dayÿ1) for all crops were
found at the last measurement.
Fig. 5. Evaporation from soil beneath early- and late-sown crops in (a) 1990 and (b) 1991. Bars indicate one
standard error of the mean.
213
J. Eastham et al. / Agricultural Water Management 42 (1999) 205±218
Table 1
Evaporation from lysimeters in cropped soil not enclosed by ventilated chambers (E), evaporation estimated by
the Cooper method (Ec), and water content of fallow soil (f)
DAFS
74
90
104
118
132
Evaporation (mm dayÿ1)
Early
E
Wheat
Ec
Late
E
Wheat
Ec
Early
E
Lupin
Ec
Late lupin
E
Ec
f
0.40
0.48
0.83
0.19
0.17
0.23
0.29
0.32
0.21
0.76
0.97
0.45
0.21
0.59
0.57
0.44
0.27
0.52
0.67
0.23
0.18
0.75
0.43
0.36
0.34
0.32
0.68
0.72
0.32
0.18
0.21
0.16
0.15
0.11
0.08
0.68
0.67
0.42
0.36
For the first measurement in both years, the greater GAI of the early crops (Fig. 1)
significantly reduced evaporation from soil compared with the late-sown crops
(Fig. 5(a,b)). In 1990, E for early wheat was reduced by 27% compared with the late
wheat at 60 DAFS, and in 1991 early sowing reduced E by 48% for wheat and by 54% for
lupin at 74 DAFS. By the second measurement and thereafter E from early- and late-sown
crops was similar despite differences in GAI (Fig. 1) because surface soil water contents
limited evaporation more than the availability of radiant energy. Measurements with
microlysimeters in uncropped soil at the site indicated that the transition from first to
second stage evaporation (Philip, 1957) occurred when surface water contents were
The influence of early sowing of wheat and lupin crops
on evapotranspiration and evaporation from the soil
surface in a Mediterranean climate
J. Easthama,*, P.J. Gregorya,1, D.R. Williamsonb, G.D. Watsonb
a
CSIRO Division of Plant Industry, Private Bag, P.O., Wembley, WA 6014, Australia
b
CSIRO Land and Water, Private Bag, P.O., Wembley WA 6014, Australia
Accepted 21 January 1999
Abstract
The losses of water by evapotranspiration and evaporation from soil were investigated during two
seasons from wheat and lupin crops sown at two times. Evapotranspiration was measured using
ventilated chambers and microlysimeters were used within the chambers to measure evaporation
from the soil surface. These techniques allowed the partitioning of evapotranspiration into its two
components. In the early part of the season, evaporation from the soil surface was greatest beneath
late-sown crops. Larger canopies, associated with early sowing, reduced evaporation during the
energy-dependent first stage. The greater losses beneath late-sown crops were not sustained as
surface soil water contents declined, decreasing the influence of canopy area on evaporation. Early
sowing may increase evapotranspiration early in the season and thereby decrease the risk of
drainage losses contributing to groundwater recharge. However, the magnitude of the hydrological
advantages from early sowing is likely to vary each year according to seasonal climatic conditions.
# 1999 Elsevier Science B.V. All rights reserved.
Keywords: Evapotranspiration; Soil evaporation; Crop management; Groundwater recharge
1. Introduction
A large proportion of the grain produced in Australia is grown in areas with a
Mediterranean climate characterised by hot, dry summers and cool, wet winters. Under
* Corresponding author. Present address: The University of Western Australia, Faculty of Agriculture,
Nedlands, WA 6907, Australia. Tel.: +61-89-380-2491; fax: +61-89-380-1108
E-mail address: jeastham@cyllene.uwa.edu.au (J. Eastham)
1
Department of Soil Science, The University of Reading, Whiteknights, P.O. Box 233, Reading, Berkshire,
RG6 6DW, UK.
0378-3774/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 0 3 6 - 0
206
J. Eastham et al. / Agricultural Water Management 42 (1999) 205±218
these conditions, crop growth is limited by water and yields are often positively related to
rainfall during the growing season. Moreover, 30±60% of the seasonal evapotranspiration
may be lost as evaporation from the soil surface (Siddique et al., 1990). This large loss
occurs because, during early winter, crops have low leaf area indices and the soil surface
is frequently wetted by rainfall. Evapotranspiration during this period is dominated by
soil evaporation (Yunusa et al., 1993a) but if this water could be transpired, growth and
grain yields of crops may be increased. Plant characteristics such as early vigour (Turner
and Nicolas, 1998) and management practices such as early sowing, increased fertiliser
input and planting density which increase early growth, have been shown to increase crop
yields through improved water use efficiency (Anderson, 1992; Anderson et al., 1992;
Connor et al., 1992). Additional benefits of increasing the early growth and water use of
crops may be derived through reducing deep drainage losses contributing to salinity and
waterlogging in the Western Australian wheatbelt. Greenwood et al. (1992) proposed early
sowing of agricultural crops as a strategy for increasing evapotranspiration from catchments.
Several approaches have been used to separate the evaporation and transpiration
components of evapotranspiration (Wallace, 1991). Some have measured the total
evapotranspiration and transpiration components (Azam-Ali, 1983; Wallace et al., 1990)
and obtained evaporation by difference, while others have used physically based process
models to estimate the components (Ritchie, 1972; Lascano et al., 1987). While evaporation from bare soil can be estimated with relative ease, root water uptake and modification
of the microclimate by canopies complicate determination of evaporation from soil beneath
crops. Direct measurements of soil evaporation have been made using small lysimeters
placed between rows (Boast and Robertson, 1982; Allen, 1990; Villalobos and Fereres, 1990;
Yunusa et al., 1993a, b) but the method is time-consuming and care must be taken to
ensure that the soil surface is undisturbed and representative of the surrounding soil. The
technique proposed by Cooper et al. (1983) for estimating evaporation from soil assumes
that the principal factor controlling evaporation from the soil surface beneath a crop is
the proportion of radiant energy reaching the soil. Evaporation can be estimated as the
product of the fraction of radiation transmitted through the crop canopy and evaporation
from a bare plot. Such measurements are relatively easy to obtain over long periods.
The present experiments formed part of a broader study to investigate the potential for
agronomic management to increase water use and yields of wheat and lupin crops and to
decrease groundwater recharge in the wheatbelt of Western Australia. Gregory and
Eastham (1996) describe the growth, radiation interception and yields of the crops. This
paper reports an investigation of the effects of early sowing on evapotranspiration and
evaporation from soil beneath wheat and lupin crops. A comparison is made between
direct measurement and predictive methods of estimating evaporation.
2. Materials and methods
2.1. Experimental site
The experiments were carried out in 1990 and 1991 at the East Beverley Research
Annex, approximately 100 km east of Perth in Western Australia (328080 S; 1178100 E).
J. Eastham et al. / Agricultural Water Management 42 (1999) 205±218
207
The climate is Mediterranean with a mean annual rainfall of approximately 380 mm. The
soil is a yellow duplex soil with a sand layer approximately 0.35 m deep overlying
kaolinitic clay (Northcote, Dy 2.82; USDA, Typic Natrixeralf); see Gregory and Eastham
(1996) for details.
2.2. Crop growth
Wheat (cv. Kulin) was sown on 25 May and 14 June 1990 and both wheat (cv. Kulin)
and lupins (cv. Gungurru) were sown on 31 May and 28 June 1991. Crops sown in May
are referred to as the early-sown crops, and those in June as the late-sown crops. The
experiment was planted in a randomised block design with six replicates, each plot being
10 60 m2. In both years, crops were sampled coincident with evapotranspiration
measurements from six adjacent 1 m rows in each plot. The area of green leaf and stem
was measured using a planimeter. Gregory and Eastham (1996) give further agronomic
details of the experiment and climatic data for the site.
2.3. Evapotranspiration
Evapotranspiration (ET) was measured using ventilated chambers in three replicates of
each treatment for four 48 h periods in 1990, and in two replicates for five 24 h periods in
1991. In 1990, measurements commenced on July 24, August 21, September 18 and
October 3, which were 60, 88, 116 and 130 days after the first sowing (DAFS)
respectively. In 1991, measurements began on August 13 and 29, September 10 and 29
and October 8 which were 74, 90, 104 118 and 132 DAFS, respectively. The design and
operation of the chambers are described in Farrington et al. (1992). The chambers were
5.25 m long, 3.0 m wide and 1.7 m high, and were made from clear high-density
polyethylene 200±250 mm thick which transmitted 78% of the incoming solar radiation.
Air was blown from an axial electric fan located at one end of the chamber and passed
through a baffle before moving horizontally across the chamber and venting through an
orifice at the other end of the chamber. The size of the orifice was adjusted to maintain a
chamber pressure of 0.3 mbar. Measurements with a pitot tube in the inlet fan duct gave a
mean air velocity of 0.3 m sÿ1. Samples of air from the inlet and outlet of the chamber
were pumped through insulated and heated lines and a heated homogenising container to
an infra-red gas analyser operating in the differential mode. The rate of evapotranspiration was calculated from the difference in vapour pressure between the air entering and
leaving the chamber. Air was sampled and analysed for approximately 6 min from each
chamber before switching to the next chamber. Evapotranspiration was expressed as a
daily rate (mm dayÿ1) by calculating the mean rate of evapotranspiration measured over
each 24 or 48 h period of sampling.
2.4. Evaporation measured by microlysimeter
Evaporation of water from the soil surface within each chamber was measured using
three microlysimeters containing undisturbed samples of soil. Lysimeters were made
from PVC drainage pipe and were 0.064 m in diameter and 0.15 m deep. In 1990,
208
J. Eastham et al. / Agricultural Water Management 42 (1999) 205±218
lysimeters were filled by forcing them into the soil just before each measurement. In
1991, 0.012 m diameter holes were drilled in the lysimeter walls (removing 10% of the
area) allowing roots to penetrate and take up water from within the lysimeter; these
lysimeters were installed between rows soon after crop emergence. Immediately before
use, the lysimeters were excavated, sealed at the base and in 1991 the sides were sealed
with adhesive tape. Lysimeters were weighed and then placed in sleeves in the ground
within each chamber with their surface at the same level as the surrounding soil. They
were re-weighed after 24 h to determine the water loss by evaporation. In 1991, the water
content of the upper 0.1 m of soil in each lysimeter was subsequently determined
gravimetrically by oven-drying. Gravimetric water content was converted to volumetric
water content assuming a mean bulk density of 1.64 t mÿ3, determined by oven-drying
eight undisturbed soil cores of known volume.
In 1991, measurements of evaporation from fallow soil and cropped soil not enclosed
by chambers were made for all but the first period of measurement to allow evaluation of
the Cooper method of estimating evaporation. A microlysimeter was installed in each of
the six replicate plots of the cropped soil (i.e. 6 microlysimeters per treatment), and four
microlysimeters were used for fallow soil.
2.5. Evaporation estimated by the Cooper method
In 1991, estimates of evaporation were made using the method of Cooper et al. (1983).
Evaporation from fallow soil (Ef) and within the chambers was measured concurrently
using microlysimeters, and evaporation from cropped soil (Ec) was estimated using:
Ec Ef 1 ÿ i
where i is the proportion of radiation intercepted by the crop (Cooper et al., 1983). Radiation
interception was recorded continuously in 1991 using Delta-T tube solarimeters in three
replicate plots of each treatment. Solarimeters were placed close to the soil surface across
six rows of each crop and the integrated reading recorded weekly. One solarimeter was
placed above the crop canopies to measure the incident radiation so that the fraction of
incident radiation intercepted by the canopy could be calculated. Radiation interception
data for each treatment is shown in detail in Gregory and Eastham (1996).
2.6. Soil water content
Soil volumetric water contents were measured in each plot using a neutron water meter
at sowing and on the same days as ventilated chamber measurements. Readings were
taken at 0.1 m depth intervals from 0.1 to 0.7 m and at 0.2 m intervals from 0.9 to 1.7 m.
Separate calibrations for the 0.1, 0.2 and >0.3 m depths were used to convert counts to
volumetric water content.
2.7. Statistical analysis
Student t-tests were undertaken to determine the significance of differences
between the treatment means. Evapotranspiration data were grouped into periods
J. Eastham et al. / Agricultural Water Management 42 (1999) 205±218
209
of high and low soil water content and analysis of variance was undertaken to
determine the significance of differences between treatments in early and late season
evapotranspiration. All differences reported in the text are significant at the 5% level
or greater.
3. Results
3.1. Canopy development
In both 1990 and 1991, time of sowing significantly influenced the size of the
crop canopies on each day of measurement (Fig. 1). Earlier canopy development
resulted in a greater green area index (GAI) for early-sown wheat for the first part
of both seasons (Fig. 1). However, senescence of early-sown wheat occurred sooner,
so that later in the season its GAI was reduced to less than that of the late-sown
wheat.
Vegetative growth of wheat was greater in 1990 than 1991 with a maximum GAI for
early-sown wheat of 2.2 in 1990 and 1.8 in 1991; comparable values for late-sown wheat
were 3.0 and 1.0. Canopy development of wheat was more rapid than that of lupins in
1991 so that GAI was greater than for the corresponding lupin crop until 104 DAFS.
Senescence of wheat occurred before that of lupin and by the final measurement in 1991
both lupin crops had a greater GAI than wheat crops. The maximum GAIs for early- and
late-sown lupins were 1.5 and 1.2 respectively.
Fig. 1. Green area indices of early- and late-sown crops for each ventilated chamber measurement in (a) 1990
and (b) 1991. Bars indicate one standard error of the mean.
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Fig. 2. Mean volumetric water contents of the upper 1.2 m of soil beneath early- and late-sown crops
for each ventilated chamber measurement in (a) 1990 and (b) 1991. Bars indicate one standard error of
the mean.
3.2. Soil water content
Ventilated chamber measurements commenced when soil water was close to its
maximum for the season following recharge of the profile by winter rainfall. Mean
volumetric water contents to a depth of 1.2 m decreased progressively with time beneath
all crops from the first to the last measurement in both 1990 and 1991 as soil water was
depleted (Fig. 2). At any given time in both seasons, there was no significant difference in
mean soil water content beneath early- or late-sown crops, or beneath either wheat or
lupin crops in 1991.
Fig. 3 shows surface soil water contents at 0.1 m measured by neutron moderation in
1990 and gravimetrically in 1991. In both years, surface water contents decreased
progressively with time under each crop from the first to the last measurement. In 1990,
surface water contents were similar beneath early- and late-sown wheat for each of the
four measurements. In 1991, water contents were similar beneath all treatments at 74 and
90 DAFS. However, for the final three measurements the soil surface was significantly
drier under the early- compared with the late-sown wheat, suggesting more rapid
depletion of water.
3.3. Evapotranspiration
In 1990, the maximum rate of ET occurred for both early- and late-sown wheat at 88
DAFS (Fig. 4) and subsequently decreased with time as rainfall decreased and soil water
was depleted (Fig. 2). Similarly, the maximum rate of ET for all crops, except late-sown
J. Eastham et al. / Agricultural Water Management 42 (1999) 205±218
211
Fig. 3. Mean volumetric water contents of the surface soil beneath early- and late-sown crops for each ventilated
chamber measurement in (a) 1990 and (b) 1991. Bars indicate one standard error of the mean.
Fig. 4. Evapotranspiration from early- and late-sown crops in (a) 1990 and (b) 1991. Bars indicate one standard
error of the mean.
lupins, occurred at 90 DAFS in 1991, followed by subsequent decrease (immediately in
early-sown crops but after 118 DAFS in late-sown crops). ET was lowest for all crops on
the final day of measurement in each year.
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In 1991, when ET was measured for both wheat and lupin crops, there was generally no
significant difference in ET between either crop sown on the same date, despite
differences in their GAI caused by more rapid canopy development in wheat. At 90
DAFS in 1991, ET from both early-sown crops was greater than that from late-sown crops
by >1 mm dayÿ1 due to their larger canopies (Fig. 1), but for all other measurements in
both years there was no significant difference in ET despite differences in GAI. When
measurements from both seasons were grouped into high and low mean soil water
contents, analysis of variance indicated significantly greater ET from early crops when
soil water availability was high early in the season. There was no effect of time of sowing
on ET when soil water availability was low towards the end of each season. High and low
mean soil water contents were defined as water contents either greater than (60 and 88
DAFS for 1990, and 74, 90 and 104 DAFS for 1991) or less than was measured when the
early crop was sown.
3.4. Soil evaporation
In both years, E measured in ventilated chambers was highest for the first measurement
of the season when the surface soil was wettest (Fig. 5). Maximum E in 1990 was 1.0 mm
dayÿ1 from early-sown wheat and 1.4 mm dayÿ1 from late-sown wheat (Fig. 5(a)).
Maxima in 1991 were 0.8 mm dayÿ1 for both early-sown crops and 1.6 mm dayÿ1 for
late-sown crops. These differences between treatments were significant in both years. In
both seasons, E from each crop generally decreased with time as surface soil water
became depleted, and minimum values for E (0.1±0.4 mm dayÿ1) for all crops were
found at the last measurement.
Fig. 5. Evaporation from soil beneath early- and late-sown crops in (a) 1990 and (b) 1991. Bars indicate one
standard error of the mean.
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Table 1
Evaporation from lysimeters in cropped soil not enclosed by ventilated chambers (E), evaporation estimated by
the Cooper method (Ec), and water content of fallow soil (f)
DAFS
74
90
104
118
132
Evaporation (mm dayÿ1)
Early
E
Wheat
Ec
Late
E
Wheat
Ec
Early
E
Lupin
Ec
Late lupin
E
Ec
f
0.40
0.48
0.83
0.19
0.17
0.23
0.29
0.32
0.21
0.76
0.97
0.45
0.21
0.59
0.57
0.44
0.27
0.52
0.67
0.23
0.18
0.75
0.43
0.36
0.34
0.32
0.68
0.72
0.32
0.18
0.21
0.16
0.15
0.11
0.08
0.68
0.67
0.42
0.36
For the first measurement in both years, the greater GAI of the early crops (Fig. 1)
significantly reduced evaporation from soil compared with the late-sown crops
(Fig. 5(a,b)). In 1990, E for early wheat was reduced by 27% compared with the late
wheat at 60 DAFS, and in 1991 early sowing reduced E by 48% for wheat and by 54% for
lupin at 74 DAFS. By the second measurement and thereafter E from early- and late-sown
crops was similar despite differences in GAI (Fig. 1) because surface soil water contents
limited evaporation more than the availability of radiant energy. Measurements with
microlysimeters in uncropped soil at the site indicated that the transition from first to
second stage evaporation (Philip, 1957) occurred when surface water contents were