The influence of early sowing of wheat and lupin crops
on evapotranspiration and evaporation from the soil
surface in a Mediterranean climate
J. Eastham
a,*, P.J. Gregory
a,1, D.R. Williamson
b, G.D. Watson
b aCSIRO Division of Plant Industry, Private Bag, P.O., Wembley, WA 6014, AustraliabCSIRO 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)
1Department of Soil Science, The University of Reading, Whiteknights, P.O. Box 233, Reading, Berkshire,
RG6 6DW, UK.
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 evapora-tion from bare soil can be estimated with relative ease, root water uptake and modificaevapora-tion 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 (328080S; 117
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
1060 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 evapotranspira-tion 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
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:
EcEf 1ÿi
whereiis 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
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.
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
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.
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.
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 dayÿ1due 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,Emeasured in ventilated chambers was highest for the first measurement
of the season when the surface soil was wettest (Fig. 5). MaximumEin 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 dayÿ1for both early-sown crops and 1.6 mm dayÿ1for
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.
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 reducedEby 48% for wheat and by 54% for
lupin at 74 DAFS. By the second measurement and thereafterEfrom 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 <0.15 m3mÿ3(Eastham and Gregory, 1994).
3.5. Estimation of evaporation using the Cooper method
Table 1 compares E from cropped soil not enclosed by ventilated chambers with
estimates obtained by the Cooper method (Ec). Ec was less than E from each crop
on 90 and 104 DAFS (except late-sown lupin on 90 DAFS) when surface soil water contents of fallow (Table 1) and cropped (Fig. 3) plots were similar. At 118 and 132 DAFS surface soil water contents in all treatments (Fig. 3) were significantly lower than in fallow plots (Table 1), and this led to evaporation being overestimated by the Cooper method.
4. Discussion
4.1. Evapotranspiration
Several studies in Western Australia have used ventilated chambers to compare ET from different crops, or to investigate the effects of crop management on ET. For example Farrington et al. (1992) compared ET from wheat, lupins, pasture and native plants using ventilated chambers. The range in ET measured over the season was similar to the values
reported here, but unlike our study ET from lupins (3.6 mm dayÿ1) was found to be
greater than from wheat and pasture (both 2.1 mm dayÿ1) for one of four periods of
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 Evaporation (mm dayÿ1)
Early Wheat Late Wheat Early Lupin Late lupin
E Ec E Ec E Ec E Ec f
74 0.40 0.75 0.21
measurement. Greater ET from lupins was attributed to their more advanced canopy development as they were sown 3 weeks earlier than wheat. This observation is consistent with the finding of our experiments in which ET early in the season was greatest for early-sown crops.
A similar conclusion of increased ET from early sown crops was found when ventilated chambers were used to compare ET from lupins planted at two different dates in 1988 at a site close to the experiments reported in this paper (Greenwood et al., 1992). ET was comparable to the range measured on similar dates in this study in 1991 (90, 104 and 118
DAFS), with mean ET of 2.49 and 2.16 mm dayÿ1 measured for early- and late-sown
lupins, respectively, between late August and late September.
4.2. Evaporation
Evaporative losses from soil under crops growing in Mediterranean climates have been investigated using different techniques. Several studies have used Cooper's method (Cooper et al., 1983) to estimate evaporation losses from soil (Allen, 1990; Siddique et al., 1990; Gregory et al., 1992; Yunusa et al., 1993b). Allen (1990) compared evaporation measured by lysimeters and estimated using the Cooper method in Syria, and concluded that agreement between the two methods was generally good. Gregory et al. (1992) found that Cooper's method overestimated evaporation beneath barley and wheat crops in Western Australia towards the end of the season when the water content of fallow plots was greater than that of cropped soil; this was also found in the present study. Yunusa et al. (1993b) found that during wet periods the Cooper method provided good agreement with evaporation measured by microlysimeter under wheat grown in a low rainfall zone in Western Australia, but under dry conditions the method underestimated soil evaporation. In summary, the Cooper method is reliable under wet conditions, but as
the soil driesEcan be either over- or under-estimated, depending on whether the surface
water content of bare soil is greater or less than that of cropped soil. For this medium rainfall zone of Western Australia, the Cooper method does not provide a good estimate of evaporation when the soil surface dries.
In the study of Greenwood et al. (1992),Ewas estimated as the difference between ET
and transpiration measured in lupins using the heat balance technique.Ewas found to be
inversely related to leaf area, and was generally greater thanEmeasured in this study.
These findings suggest that surface soil water contents were sufficiently high thatEwas
in the energy-dependent first stage throughout the measurements, whilst in our study this was true only for the first measurement. Thus the magnitude of the gains to be made by
early sowing to reduceEmay be expected to vary from season to season depending on
rainfall conditions, with greater savings possible in seasons characterised by frequent rainfall events.
Yunusa et al. (1993a) used lysimeters similar to those used in this study to measureE
beneath wheat at variable row spacing in Western Australia at a site drier than East
Beverley. They found that row spacing did not influenceEin the early season during the
conditions much of the evaporation was in the second stage, so E was unaffected by changing row spacing. Thus the findings of Yunusa et al. (1993a) that increased GAI later in the season are ineffective in reducingEare consistent with those of our study, and
point to the need for early increases in GAI to reduceE when soil water contents are
high andEis energy-dependent.
4.3. Limitations to ventilated chambers for measuring ET
The ventilated chamber was chosen in this study as a direct method of measuring ET because of the difficulty in estimating the lateral and vertical drainage components necessary for undertaking a water balance approach on the duplex soil. Micrometeor-ological methods such as eddy correlation and Bowen ratio techniques may have provided a more reliable estimate of ET, but were not applied because of limitations in equipment availability.
Although the ventilated chamber provides a direct method of measuring ET, there are several factors which limit its usefulness. The microclimate within chambers may differ from external conditions because the chamber may alter radiation, humidity, temperature and air flow. Some combinations of environmental and physiological factors could cause chamber transpiration rates to be affected by up to 100% (Denmead, 1984). Dunin et al. (1989) compared the ventilated chamber, infra-red thermometry and the Bowen ratio methods of measuring ET from a lupin crop in Western Australia. Dunin and Greenwood (1986) also evaluated the ventilated chamber for measuring ET from a forest by comparing it with measurements by the Bowen ratio method, soil water balance and weighing lysimeter. Both studies found that chamber measurements generally agreed well with other measurements because of the frequent occurrence of near-equilibrium evaporation conditions under which ET is independent of windspeed. Although chamber ET was found to be enhanced during periods of low ambient windspeeds in both studies, overestimation of ET was generally <5% for the lupin crop.
Under the climatic conditions that these experiments were undertaken, ET within the chambers may have been enhanced through an increase in the contribution of the aerodynamic component of ET. An analysis of the relative contribution of radiation and aerodynamic terms to ET from short vegetation growing in Western Australia under similar climatic conditions suggests that the aerodynamic term contributes 69% of ET (Stewart, 1984). The chambers can be useful to compare the potential water use (as determined by soil water availability and canopy size) for different crops, but this potential can only be achieved under conditions where the evaporative demand is similar to that imposed by the chamber.
Use of ventilated chambers to measure ET may result in a change in the partitioning
of ET between E and transpiration, because E measured within ventilated chambers
may either be suppressed or enhanced by excess or low chamber air pressures (Denmead, 1984). He reported that a pressure deficit of 1 mb in chambers resulted in a ten-fold increase in the rate of gaseous emissions from the soil (Denmead, 1979).
In this study, comparison of E both within (Fig. 5(b)) and outside chambers
(Table 1) suggests that the pressure of 0.3 mbar did not suppressE maintained within
4.4. Limitations to estimating seasonal ET using ventilated chambers
The scope to replicate chamber measurements both in space and in time is limited by the physical dimensions of the chamber which make it cumbersome to move, and by the intensity of measurements necessary to measure ET, making it expensive and time-consuming to run. Periodic measurements with ventilated chambers have been used to compare seasonal ET from different plant species in Western Australia to evaluate their potential contribution to groundwater recharge (Nulsen, 1984; Farrington et al., 1992; Scott and Sudmeyer, 1993). These studies used ventilated chambers to measure ET, with no spatial replication and with a minimal number of measurements (as few as 4) for each species over the season. There is considerable scope for error in extrapolating measure-ments made on small plots with limited replication to larger areas, and for interpolating through time without regard for changing soil water availability or evaporative demand. No estimates of seasonal ET or deep drainage loss for different treatments were undertaken in this study because of these limitations to the ventilated chamber technique.
5. Conclusions
In these experiments conducted over two seasons, greater losses of water by evaporation from soil were observed in both years in the early part of the season beneath late- compared with early-sown crops. More advanced canopy development associated
with early sowing reducedEduring the energy-dependent, first stage of evaporation. The
greater losses beneath late-sown crops were not sustained as surface soil water contents declined, decreasing the influence of canopy area on evaporation. Greater grain yields for early-sown wheat in 1990 and early-sown lupin in 1991 (Gregory and Eastham, 1996) were probably related to better water use efficiency due to less early season evaporation for early-sown crops (Anderson, 1992; Connor et al., 1992).
In both seasons, ET for early-sown crops was greater than that from late-sown crops when soil water availability was high. This suggests that early sowing can increase early season water use and thus is more likely to decrease recharge losses compared with late-sown crops. However, the size of the difference in water use between early- and late-late-sown crops may be less than that measured in this study because of the possibility of enhanced evaporative demand within the chambers. Seasonal variation in climatic conditions is likely to influence the size of the hydrological and yield advantages of early sowing. In particular, temperature and rainfall conditions early in the season are likely to have the greatest influence through their impact on early crop development and on the probability of large evaporative and drainage losses occurring within a season.
Acknowledgements
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