| When cotton is irrigated
a proportion of the water that infiltrates the soil moves too deep to
be used by the crop. This proportion of water is commonly referred to
as deep drainage and its volume and fate has become of particular interest
to the Australian Cotton industry.
Excessive volumes or volumes greater than what is required to keep salts
out of the root zone not only represent a lost economic opportunity
but may raise the water table and the reduce the quality of ground water.
Irrigators in St George participating in the Rural Water Use Efficiency
Initiative (RWUEI) recognised these potential issues and were keen to
find out how much deep drainage was occurring and what methods are available
to monitor it.
A trial was set up that compared four strategies for measuring deep
drainage. These strategies were:
• Water balance;
• Chloride mass balance;
• Continuous soil moisture monitoring at depth; and,
• Lysimetry.
By investigating the four strategies, a cross check for the measured
volume was achieved.
It is worth noting that the average annual deep drainage at this trial
site was estimated prior to the commencement of this demonstration using
EM surveys, soil sampling and SALF to be 21 mm with a range of 10 to
50 mm across the field. SALF is a computer model developed by Queensland
Department of Natural Resources and Mines (NRM) to predict long term
deep drainage rates based on soil properties that are easily determined.
Water balance
Water is removed from the soil by plants as they transpire and by evaporation
from the soil surface. These volumes are usually combined and referred
to as evapotranspiration or ET. The international standard for estimating
ET known as the Modified Penman Monteith Equation was used at this site.
This model calculates ET by estimating the environmental demand based
on the prevailing weather conditions and making adjustments according
to crop stage and vigour.
Irrigations occur when a predetermined amount of water has been removed
from the soil by ET and this volume of water is referred to as the irrigation
deficit. The irrigation deficit is therefore the target volume of irrigation
water that is required to enter the soil in order to refill the profile.
Any more is deep drainage and any less will stress the crop.
If you know how much water was supplied to the field in an irrigation
event and how much water ran off as tail water then the difference is
the volume that infiltrated the soil. So deep drainage can be calculated
as the difference between ET and the infiltrated volume and this is
known as the water balance method.
Sometimes ET can be replenished by rain. Effective rainfall is the proportion
of rain water that infiltrates the soil. In this cotton season all the
rain that fell was effective.
The field where the trial occurred was supplied either by irrigation
or rainfall with 12.8 megalitres per hectare and of this 4.4 megalitres
per hectare was collected as tail water. ET was estimated to be 7.8
megalitres per hectare. This leaves 0.6 megalitres per hectare unaccounted
for. Due to the position of the meters this 0.6 megalitres per hectare
includes deep drainage and evaporation and seepage losses in the head
ditch and tail drain. So using the water balance method it can be concluded
that deep drainage is less than 0.6 megalitres per hectare (or 60 mm).
Chloride mass balance
The chloride mass balance model uses the change in concentration of
chloride in the soil profile over time to estimate deep drainage. Because
chloride undergoes no chemical transformations in the soil, losses or
gains of chloride from the soil profile may be assumed to be due to
the movement of water containing dissolved chloride.
Changes in the total amount of chloride in the soil and the concentration
of chloride in water draining from the root zone over time are used
to calculate how much water (rain and/or irrigation) moved through the
soil profile during a specified time period.
Soil cores were taken before and after the irrigation season and analysed
for chloride. Water was sampled from the head ditch during each irrigation
to measure the chloride concentration in the irrigation water.
Table 1 presents the results that were produced by the Chloride mass
balance. Unfortunately, some soil was lost in the laboratory so the
only information about the chloride profile at this site at the start
of the cotton season was to 90 cm. As such the drainage past 120 cm
can only be estimated.
The data in Table 1 indicates that 90 per cent of ET was extracted from
the top 60 cm of the profile. There is a rapid drop off in drainage
after 60 cm which indicates the presence of root zone limitations in
this soil type. The drainage through the next two depths in the soil
profile are similar. This indicates that it is likely that the soil
below 90 cm is saturated throughout the season and draining.
Continuous soil moisture monitoring at depth
In cotton, soil moisture monitoring for irrigation scheduling commonly
occurs in the top 80 cm of the soil profile. But if soil moisture was
monitored at depth some conclusions can be drawn about the depth of
the roots and the occurrence of drainage.
So a C-Probe was installed to record soil moisture levels at every 10
cm depth between 100 and 150 cm on a continuous basis. The following
observations can be made about the graphical data displayed in Figure
1.
The graph can be divided into four periods. The first is between installation
and the last crop irrigation on January 30, 2003. During this time the
water content below 130 cm was gradually increasing while the water
content between 100 and 120 cm was gradually decreasing. This trend
continued through one rainfall event and three crop irrigations which
are numbered 2 to 4 in Figure 1.
The second stage commences with the last crop irrigation where deep
cracking allowed water to wet the soil to 130 cm. Some water use occurred
between 100 and 110 cm from this irrigation until harvest which is shown
by the vertical dash line at ‘7’.
The next two stages were a result of an effort to manipulate soil moisture
contents in the interest of developing a local C-probe calibration.
Firstly the cotton was left to grow and dry the soil out more thoroughly
than occurs during a fully irrigated crop. From Figure 1 it can be seen
that between picking and June 26, the cotton regrowth dried all depths
between 100 and 150 cm.
In the last section after point ‘10’ in Figure 1, the profile
was rewet by ponding the water to wet up a small plot of wheat for the
C-probe calibration. The site was wet again at point 11 when soil moisture
at 110 cm increased dramatically. Through this section there has been
a steady increase between 100 and 140 cm and by the time of cotton pre-irrigation,
all depths except 150 cm again appeared close to saturation.
The C-probe graph in Figure 1 confirms that there is an opportunity
for deep drainage during the cotton season as the lower parts of the
profile remain saturated. It also shows that it is possible to manipulate
soil moisture contents at depth as shown by the drying of the cotton
regrowth and the wetting by long inundation.
Lysimetry
A lysimeter captures the water draining over an area of soil and this
water is then pumped out and measured. The pump is designed to not apply
more suction than that being experienced by the soil water at that depth
so that water that would not usually drain is not pumped out.
This location was not an official NRM trial site but they generously
made available a lysimeter which was installed in the head ditch of
this field. The top of this lysimeter was 120 cm below the average soil
surface and its base was at 150 cm. Although extreme care is taken to
minimise the soil disturbance associated with installation there will
be a period where the soil will need to resettle around the lysimeter.
So the readings last season are only preliminary.
NRM did not record any drainage during the cotton season. Throughout
the cotton season there was never any water in the collection bottle
until 2.5 mm was captured after the wheat was watered for two days.
The C-probe indicated that the soil profile was wet to 150 cm as a result
of this irrigation event.
Conclusions
Table 2 compares the methods which were used at this site to estimate
deep drainage.
The water balance method estimated the highest deep drainage losses
but these include evaporation and seepage losses from the head ditch
and tail drain.
The chloride mass balance method and the SALF model produce similar
estimates of deep drainage. The chlorine mass balance method looks promising
‘because’ it seems quite accurate and is fairly easy to
implement.
The lysimeter gave the smallest estimates but it may take a season of
wetting and drying to settle in properly. The C-probe did not measure
the amount of deep drainage but clearly showed that deep drainage occurred
when the wheat was irrigated. It also showed that there was an opportunity
for drainage during the cotton season due to the saturated soil profile.
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