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Deep drainage myth-busters

VIEWS: 8 PAGES: 8

Deep drainage myth-busters

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									                      deepdrainage               COTTON CRC WATER TEAM



Deep drainage myth-busters
Emma Brotherton1,                   This article explores the following myths:
Graham Harris1, Peter
                                    Myth 1: When I irrigate throughout the season the soil seals,
Smith2 and David
                                    reducing infiltration - so how can I have Deep Drainage? – Page 2
Wigginton2
DPI&F1 and NSWDPI2
Background                          Myth 2: Groundwater tables have been falling over the last few
For many years it was believed      years, how can we have deep drainage? – Page 4
that deep drainage from furrow
irrigation was not an issue on
                                    Myth 3: My soil moisture measurement tool does not show a
the cracking clays
predominantly used to grow
                                    change in soil water levels at depth throughout the season – this
cotton in Australia. Results from   means deep drainage is not occurring. – Page 5
current Cotton Catchment
Communities CRC deep                Myth 4: My water storage does not leak so deep drainage is not an
drainage research projects and
                                    issue when I irrigate. – Page 7
evaluations of commercial
irrigations are challenging this
belief. At the same we often        Myth 5: I irrigate efficiently by maintaining high heads and pulling
hear statements providing           siphons as they come through so no deep drainage occurs. – Page 8
“evidence” that deep drainage is
not occurring when irrigating.

The Cotton CRC Water Team
sought the response of some of
the industry’s leading deep
drainage researchers on these
Deep Drainage “myths” -
Anthony Ringrose-Voase and
Richard Stirzaker (CSIRO),
Willem Vervoort (Sydney
University), Des McGarry, Mark
Silburn and Jenny Foley
(QNRW).




1
Myth 1: When I irrigate                   this is not true for the majority of cotton growing soils. For example, of the
                                          10 sites in a current Deep Drainage project (Project 1.02.04 ‘Deep drainage
throughout the season the soil            under irrigated cotton’ - Des McGarry), only 3 have a tendency to seal after
seals, reducing infiltration - so         rain or irrigation. Several soil properties act together to cause this sealing
how can I have Deep Drainage?             action. Soils with more than 45% clay and high sodium levels (as a
Once water enters the soil it will fill   proportion of the exchangeable cations) tend to seal as the sodium breaks
the soil profile in a non-uniform         up soil aggregates, when wetted by rain or irrigation. If fine sand is also
way, with some rapid preferential         present (more than about 20% of the soil mineral fraction) then the
flow down larger cracks and pores.        tendency to seal is even greater. Low organic matter levels add to the risk.
Vertosols, commonly used for              If you have soil with these properties, adding gypsum and/or organic
irrigated cotton production, are          matter (crop residues) may reduce the problem.
often considered to be non-
draining but in reality water will
drain beyond the root zone,
becoming potential deep drainage
(DD), both during the early stages
of irrigation due to preferential
flow, and in the later stages of
irrigation if too much water is
applied. Even the heaviest clay
soils still have water moving
through them, and infiltration is
never zero during irrigation. It
takes about 1 hour for water to
travel 1 metre down the profile of
an initially moist Black Vertosol
without large cracks. Drainage                           Installation of lysymeters to measure deep drainage
beyond the root zone may occur
                                        The decrease in DD as the season progresses was measured at almost all
within a few hours of the start of
                                        sites in all years of the project. However, this is not necessarily because
irrigation.
                                        water infiltration lessens or because the soil begins to seal over. Rather, as
                                        the cotton season progresses, the above-ground biomass and the plant
In heavy clay soils the infiltration
                                        root system of the cotton crop develop and grow larger, and the day time
rate decreases as the soil wets up
                                        temperatures greatly increase. The cumulative effect is greatly increased
due to swelling of the clay particles.
                                        evapotranspiration (water demand). Though the irrigation frequency
It is important to separate
                                        increases to match the increased evapotranspiration (if water is available),
infiltration (the water moving into
                                        the crop demand is so large and daytime temperatures so high that almost
the soil from the top) from DD (the
                                        all the infiltrated water is rapidly used. SIRMOD analysis, measured across 4
movement of water beyond the
                                        of the current DD trials clearly shows decreased infiltrated depths of
reach of crop roots). The two may
                                        irrigation water during the last few irrigation events of the season (see 4th &
be related; DD often occurs much
                                        5th irrigations in Figure 1). Data from the current DD trials show that,
later and continues much longer
                                        commonly from the 3rd or 4th in-crop irrigation, there is no measured DD.
than infiltration as water slowly
moves through the profile. DD is
                                        Some soils used for irrigation do exhibit this “sealing up during the season”
generally much less than
                                        behaviour, causing infiltration rate per irrigation to decline through the
infiltration as part (hopefully most)
                                        season and difficulties with subbing-up. A common management
of the infiltrating water is stored for
                                        response is to run furrow irrigation for longer (sometimes much longer),
use by plants.
                                        sometimes generating a large volume of tail-water. However, you still
                                        won’t know for sure that DD is not occurring, especially in the first
Some irrigated soils under cotton
                                        irrigation or in irrigations with long run times, unless you do in-field
do have a tendency to seal (close
                                        measurements. It is likely that deep drainage does occur but it may be less
over or form a surface crust) but
                                        than for soils with higher infiltration rates.


2
.


                             200
                                   (b) 2004-05 (Cotton)                                       Pre-irrigation (170;155;135 mm)
                                                                                              1st (167;152;73 mm)
    Infiltrated depth (mm)




                             150                                                              2nd (155;130;0 mm)
                                                                                              3rd (59;59;57 mm)
                                                                                              4th (58;58;56 mm)
                             100                                                              5th (35;36;36 mm)


                             50

                                      Head                Mid                       Tail
                              0
                                    0 100 200 300 400 500 600 700 800 900

                                  Distance down furrow (m)
Figure 1 Infiltration uniformity as simulated from SIRMOD analysis down the furrow on the Goondiwindi site for 6 irrigation events in the
2004-5 cotton season. The modelled amounts of water infiltrated (in millimetres) at each of head, mid and tail locations are shown (in
brackets) in the legend (Source: T. Gunawardena, NRW 2008)

.




                                                                Surface irrigation event




3
Myth 2: Groundwater tables have been falling over the last few years, how can we have deep
drainage?
Soils drain and recharge aquifers. Aquifers in turn can discharge to rivers. Most irrigation areas have at least two
aquifer systems. The top aquifer is a water table aquifer, and the quality of this aquifer is often marginal to saline.
This can be easily derived from the bore logs in the areas. For example, in the Namoi and Gwydir valleys this
upper aquifer is called the Narrabri formation. In contrast, most groundwater for irrigation is extracted from the
deeper aquifer, which are often much better in quality. In the Namoi and Gwydir valleys this is the Gunnedah
formation. The groundwater levels in the deeper aquifers have been falling due to extraction. We don’t know
exactly what is happening in the upper aquifer (Narrabri formation) as very few measurements are taken in this
aquifer but recent research (Project 1.02.07 ‘Hydrological and geophysical characterisation of Palaeochannels in
northern NSW’ - Chris Vanags) indicates that the upper aquifer (Narrabri formation) responds rapidly to rainfall
and irrigation.




Figure 2. Average daily water levels for the piezometers in the Narrabri formation for 2005 (irrigated) and 2006 (not irrigated) in a field
containing a palaeochannel. Plotted data are calculated as the average from the 15 minute observed data over 24 hours. Black bars are
recorded rainfall in cm, while the grey bars are irrigation events of approximately 1 ML ha-1. Over the course of the 2005 irrigation season,
the water table rose 14 times in the wells inside and below the palæochannel on the SE side (piezometers 2, 3, 5 and 6) (Vanags et al.
submitted to Australian Journal of Soil Research).

In addition, groundwater levels decline when the natural outflows plus pumping volumes exceed the inflows
plus recharge. Recharge can come from rainfall, irrigation fields and structures, and from rivers and floods,
depending on where you are. Thus there could indeed be deep drainage on your farm and falling groundwater
levels if groundwater use/outflow exceeds inflows and recharge. The response of water tables to recharge from
deep drainage is complicated as there can be a prolonged delay between drainage occurring and recharge to a
water table, and this recharge may not be from directly above. If the aquifer was a “closed bucket” under a farm
which was entirely irrigated from that aquifer, and 5 ML/ha was pumped for irrigation and 10% went to deep
drainage (0.5 ML/ha) each year, then outputs exceed inputs by 4.5 ML/ha and the water level in the aquifer
would fall over time. If the aquifer held only 10% water (the rest being solids and water that can’t be pumped),
the water table would be lowered by 4.5m each year even though deep drainage is occurring!


4
Myth 3: My soil moisture                 probes, soil coring, neutron meters). The first type of device is very useful
                                         for indicating whether the soil is “wet” and therefore draining i.e. when the
measurement tool does not                soil water potential is between 0-0.1 bar (0-10 kPa). They give a very good
show a change in soil water              idea of when and for how long drainage occurs, but not how much has
levels at depth throughout the           drained.
season – this means deep
                                        The second type of device measures the amount of water held in the soil,
drainage is not occurring.              however this does not, in itself, tell the irrigator if the soil is “wet” or “dry”.
This comment is based on the
                                        Generally, a dry soil below drained upper limit (DUL) is not draining,
assumption that little change in soil
                                        whereas a wet soil above DUL is draining. Soil water content
moisture measurements over a
                                        measurements alone will not tell the irrigator this, and related soil
period of time shows that water is
                                        information is needed to interpret the measurements. Note: if a device is
not moving past the measured
                                        measuring constant soil water content over time and the soil is above DUL,
depth. Water movement is driven
                                        then that soil is draining, and at a constant rate.
by energy differences rather than
by water content. The energy
                                        Water content in heavy clay soils may vary only a few percent (2-4%)
differences are determined by two
                                        between saturation and DUL i.e. from maximum drainage rates to no
components, one is the soil
                                        drainage. Such a small change may be overlooked, or lost within the error
moisture (wetter soil is higher in
                                        component of the device. Even when such a change is detected, its
energy than drier soil), the other is
                                        meaning is difficult to interpret because soil moisture content does not
gravity (water will more easily
                                        indicate the absolute moisture status of the soil. Soils can be saturated at
move down than move up). So
                                        moisture contents of 0.4 to 0.6 v/v (the water volume is 40% to 60% of the
even if the soil moisture difference
                                        soil volume), depending on their bulk density. Thus a soil moisture
is negligible (and thus the energy
                                        content of 0.35-0.40 v/v would be approaching saturation (and drainage
difference is negligible) the gravity
                                        would be occurring) in a dense soil (e.g. a Grey Vertosol) but would be near
can still drive water movement.
                                        wilting point in a swelling, high clay, Black Vertosol.
And this would have very little
effect on the soil moisture content.
This means deep drainage may be
occurring even if the soil moisture
measurements do not change over
time. The lost water is replaced
with water from above. At depth
the subsoil is near saturation so it is
not possible to detect any increase
in soil moisture with deep
drainage.

Soil moisture monitoring devices
don’t measure drainage.

Depending on the type of device,
they may indicate if drainage is
likely to be occurring. There are
two types of devices, those that
                                                                      Neutron moisture meter
measure the pressure (energy) at
which the water is held within the
soil (tensiometers, gypsum blocks,
Watermarks) and those that
measure the water content of the
soil (capacitance devices, TDR



5
Other reasons why soil moisture monitoring tools (like capacitance and neutron probes) may give poor insight
into DD include:
    •    The output from these devices shows change in soil water content (between times). That the line does
not change, at say 100 cm depth, throughout a season, shows only that there has been no change in soil water
content at that depth. However, the water at that depth may well be flowing through the soil, by-passing the
root zone as DD. This bypass flow can occur down cracks or old root holes and will not be picked up by a soil
moisture monitoring tool.
    •    Also, data from Des McGarry’s DD project shows large variation in DD within any one paddock as well as
between seasons, and certainly during the season. The largest values of DD measured have been early in the
cotton season at the pre- and first-irrigations. Most soil moisture monitoring tools are only installed when the
crop emerges – to ensure proximity of healthy plants to the probe. In this way, the early season events (with
large water application at the pre-irrigation, low temperatures and small transpirational demand) that
contribute greatly to DD are missed.
    •    Depending where the soil moisture monitoring tool is located, it may or may not show change (at
depth) in soil water content. Unless moisture probes are installed at head ditch, middle of the paddock and tail
ditch locations, and monitored throughout the season, including before and after pre-irrigations, it is not
reasonable to conclude that there is no DD. Unlike soil moisture monitoring tools, the device used in the Des
McGarry’s DD project to provide DD data is a drainage lysimeter. This is not a soil water content measuring
device - rather it continuously collects the actual DD water arriving at 150 cm depth. The amounts collected are
recorded electronically and the actual DD water is collected routinely (for salinity assessment) during site visits




                                 A drainage lysimeter setup to measure deep drainage
.




6
Myth 4: My water storage does
not leak so deep drainage is not
an issue when I irrigate.
This statement assumes that if a
storage does not leak then soils
irrigated nearby will not lose water
through deep drainage. Firstly, all
storages leak to some degree, and
the greater the head in the storage,
the greater the leakage. Secondly,
a properly constructed storage with
the right level of compaction and
construction material is quite
different from a field being
irrigated – it is not sensible to
compare drainage losses from a
storage with those from an
irrigated field.

A complication is that the water
level in storages falls from
pumping, evaporation and
drainage. Without accurate storage
measurement, it is not possible to        Storages can be evaluated for seepage using evaporation/seepage meter
ascertain how much is due to
evaporation and seepage loses.
And if it is leaking at less than a few
mm per day, it would be difficult to
separate seepage from
evaporation. So how can you be
confident that your storage is not
leaking?

In addition, similar to irrigated
fields, storages are known to be
highly variable in terms of their
“leakiness” and where it may occur.
In leaky storages, the whole floor of
the storage probably does not leak
equally. Sand lenses just under the
storage floor may give a leakage
“hotspot”.




                                                           Installation of drainage lysimeter




7
Myth 5: I irrigate efficiently by       irrigation are well over the required depth contributing to deep-drainage.
                                        In conclusion, a general result from the current project is that around 70%
maintaining high heads and              of measured DD can be attributed to the pre-irrigation and the first two in-
pulling siphons as they come            crop irrigations.
through so no deep drainage
occurs.                                 An examination of 79 furrow irrigation evaluations in Queensland cotton
                                        fields by Rod Smith, Steve Raine and John Minkevich found:
This statement poses two
questions:                                   •    Irrigation application efficiencies varied widely from 17-100% with
1. How did you measure your             an average of 48%,
     efficiency?                             •    Deep percolation (drainage) losses averaged 42.5 mm per
2. How do you compare with              irrigation, representing an annual loss of up to 2.5 ML/ha,
     other irrigators?                       •    Irrigation application efficiencies in the range of 85-95% were
Without measuring irrigation            achievable by optimising furrow irrigation in all but the most adverse
performance of a field it is not        conditions.
possible to know the efficiency
with which it is being irrigated.       Efficient furrow irrigation is possible by fine tuning your irrigation
Rule-of-thumb irrigation                applications to your soils and fields using a surface irrigation evaluation.
management strategies applied
without measurement may be              One of the key principles of efficient irrigation is that extra water is applied
efficient - but you don’t know for      to leach salts which are inevitably added with irrigation water and
sure.                                   fertilisers. If no DD occurs, salinisation of the soil would occur. However,
                                        DD during rainfall events on irrigated fields will generally provide enough
Two indicators can be used to           leaching, unless the irrigation water is particularly saline. While some DD
measure the efficiency of an            will always occur under irrigated fields the key is to make sure it does not
irrigation event: Application           occur due to irrigation to avoid the cumulative effect of degrading the
efficiency (Ea - the percentage of      environment at both the local and catchment scales.
water applied that infiltrates into
the soil) and Distribution              Conclusion
Uniformity (DU - the uniformity of      Deep drainage is a reality in furrow irrigated fields within our cotton-grain
infiltrated depth down the              farming systems. Some deep drainage is necessary to remove salts from
paddock from head ditch to tail         the root zone of our irrigated crops. The extent of deep drainage is
drain). These are illustrated in the    dependent upon the soil type and uniformity and the condition of the field
data given in Figure 1. At the first    being irrigated, the irrigation management practices being used and the
irrigation, the Ea was 54% with a       seasonal conditions experienced. It is not something that necessarily
DU of 71%. That is 54% of the           happens with every irrigation event and the magnitude of it can be
irrigation water applied actually       managed through improved irrigation practices – many of which are now
infiltrated, so was available for the   being implemented within the industry.
crop to use. By the 5th irrigation
event, however, Ea was only 35%
showing less water available for        Acknowledgements
crop use (from drier soil at time of    This article is a synthesis of responses to the five deep drainage myths by
irrigation, from large                  Dr Jenny Foley, Dr Des McGarry, Thusitha Gunawardena and Dr Mark
evapotranspiration mid-season, as       Silburn, NRW; Dr Anthony Ringrose-Voase and Dr Richard Stirzaker, CSIRO;
well as less water applied) but DU      and Dr Willem Vervoort, Sydney University . We thank them for their
was 99%, showing an almost              valuable input based on the many years of deep drainage research they
uniform distribution down the           have been undertaking through funding from the CRDC, Cotton
paddock (though with far less           Catchment Communities CRC and the CRC for Irrigation Futures.
water than at the start of the
season). In the same figure,            Thank you to Graham Harris and Janelle Montgomery for kindly supplying
infiltrated depths at all head, mid     the photographs used in this article.
and tail locations for the pre-



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