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    water use efficiency
    fact sheet
                                                                                                                                                         2009


                            nORTHERn REGIOn
                            Converting rainfall to grain
                            Increasing the amount of water stored in fallows is an important strategy in managing the
                            risks associated with highly variable rainfall in the northern region to improve water use
                            efficiency and potential crop yields.

                                                                       What is water use                                 proportion of rainfall that crops use,
                                                                                                                         and minimise water lost through
                                                                       efficiency?                                       runoff, drainage and evaporation
                                                                                                                         from the soil surface and to weeds.
                                                                       Water use efficiency (WUE) is the
                                                                       measure of a cropping system’s                    Rainfall is more summer dominant
                                                                       capacity to convert water into plant              in the northern region, and both
                                                                       biomass or grain. It includes both the            summer and winter crops are
                                                                       use of water stored in the soil and               grown. However, rainfall is highly
                                                                       rainfall during the growing season.               variable and can range, during each
                                                                                                                         cropping season, from little or no
                                                                       Water use efficiency relies on:                   rain to major rain events that result
                                                                       ■ the soil’s ability to capture and store         in waterlogging or flooding.
                                                                         water;
                                                                       ■ the crop’s ability to access water              Storing water in fallows between
                                                                         stored in the soil and rainfall during          crops is the most effective tool
                                                                         the season;                                     growers have to manage the risk
                                                                       ■ the crop’s ability to convert water             of rainfall variability, as in-season
                                                                         into biomass; and                               rainfall alone – in either summer or
                                                                       ■ the crop’s ability to convert biomass           winter – is rarely enough to produce
                                                                         into grain (harvest index).                     a profitable crop, especially given
                                                                                                                         high levels of plant transpiration and
      PHOTO: EmmA LEOnARd




                                                                       Water is the principal limiting factor            evaporation.
                                                                       in rain-fed cropping systems in
                                                                       northern Australia.                               Fortunately, many cropping soils
                                                                                                                         in the northern region have the
                                                                       The objective of rain-fed cropping                capacity to store large amounts of
                                                                       systems is to maximise the                        water during the fallow.


                             Key POints                                As northern cropping systems consist of a sequence of fallows
                                                                       and crops, three components of water use efficiency (WUE) are
                             ■ fallow management, including            important:
                                the sequencing of fallows
                                                                       Fallow efficiency:   he efficiency with which rainfall during a fallow period is stored
                                                                                           t
                                and crops, offers the greatest
                                                                                           for use by the following crop.
                                opportunity to improve water
                                availability to plants and increase    Fallow efficiency (%) = change in plant available water during the fallow x 100
                                water use efficiency in the
                                northern region.                                                                   fallow rainfall (mm)
                             ■ a realistic understanding of the        Crop water use efficiency:   he efficiency with which an individual crop converts
                                                                                                   t
                                soil’s capacity to store water and                                 water transpired (or used) to grain.
                                assess available water prior to
                                planting will help identify planting   Crop WUE (kg/ha/mm) =                       grain yield (kg/ha)
                                opportunities and potential crop
                                yields.
                                                                                                   crop water supply (mm) – soil evaporation

                             ■ Good agronomic practices that           Systems water use efficiency:   he efficiency with which rainfall is converted to
                                                                                                      t
                                maximise plant health and growth                                      grain over multiple crop and fallow phases.
                                will allow crops to use water
                                more effectively, improving yield
                                                                       SWUE (kg grain/mm rainfall) =               total grain yield (kg)
                                potential.                                                                           total rainfall (mm)


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                                                                          PAGE 2




 An understanding of soils
 and available water can help
 growers take advantage of
 opportunities for continuous
 cropping.




                                                                                                                                                              PHOTO: REBECCA THyER
 Table 1 Key beST pracTiceS for maximiSing The converSion of rainfall inTo grain
 Practice                                                                      Impact
                                                                               Maintain high cover levels, maximise water infiltration and minimise
 Zero tillage                                                                  evaporation and soil loss, extend planting windows

 Crop rotation                                                                 Regularly include crops that produce high stubble cover, for example
                                                                               cereals on narrow row spacing or a summer cover crop
 Controlled traffic farming                                                    Reduced soil compaction maximises water infiltration and plant growth
 Opportunity cropping                                                          Capitalise on planting opportunities and minimise fallow length
 Know your plant available water (PAW) and plant available
 water capacity (PAWC)                                                         Assist in determining yield potential

 Assess yield potential and likely crop profitability at planting              Assist in making the decision whether to plant now or wait until the next
                                                                               opportunity
                                                                               Enable planting of adequate areas when opportunities arise – requires
 Invest in a planter with the required capabilities and capacity               capability to handle stubble, moisture seek, achieve good establishment in
                                                                               less than optimal conditions and apply required fertilisers at planting
 Manage weeds in fallow and crop                                               Maximise fallow efficiencies and minimise in-crop competition for water
 Good management of other aspects of crop agronomy                             Maximise crop water use efficiency

Soils and stored moisture                                    are also able to access deeper                  FIGURE 1 A TYPICAL STORAGE
                                                             subsoil moisture.                               PROFILE FOR A HEAVY-TEXTURED
Plant available water                                    ■ Soil chemistry affects crops                      SOIL SHOWING THE POTENTIAL
capacity                                                     differently – for example the                   WATER STORAGE OF THE SOIL
Plant available water capacity                               presence of chlorides reduces                   (PAWC) AS DEFINED BY THE
(PAWC) is the total amount of water                          PAWC for chickpeas to a greater                 DRAINED UPPER LIMIT (DUL),
a soil can hold that a particular crop                       extent than for wheat.                          CROP LOWER LIMIT (CLL) FOR AN
can extract from a particular soil.                      ■ The PAWC of cropping soils in the                 INDIVIDUAL CROP, ie WHEAT, AND
PAWC is related to the soil type and                         northern region can vary between                SATURATION (SAT)
the crop being grown on that soil.                           100 and 350 millimetres.                        Depth (cm)
PAWC is less than the total water                        ■ Knowledge of PAWC, and in                          0
held in a saturated soil (Figure 1)                          particular CLL, is necessary to
and varies between crops grown on                            calculate available soil water at               30
the same soil type.                                          planting from soil sample results.
                                                             not all water stored in the soil will           60
It is defined by a soil’s drained upper                      be available to plants.
limit (dUL) (the water content of a soil
when it is fully wet but drainage has                    ■ Information on 500 different soil                 90
ceased) and its crop lower limit (CLL)                       types and their PAWC is available
(the water content of a soil when a                          from the APSoil database                       120
crop has extracted as much water                             (www.apsim.info) and Australian
as it can). not all soils are capable of                     Soil Resource Information
                                                             System (ASRIS) website                         150
storing the same amount of water, or
of releasing stored water to plants.                         (www.asris.csiro.au/index_other.
                                                             html). Geospatial location of the              180
■ Some crops are more efficient                              soils can be viewed and data                      10        20      30      40       50     60
   than others at extracting soil                            downloaded using the Google                                      Volumetric water (%)
   moisture; crops with deeper roots                         Earth link available at these sites.                      CLL    DUL      SAT
                                                          PAGE 3




 Compaction is a soil
 constraint that reduces water
 infiltration and the crop’s
 ability to extract water. It
 can be addressed with the
 introduction of controlled-
 traffic farming systems.




                                                                                                                                       PHOTO: EmmA LEOnARd
Available soil moisture                      region, although this value can          FIGURE 2 YIELD DISTRIBUTION
                                             vary between 0 and 50 per cent,                   FOR SORGHUM AT
Plant available water (PAW) is the           depending on rainfall patterns,
amount of water in the soil actually                                                           DALBY, PLANTED 15 SEPT,
                                             fallow length, weed control
available for crop production – often        and other fallow management
                                                                                               240 MM PAW SOIL
far less than PAWC, given rainfall                                                    Grain yield (kg/ha)
                                             practices (see page 4, Improving         11000
and moisture extracted by previous           water capture and storage).
crops. It is also less than total water                                               10000
stored in the soil, as various soil
physical and chemical constraints         Estimating yield potential                   9000
will prevent plants accessing all soil                                                 8000
                                          With an assessment of stored soil
moisture. A number of methods can
                                          moisture prior to planting, growers          7000
be used to estimate PAW:
                                          can estimate potential yields for their
                                          crop, and therefore potential return         6000
■ soil cores can be taken and
   dried to determine water content       on their investment, although the high       5000
   at various depths and PAW              degree of variability in rainfall during
                                                                                       4000
   calculated using a knowledge of        the growing season makes it difficult
   CLL for that soil (see Soil matters    to accurately predict crop yields (see       3000
   under ‘Useful resources’, page 6);     page 4, the French-Schultz approach).        2000
■ a simple push probe can be used                                                      1000
   to determine depth of wet soil.
                                          Crop simulation models
                                                                                           0
   As a rule of thumb, vertosols          more sophisticated crop simulation                     80            160          240
   hold around 2mm of PAW per             models such as APSIm, and its                               PAWC at planting (mm)
   centimetre of wet soil, though         commercial derivatives yield                                                 SOURCE: QPI&F
   this factor will vary with soil and    Prophet® and WhopperCropper®, can
   crop type. Conversion factors          also combine detailed data about            Note on reading box plots - solid line is
   for a range of crops and soils are     soil types, PAWC, soil moisture at          median yield, dotted line is average yield,
   available (see Soil matters under      planting, long-term climate data and        bottom of box is yield exceeded in 75% of
   ‘Useful resources’, page 6);           current weather information to help         years, top of box is yield exceeded in 25% of
                                          assess potential crop yields based          years, bottom whisker is yield exceeded in 95
■ the software program ‘HowWet’                                                       % of years, top whisker is yield exceeded in
                                          on available moisture and additional
   can be used to estimate PAW            inputs such as fertiliser (Figure 2).       5% of years
   during a fallow period from daily
   rainfall figures input by the user     This can provide a more accurate
   (www.apsim.info); and                  cost-benefit analysis for management
                                          decisions and allow growers to test
■ PAW at the end of a fallow can
                                          different management scenarios
   be estimated directly from fallow      before putting them in place.
   rainfall using an estimate of fallow
   efficiency (FE). FE averages           Given rainfall variability, soil moisture
   around 20 to 25 per cent for           at planting is the most critical factor
   ‘normal’ fallows in the northern       in deciding to plant a crop.
                                                              PAGE 4




Improving water capture                   crops are low (for example, after                        Improving crop WUE
and storage (or improving                 a skip row sorghum crop), cover
                                          crops can improve surface cover                          The healthier a crop is the more
fallow efficiency)                        and fallow efficiency. A cover crop                      able it is to extract and use water
Soil texture, structure, organic matter   is grown for a short period of time                      from the soil. All aspects of good
and beneficial microbial activity         (three to six weeks) and then sprayed                    agronomy – such as timeliness of
all contribute to the soil’s ability to   out to provide extra surface cover                       planting, weed control, appropriate
capture water and store it for use by     while minimising the amount of water                     nutrition, and control of pests and
plants, to provide nutrients to crops,    used to grow the cover crop.                             diseases – help a crop capture and
and allow plants to develop stronger                                                               use as much of the seasonal water
root systems.                             Research as part of the Central                          supply as possible. Weed control
                                          Queensland Sustainable Farming                           before and during the cropping
If left uncontrolled, weeds will use      Systems project indicates that                           season is important to reduce
water from the deep subsoil – water       longer fallows are generally less                        competition for moisture. Between-
that can be extremely valuable            efficient at storing water than shorter                  season management of weeds
to crops during a dry finish to           fallows because more water is lost                       can also reduce the risk of pests
the growing season. Research at           as evaporation, runoff and deep                          and disease carried over from the
Emerald, Queensland, indicates            drainage.                                                previous season.
that during a short fallow weeds
can use up to 90mm of soil moisture
(Figure 3).                               FIGURE 3 WATER ACCUMULATION UNDER CLEAN AND WEEDY FALLOWS –
                                                   THE MEAN OF SIX FALLOWS BETWEEN 2003 AND 2005
Reducing tillage and introducing                   AT EMERALD, QLD
controlled traffic systems can help to    Fallow water accumulation & rainfall (mm)
maintain and improve soil structure,      400
improve water infiltration and reduce
water runoff (and soil loss).             350                                                              371
                                                            341
Reduced tillage or zero till systems      300
also maximise surface stubble             250
cover, which reduces the rate of
evaporation and protects the surface      200
soil from the compacting impact
of raindrops. mulch that keeps the        150
topsoil wet for longer will enable                                                 133
                                          100                                                                               114
follow-up rain to penetrate deeper                                      104
into the soil and also extends             50
planting opportunities.                     0                                                                       23
                                                                      February 1                                   May 1
In some situations, for example at
                                                                                                Date
the beginning of long fallow periods
when surface residues from previous                 Fallow rainfall     Weedy fallow       Clean fallow                           SOURCE: QPI&F




The French-Schultz approach

In southern Australia the French-         evaporation and soil water remaining                      crop’s agronomy or a major limitation
Schulz model is widely used to            at harvest is difficult. However, this                    in the environment. There could be
provide growers with a benchmark          model may still provide a guide to                        hidden problems in the soil such
of potential crop yield based on          crop yield potential.                                     as root diseases, or soil constraints
available soil moisture and likely in-                                                              affecting yields. Alternatively,
                                          The French-Schultz model has been
crop rainfall.                                                                                      apparent underperformance could
                                          useful in giving growers performance
                                                                                                    be simply due to seasonal rainfall
In this model, potential crop             benchmarks – where yields fall well
                                                                                                    distribution patterns which are
yield is estimated as:                    below these benchmarks it may
                                                                                                    beyond the grower’s control.
Potential yield (kg/ha) =                 indicate something wrong with the
WUE (kg/ha/mm) x (Crop water
supply (mm) – estimate of soil
evaporation (mm))                           Typical parameTerS ThaT could be uSed in ThiS equaTion are:
(where crop water supply is an              Crop                                         WUE (kg/ha/mm)           Soil evaporation (mm)
estimate of water available to the
crop, that is, soil water at planting       wheat                                             18                           100
plus in-crop rainfall minus soil water      Chickpea                                          12                           100
remaining at harvest).                      Sorghum                                           25                           150
In the highly variable rainfall             Note – these parameters will vary with location, management and season are a guide only.
environment in the northern region,         WUE values listed are those that could be achieved in the most favourable seasons.
estimating in-crop rainfall, soil
                                                          PAGE 5
                                                          PAGE 5




 Early planting with
 appropriate nutrition and
 effective control of root
 diseases is key to improving
 water use efficiency. The
 use of no-till and stubble
 retention helps improve
 rainfall capture and retention.




Planting
■ Select varieties best suited to          effecting early sown crops, versus        Canopy management aims to improve
   local conditions, including soil        the potential yield increase.             the balance between water use before
   types, climatic conditions and                                                    and after flowering, and can improve a
   with resistance to likely pests and                                               crop’s harvest index. This is generally
   diseases, including root diseases,
                                           Improving conversion of                   achieved by limiting canopy growth
   to increase the likelihood of the       biomass to grain                          early in the season, which reduces
   crop thriving and making best use       Low WUE can be the result of a crop       early water use, and makes more
   of available soil moisture.             failing to convert biomass into grain.    water available during the critical
                                                                                     stages of flowering and grain filling.
■ For winter crops, earlier planting       distribution of rainfall can also
   improves the conversion of water        reduce WUE. Rain falling at flowering
   into biomass and grain because it       or early grain-fill can make a            Canopy management
   allows the crop to mature in more       significant contribution to yield; rain   techniques include:
   favourable climatic conditions in       falling at the end of grain filling may   ■ for winter crops, planting earlier in
   early spring. This benefit of early     make none.                                    the season, which brings forward
   planting needs to be balanced                                                         flowering and grain filling to a
   against the increased risk of frost     The harvest index – the proportion            time when more soil moisture is
   associated with early planting.         of a crop’s biomass that is                   potentially available to support
                                           converted into grain – provides               crops at these critical stages.
■ For summer crops, planting times         an indication of a crop’s ability to          However, it also increases the
   should be selected to avoid the         convert plant biomass into grain              chance of frost damage;
   crop flowering during periods of        yield. A good harvest index for
   potential heat stress.                  wheat is about 40 per cent in a well-     ■ matching seed rates and row
                                           managed crop.                                 spacing to planting date, region
■ Faster, larger and better-                                                             and yield potential, to reduce the
   designed farm machinery allows          Heat, frost and water deficits at             size of the crop canopy early in
   more timely seeding and other           critical periods can prevent plants           the growing season; and
   operations. These outcomes can          converting biomass to grain.
   improve water use efficiency                                                      ■ wide rows and skip rows in
   through either earlier seeding or a     Conditions that are too good during           sorghum can help conserve
   healthier crop.                         the crop’s vegetative growth can              water in the inter-row areas
                                           also reduce yield because the crop            during the early stages of crop
The most water efficient crop is not       canopy becomes too large and not              development, which is then
necessarily the most profitable and        enough water is left at flowering and         available to the crop during
growers need to evaluate the risks of      grain filling. Such crops may produce         grain fill. Skip rows can reduce
their decisions against the potential      large amounts of biomass but suffer           yield variability and can increase
benefits, for example, the risk of frost   from a low harvest index.                     yields in low-yielding years.
                                                                                                    PAGE 6




                                                                                  A well-managed fallow
                                                                                  can help improve soil
                                                                                  moisture available to
                                                                                  following crops.

Crop sequences
The sequence of crops and fallows
impacts on the WUE of the whole
cropping system. Conservative
cropping systems, where crops are
only planted when soil moisture
levels are high, will result in high
individual crop yields, but relatively
fewer crops and long, inefficient
fallow periods. more aggressive                           PHOTO: REBECCA THyER
cropping systems that include
double cropping will result in a
greater number of lower-yielding
crops and generally more efficient
use of available rainfall. The
appropriate balance between
aggressive and conservative
systems will depend on a whole                                                     Table 2 effecT of Soil WaTer ThreShold for planTing on
range of factors including a                                                               SySTemS WaTer uSe efficiency and oTher SySTem
grower’s attitude to risk, and is the                                                      performance parameTerS*
subject of ongoing research. Table
2 (for a representative situation                                                                      System                         Conservative        Moderate           Aggressive
in central Queensland) illustrates                                                Planting threshold                      mm               150               100                  50
some of the trade-offs that occur.                                                Number of crops                                          35                45                   72
Systems’ WUE                                                                      Crops/year                                              0.69              0.88                 1.41
Systems’ water use efficiency can                                                 Total grain produced                    t/ha             141               172                 197
be calculated simply as total grain
produced (kg/ha)/total rainfall (mm),                                             Average yield                           t/ha            4.04              3.82                 2.73
over single or multiple crop time                                                 Average cover                            %              40%               49%                  55%
periods. This integrates the effects
                                                                                  SWUE                                kg/ha/mm            4.55              5.53                 6.32
of fallow and crop management
and crop sequences to provide a                                                   % rainfall ending up as:
simple indicator of how efficiently                                               Transpiration                                           21%               26%                  32%
the cropping system converts
rainfall to grain. Preliminary                                                    Evaporation                                             56%               55%                  55%
research in the northern region                                                   Run-off                                                 18%               16%                  11%
indicates a system WUE of 6kg                                                     Drainage                                                 5%                3%                  2%
grain/ha/mm rainfall may be an
appropriate target.                                                              * This table presents the results of a simulation modelling analysis for a cropping system at
                                                                                   Emerald from 1955 to 2006.


Useful resources
■	   Australian Soil Resource Information System (ASRIS) website                                                        www.asris.csiro.au/index_other.html 
■	   APSoil database of soil water characteristics                                                                      www.apsim.info
■	   Soil matters – Monitoring soil water and nutrients in dryland farming                                              www.apsim.info
■	   Healthy Soils for Sustainable Farms project website                                                                www.soilhealthknowledge.com.au 
■	   Subsoil constraints to crop production in north-eastern Australia                                                  www.grdc.com.au
■	   Estimating Plant Available Water Capacity, GRDC 2009                                                               Ground Cover Direct, 1800 11 00 44
■	   Yield Prophet® website                                                                                             www.yieldprophet.com.au
■	   Whopper Cropper®                                                                                                   www.apsim.info/apsim/Products/                        
                                                                                                                        NationalWhopperCropper.pdf
                                                                                                                                                                                                www.coretext.com.au




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Acknowledgements: Richard Routley, QPI&F; Neil Dalgleish, CSRIO; John Kirkegaard, CSIRO;  
John Passioura, CSIRO; and James Hunt, CSIRO. 

				
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