PAGE 1 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) Level 1, Tourism House | 40 Blackall Street, Barton ACT 2600 | PO Box 5367, Kingston ACT 2604 | t. +61 2 6166 4500 | f. +61 2 6166 4599 | e. firstname.lastname@example.org | w. www.grdc.com.au 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 Disclaimer Any recommendations, suggestions or opinions contained in this publication We do not endorse or recommend the products of any manufacturer referred to. do not necessarily represent the policy or views of the Grains Research and Other products may perform as well as or better than those specifically referred Development Corporation. No person should act on the basis of the contents of to. The GRDC will not be liable for any loss, damage, cost or expense incurred this publication without first obtaining specific, independent professional advice. or arising by reason of any person using or relying on the information in this The Corporation and contributors to this Fact Sheet may identify products by publication. proprietary or trade names to help readers identify particular types of products. produced by Acknowledgements: Richard Routley, QPI&F; Neil Dalgleish, CSRIO; John Kirkegaard, CSIRO; John Passioura, CSIRO; and James Hunt, CSIRO.
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