Benchmarking - the key to improving productivity and WUE

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The old adage that ‘you can’t manage what you don’t measure’ certainly applies to water use efficiency (WUE), but the many definitions and methods of calculating WUE can make quantifying this important productivity measure confusing

PHOTO: Dr James Hunt, CSIRO

A major goal of the GRDC Water Use Efficiency Initiative was to provide a clear and consistent benchmarking approach to WUE so that growers and scientists within the network can evaluate the success of the interventions developed to raise WUE across cropping systems.

The new benchmarking system is now available for download from the CSIRO website (www.csiro.au/en/Organisation-Structure/Flagships/Sustainable-Agriculture-Flagship/WUE.aspx).

The revised system addresses commonly held misconceptions about the WUE benchmarking study of RJ French and JE Schultz (1984) and also offers simple ways to estimate on-farm water use. 

Measuring WUE

In simple terms, the WUE of wheat grain production is the grain yield of a crop divided by the water used by that crop.

The amount of water used by a crop is usually defined as transpiration (from the leaves) plus evaporation (from the soil). However, including the amount of water lost to the crop in run-off and drainage is also important as these represent unproductive water losses.

French and Schultz investigated the relationship between wheat water use and grain yield, but over the years their findings have become distorted – to the point where quoted WUE figures can be misleading.

French and Schultz assumed that under ideal conditions wheat crops could yield 20 kilograms of grain per hectare per millimetre of water transpired (transpiration efficiency). They also assumed that the least amount of evaporation possible from this system would be 110mm.

A common mistake now made is to assume that 110mm of evaporation is the average amount possible from a cropping system and not the least amount as French and Schultz stated. The second common mistake is to calculate transpiration efficiencies by subtracting 110 from an estimate of crop water use and then quote these as WUEs. This leads to errors in calculated transpiration efficiencies because evaporation is not constant but varies greatly between seasons and soil types. Also, one of the best to ways improve WUE is to decrease the amount of water evaporating relative to the amount transpired by a crop and by subtracting 110 from the crop’s total water use the parameter that is most easily improved is taken out of the equation.

Another observation of French and Schultz was that April to October rainfall could be used as a surrogate for total water use, but only if there was no stored soil water at sowing. This stored soil water is often forgotten and has led to WUE calculations being incorrect if stored water is present at sowing as the calculated WUE will be overestimated.

The new calculations build on the WUE work of VO Sadras and JF Angus (2006), who accounted for the introduction of semi-dwarf wheats and increasing atmospheric carbon dioxide – both of which increase transpiration efficiency of grain.

Putting it into practice

The benchmarking example below uses the new water use efficiency (WUE) guidelines developed by the GRDC Water Use Efficiency Initiative to compare the WUE of a current and a novel cropping practice investigated at Temora, NSW.

The comparison is between the currently grown mid-maturing variety EGA GregoryPBR logo, sown 9 May at 100 plants per square metre, with the novel practice of early sowing (15 April at 40 plants/m²) the very slow maturing variety EGA EaglehawkPBR logo – researched as part of the WUE initiative. It shows that the new early-sowing practice leads to a 21 per cent improvement in WUE (16.4 to 19.9kg/ha/mm) and achieves 98 per cent of water-limited yield potential in that season.

Table 1: Water use efficiency of a current and a novel cropping practice using the new benchmarking guidelines developed by the GRDC Water Use Efficiency Initiative
   Current practice
– EGA GregoryPBR logo
(9 May, 100 plants/m2
 Novel practice
– EGA EaglehawkPBR logo
 A: Summer-fallow rain (November to March)  510mm  510mm
 B: Growing-season rain (April to October)
 207mm  207mm
 C: Water use (A x 0.25) + B
 335mm  335mm
 D: Grain yield (12% moisture)
 5509kg/ha  6653kg/ha
 E: Potential yield = (C–60) x 22
 6050kg/ha  6050kg/ha
 F: Potential yield at 12% moisture (E x 1.12)
 6776kg/ha  6776kg/ha
 G: Per cent of potential yield at 12% (DF) x100
 81%  98%
 H: Water use efficiency = D/C
 16.4kg/ha/mm  19.9kg/mm

More information:

Dr James Hunt, research scientist, CSIRO Plant Industry,
02 6246 5066,
james.hunt@csiro.au

The guidelines can be downloaded from the CSIRO website (www.csiro.au/en/Organisation-Structure/Flagships/Sustainable-Agriculture-Flagship/WUE.aspx).

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Previous: Short and long-term focus needed to lift WUE

GRDC Project Code CSP00111

Region South, National, North