Water-use efficiency of wheat in Tasmania
TAKE HOME MESSAGES
 Water-use efficiency (WUE) in the high rainfall zone can be
extremely variable, as a result of environmental and agronomic
constraints to grain production that limit yield potential.
 Potential WUE, which takes into account losses due to runoff and
drainage, has a maximum value of around 20 kg/ha.mm.
 Gaps of 58% to 100% between attainable and potential WUE,
could be addressed by improved crop management.
 Scenarios modelled using APSIM show that yield, WUE and
economic returns could be improved by applying additional N
fertiliser with strategic irrigation, to avoid co-limitation of inputs.
 In high rainfall cropping areas such as Tasmania, N supply is a key
driver of yield and hence WUE.
Background
Emerging developments in Tasmanian grain cropping such as the expansion of low pressure overhead irrigators
and increased access to low-cost, plentiful irrigation sources in some areas will have a strong influence on WUE.
Conventional estimates of WUE that are based on grain yield per the combined amount of rainfall plus irrigation
do not account for stored soil moisture, drainage or runoff events that can be frequent in high rainfall
environments. As a result, WUE may be over-estimated. An alternative approach is to estimate WUE as grain yield
per the combined amount of surface evaporation, transpiration, drainage and runoff. Direct measurement of
these different components to quantify ‘actual’ WUE is difficult and desktop modelling is used instead. This
approach can distinguish between ‘attainable’ WUE, which is limited by crop management e.g. nitrogen fertiliser,
and ‘potential’ WUE, which is determined by climate and is free of nutrient and biological constraints.
Methods
Management and yield data for 27 wheat trials were used to configure and validate the APSIM farming system
model. Model output for key water balance elements and yield were used to estimate ‘attainable’ and potential
WUE. Further model scenarios were run to explore the response of WUE to nitrogen and irrigation management.
25
Attainable WUE
WUE (kg/ha.mm)
20
Potential WUE
15
10
5
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Dataset
Fig. 1. Predicted attainable and potential WUE across a range of datasets
Benchmark WUE
60
There is a trend for WUE to increase with
decreasing N stress (Figure 2), which indicates that
nitrogen supply and demand dynamics are an
important driver of WUE in Tasmania. The
importance of N in Tasmania is also supported by
the gap between potential and attainable WUE at
some sites (Figure 1).
50
Cumulative N Stress
The performance of the model to predict grain
yield was found to be acceptable across a range of
environments and management systems.
Maximum potential WUE is around 20 kg/ha.mm
(Figure 1). However, there is substantial variability
in potential WUE and a significant gap between
attainable and potential WUE at most sites.
y = -3.1674x + 62.876
R² = 0.4282
40
30
20
10
0
0
5
10
15
WUE (kg/ha.mm)
20
Fig. 2. Accumulated seasonal crop N stress
WUE sensitivity to nitrogen and water supply
Increasing the irrigation by 60 mm resulted in an average predicted yield gain of 602 kg/ha and an increase in WUE
of 0.4 kg/ha.mm of the control, with 50% of years generating both an economic yield gain and an increase in WUE
(Table 1). Doubling the irrigation led to a small increase in average yield but had no effect on average WUE. The
frequency of seasons with gains in both yield and WUE declined to 8. There are larger gains in WUE achieved from
irrigation in drier years.
Increasing the total N rate from 70 kg N/ha (control) to 140 N/ha (unchanged irrigation of 20 mm), resulted in yield
gains of 437 and 660 kg/ha, respectively. Corresponding gains in WUE were 0.6 and 0.9 kg/ha.mm. The frequency
of gains in both yield and WUE ranged from 46 (+70 kg N/ha) to 43% of years (+140 kg N/ha).
Increasing both irrigation and applied N resulted in much larger and more consistent gains in both yield and WUE.
For example, average yield and WUE gains of 2119 kg/ha and 1.9 kg/ha.mm were reached with an additional 120
mm irrigation and 140 kg N/ha of fertiliser with positive gains in both occurring in 73% of years.
Table 1. WUE and yield response of wheat to irrigation and N treatments from 1980 to 2010.
Irrigation
(mm)
N
(kg/ha)
Additional irrigation
60
0
120
0
Additional N
0
70
0
140
Additional irrigation + N
120
70
120
140
Average
yield
change
(kg/ha)
Years with
yield gain
(%)
Years with
economic
yield gain
(%)
Average
WUE
change
(kg/ha.mm)
Years with
WUE gain
(%)
Years with
economic
WUE gain
(%)
602
817
66
66
50
27
0.4
0.4
50
43
50
27
437
660
93
90
56
43
0.6
0.9
93
76
57
43
1577
2119
100
100
46
73
1.3
1.9
83
100
46
73
Acknowledgements: The project was funded by GRDC (Project UT00016), TIA, UTAS and CSIRO.
Authors T. Acuna, S. Lisson, M. Matuszek, R. Fish, P. Johnson, G. Dean
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