Variation in irrigation requirements of forages in Northern Victoria
The dairy industry in northern Victoria relies on irrigation water to grow a large proportion of its feed inputs. However, as a result of drought conditions in the last 15 years, annual irrigation allocations have been substantially lower and more variable than the preceding 20-30 years. This has caused dairy farmers to change their feedbase and has made it difficult for them to plan their forage mix over the following years.
In this environment of low and variable water availability, it is essential that dairy farmers have accurate estimates of the amount of irrigation water required to grow a range of forage types. The irrigation water requirements of a range of perennial forages, winter growing annual pastures and summer forage crops has been measured in experimental situations in northern Victoria. However, plant irrigation water requirements can vary markedly from year to year.
Fortunately, there are models such as "FAO-56" (Allen et al. 1998) which can use climatic data to predict the water requirements of irrigated forages. This model has been used in northern Victorian and there has been good agreement between the measured and modelled water use for most forages (Greenwood et al. 2008 and 2009). Therefore, the FAO-56 model can be confidently used to predict the total and irrigation water requirements of forages typically grown in northern Victoria using historic weather data.
This factsheet presents the probability distribution of annual irrigation water requirements, using historical weather data, for five irrigated forages, namely:
- perennial pasture,
- short season, winter-growing annual pastures (irrigated from mid March to mid October)
- long season, winter-growing annual pastures, (irrigated from mid February to mid November),
These probability distributions were determined for three locations in northern Victoria (Kerang, Kyabram and Yarrawonga). These data on irrigation water use can be used by dairy farmers when planning their management responses to limited irrigation water.
This factsheet also presents the probability distributions for irrigation water requirements for these forages at these locations for a range of climate change scenarios.
An internationally recognised model (Allen et al. 1998) was used to estimate the irrigation water requirements of the five forage types at the three locations.
These estimates were then compared to measured irrigation water use from experiments based near Kyabram (Greenwood et al. 2008, Lawson et al. 2009), with the model parameters then being adjusted if necessary to ensure consistency of modelled and measured water requirements. Historical climatic data for the three sites was sourced from the SILO website (SILO 2011) for the period 1935 to 2005. The three climate change scenarios used 70 years of data and assumed a steady-state climate in 2070. For more details of the methodology and climate data sets see Appendix 1.
Historical irrigation water use
Analysis of the 70 years of historical climate data showed that rainfall was lowest at Kerang and highest at Yarrawonga (Table 1). In contrast, the "reference crop evapotranspiration" (ETo) was highest at Kerang and lowest at Kyabram.
ETo is a description of the evaporative power of the atmosphere for a standard crop (typically a well water and well managed grass sward) – see Appendix 1 for how ETo was used to calculate irrigation water use. This resulted in the difference between average annual ETo and rainfall being higher at Kerang (920mm) than at either at Kyabram or Yarrawonga (740 mm).
Table 1. Average climatic data (1935-2005) and modelled annual irrigation water use (ML/ha) for sites in northern Victoria. Absolute range of the data is presented in brackets
|(mm)||(mm)||pasture||season annual||season annual|
These geographic differences in rainfall and ETo were also evident in modelled irrigation water use, with irrigation water use for all crop types being higher at Kerang than at either Kyabram or Yarrawonga (Table 1).
Modelled annual irrigation water use was highest for the perennial pasture (9.1 ML/ha) and Lucerne (8.7 ML/ha), intermediate for the maize (6.0 ML/ha) and long season annual (5.3 ML/ha) and lowest for the short season annual (3.2 ML/ha), when averaged over the three locations.
The large range in irrigation water use means that a simple average (and range) is a poor descriptor of the amount of irrigation water that is required for a given crop in either a wet, average or dry year.
Therefore, the probability of exceedance values were calculated for each forage at each location using the historical climatic data. (An explanation of probability of exceedance values is given in Figure 1 and the accompanying textbox).
A simple summary of these probability of exceedance values are presented in Table 2 while the complete data sets are presented in the probability of exceedance curves in Appendix 2.
Some interesting features of these probability of exceedance values in Table 2 are:
- For all forage types there was little difference in irrigation water use between Kyabram and Yarrawonga, as both rainfall and ETo are slightly higher at Yarrawonga than at Kyabram.
- The lucerne required a little less irrigation water than the perennial pasture due to the earlier finish to its irrigation season (mid April rather than mid May).
- The curves for lucerne and maize are very "stepwise" compared to those for both the annual and perennial pastures – this is a result of them having a longer irrigation interval, and hence more water applied at each irrigation, compared to the pastures.
- The range in water use for maize at each location (difference between dry and wet years) is smaller than that of the other forage types.
Understanding probability of exceedance
Figure 1. Modelled annual irrigation water use of perennial pasture at Kyabram for 1935-2005.
The probability of exceedance curve for the irrigation water use of a perennial pasture at Kyabram is shown in Figure 1.
The curve shows the proportion of years (vertical axis) for which the irrigation water use will exceed a certain level (horizontal axis). For example, annual irrigation water use will exceed approximately 5 ML/ha for 99% of years, 6.9 ML/ha for 90% of years, 8.6 ML/ha for 50% of years, and 10.3 ML/ha for 10% of years.
Consequently, annual irrigation water use for perennial pastures at Kyabram is between 6.9 and 10.3 ML/ha for 80% of years, with water use falling below this range for 10% of years and above this range for 10% of years.
Table 2. The probability of exceedance values for annual irrigation water requirements (ML/ha) for 5 forage types for Kerang, Kyabram, and Yarrawonga for wet, average and dry years as calculated using historical (1935-2005) climate data. (The complete data sets are presented in Appendix 2.)
|Seasonal Conditions1||Perennial pasture||Lucerne||Maize||Long season annual||Short season annual|
1Seasonal conditions are defined as:
Wet - irrigation water requirements were exceeded in 9 out of 10 years
Average - irrigation water requirements were exceeded in 5 out of 10 years
Dry - irrigation water requirements were exceeded in 1 out of 10 years
Climate change impacts on irrigation water use
Predicted changes to average climate conditions for the three climate change scenarios are shown in Table 3. These predicted changes involve increases in maximum and minimum temperatures and ETo and a decrease in annual rainfall. The changes were of a similar magnitude at all 3 locations (data not shown).
The predicted reduction in annual rainfall in the catchment areas is also likely to result in reduced runoff, inflows to storages and availability of irrigation water. These issues are not considered in this factsheet.
With the high climate change scenario, rainfall declined by 40-50 mm and ETo increased by 170-190 mm for each of the three locations, compared to the historic data (Table 4). The resultant modelled increase in annual irrigation water use under the high change scenario was around 2.2 ML/ha for the perennial forages, 0.9 ML/ha for maize, 1.3 ML/ha for the long season annual and 0.8 ML/ha for the short season annual. The increase in water use for all forages was higher at Yarrawonga than at either Kerang or Kyabram.
Probability of exceedance water curves for perennial pasture at Kyabram under historic and 3 climate change scenarios are shown in Figure 2. The curves show that the modelled annual water use at Kyabram is expected to increase by around 2 ML/ha for perennial pastures under the high climate change scenario.
One noteable feature is that there is no effective change in the shape of the curves (i.e. they are reasonably parallel) meaning that the modelled increase in water use in the wettest years is likely to be similar to that in the driest years. This feature also occurred for all of the forage types at all three locations (data not shown).
Table 3. Predicted changes to annual average conditions using B1 (low), A1 (medium) and A1F1 (high) climate change modelling scenarios by 2070.
|Temperature Maximum oC||Temperature Minimum oC||Rainfall|
|B1 (low)||+ 1.3||+ 1.1||- 4||+ 4|
|A1 (medium)||+ 2.5||+ 2.1||- 7||+ 8|
|A1F1 (high)||+ 4.1||+ 3.4||- 11||+ 14|
Table 4. Average climatic data and modelled annual irrigation water use (ML/ha) for three sites in northern Victoria using historic data (1935-2005) and predictions for climate change as of 2070 for the high (A1F1) climate change scenario.
|Rainfall (mm)||ETo (mm)||Perennial pasture||Lucerne||Maize||Long season annual||Short season annual|
Modelled annual irrigation water use using historical climate data was highest for the perennial pasture (9.1 ML/ha) and lucerne (8.7 ML/ha), intermediate for the maize (6.0 ML/ha) and long season annual (5.3 ML/ha) and lowest for the short season annual (3.2 ML/ha), when averaged over three locations in northern Victoria.
However, there was a large range in irrigation water use at each location, and large difference between locations. This means that probability of exceedance values will give a better description of how much irrigation water is likely to be required for a given forage at a given location, than a simple long term average.
Predicted increases in annual irrigation water use in 2070 under the high climate change scenario was around 2.2 ML/ha for the perennial forages, 0.9 ML/ha for maize, 1.3 ML/ha for the long season annual and 0.8 ML/ha for the short season annual, when averaged over the 3 locations.This data on irrigation water use and its predicted increases with climate change need to be used by dairy farmers and their advisors when planning their use of, and requirements for, irrigation water.
Figure 2. Modelled annual irrigation water use of a perennial pasture at Kyabram using historic data (1935-2005) and predictions for climate change as of 2070 for low (B1), medium (A1) and high (A1F1) high climate change scenarios.
Allen RG, Pereira LS, Raes D, Smith M (1998) 'Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements.' FAO Irrigation and Drainage Paper No. 56 (FAO: Rome)
Bethune M (2004) Towards effective control of deep drainage under border-check irrigated pasture in the Murray-Darling Basin: a review. Australian Journal of Agricultural Research 55, 485-494.
Greenwood KL, Mundy GN, Kelly KB (2008) On-farm measurement of the water use and productivity of maize. Australian Journal of Experimental Agriculture 48, 274-284.
Greenwood KL, Lawson AR, Kelly KB (2009) The water balance of irrigated forages in northern Victoria, Australia. Agricultural Water Management 96, 847-858.
Lawson AR, Greenwood KL, Kelly KB (2009) Water productivity of winter-growing annuals is higher than perennial forages in northern Victoria. Crop and Pasture Science 60, 407-419.
Weeks A, Christy B, O'Leary G (2010) Generating daily future climate scenarios for crop simulation. In "Food Security from Sustainable Agriculture" Edited by H Dove and RA Culvenor. Proceedings of 15th Agronomy Conference 2010, 15-18 November 2010, Lincoln, New Zealand.
Appendix 1 – Methodology for estimating irrigation water requirements
Simplification of validated, dual crop coefficient model
The FAO-56 single crop coefficient model (Allen et al. 1998) was used for this long-term modelling project as the actual dates of irrigation and grazing or harvesting are not known. The FAO-56 process uses climatic data (ETo) and crop characteristics (Kc) to calculate the water use of a crop. These terms are defined as:
- ETo is the "reference crop evapotranspiration" and is a description of the evaporative power of the atmosphere for a standard crop (typically a well-watered and well-managed grass sward).
- Kc is the "crop coefficient" and is a description of how much water a given crop will use compared to the standard crop.
The "crop water use" (ETc) of a well water and well managed crop is given by:
ETc = ETo x Kc
Selection of climatic data
Historical climatic data for Kyabram, Kerang and Yarrawonga was sourced from the SILO website (SILO 2011) for the period 1935 to 2005. SILO data was used as it is readily accessible and is a relatively clean and comprehensive data set. The period 1935 to 2005 was selected as it is the same period of data used to generate climate change forecasts by the Intergovernmental Panel on Climate Change (IPCC).
The data for three climate change scenarios, based on the IPCC low, medium and high climatic change predictions (Weeks et al. 2010), were also used to model irrigation water use. The future climatic data used 70 years of data and assumed a steady-state climate in 2070.
All forages were irrigated using border-check irrigation. It was assumed that all run-off from irrigation was captured and reused on the farm. However, no allowance was made for the capture of run-off from rainfall.
The irrigation interval for each forage was defined in terms of cumulative ETo-R since the last runoff event, where:
- ETo is as defined above,
- R is effective rainfall, ie. Rainfall less any runoff resulting from that rainfall.
The 5 forages modeled and their irrigation intervals (given in brackets) were:
- Perennial pasture (ETo-R>45 mm),
- Lucerne (ETo-R>75 mm),
- Maize (pre-irrigated 15 November, sown 22 November, 110 day growing season, last irrigation 22 March), (ETo-R>60 mm),
- Short season annual pasture (ETo-R>45 mm), irrigated from 15 March—before 20 Oct,
- Long season annual pasture (ETo-R>45 mm), irrigated from 15 Feb to before 20 Nov
None of the forages were irrigated between 15 May and 15 August, with the last irrigation for the lucerne being prior to 15 April.
An allowance was made for deep drainage during times when the model indicated surface ponding. The estimates from using this approach were consistent with the findings of Bethune (2004), who estimated the discharge to the deep aquifer in the Goulburn Valley at 30–40 mm/year.
Water intake at initial irrigation of winter annuals and maize
The FAO-56 methodology cannot predict water intake at the initial irrigation of annual forages. Therefore, the method derived by Lawson et al. (2009), based upon a relationship between cumulative E-R since 1 December and intake at the initial irrigation, was used to predict intake at the first irrigation for the long and short season annuals. This method sets an upper limit of 1.5 ML/ha of water at the initial intake, which may underestimate intake for some grey cracking soils that are often found around Kerang.
For the initial water intake of maize it was assumed that the site was irrigated in spring prior to the establishment of the maize – hence water intake at the initial irrigation was set at 80 mm.
Validation of modelling outputs
Two water use experiments conducted near Kyabram measured the water use of three maize crops (Greenwood et al. 2008) and seven forage systems over a three year period (Greenwood et al. 2009, Lawson et al. 2009). The modelled water use values were compared to the measured water use values to ensure consistency. Where necessary, the model was adjusted to ensure that the modelled water use agreed with the measured water use.
Appendix 2 – Probability of exceedance curves using historical (1935-2005) climatic data
Modelled annual irrigation water requirements for perennial pasture (PP), lucerne, maize, long season annual pasture (LSAP) and short season annual pasture (SSAP) for Kerang, Kyabram, and Yarrawonga. See Figure 1 and the accompanying text for an explanation of the curves.
Contact: Kevin Kelly or Alister Lawson
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Published by the former Department of Primary Industries, Future Farming Systems Research, September 2011
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