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Dividing the droplet – A water balance study for lucerne seed production resourced by an underground aquifer by James De Barro
August 2005
RIRDC Publication No 05/116 RIRDC Project No. DEB-3A
With the impending changes to irrigation licences posing questions over the sustainability of irrigation practices and the aquifer, this research serves both immediate and longer term requirements for improved understanding of the current irrigation systems used.
Since the commencement of irrigated lucerne seed production in the Keith region, no recorded attempt has been made to quantify or qualify the irrigation systems used. Such information is the foundation data required for assessing the systems’ efficiency and any areas of likely improvement as well as the impact of irrigation on the quality and quantity of the aquifer.
The aim of the research was to determine what does happen to the water pumped from the aquifer for the purpose of lucerne seed production. The research methodology used is simple and repeatable. The methodology for this indicative type of research project amalgamates the use of several commercially available instruments into an excellent process for quantifying the lucerne water balance. The methods used were so successful that they have become the model for the South Australian Department of Water, Land and Biodiversity Conservation Volumetric Conversion Project for the entire South East region of South Australia.
Additionally the research methodology can be used for extended irrigation efficiency studies that could result in improvements in irrigation delivery and crop yield. Whilst the project was not aimed to investigate irrigation efficiency, it did by design, assess the efficiency of irrigation delivery. The following general observations were made from the findings.
The research showed that, specifically for lucerne seed production, a fixed centre pivot in good working order that was irrigated according to crop water requirements and active root depth was 36.4% more efficient than the most efficient flood irrigated lucerne seed crop and about 70% more efficient than the worst performed flood irrigation.
Provided water quality is not in excess of 3000 ppm total dissolved salts a conventional centre pivot or lateral flow could be an alternative to flood irrigating lucerne seed crops. Compromises to this include capitalising the conversion of irrigation delivery and environmental factors such as tree removal. Modifications to centre pivots such as drop tubes could present opportunities for their use with water of significantly higher salinities.
The efficiency of a shiftable, shared centre pivot declines significantly compared to a fixed system due to compromises in irrigation timing.
Flood irrigation efficiency is expressed as a high percentage of pumped water that is used by the crop. This depends largely on the flow rate of the water pumped, sealing properties of the delivery channel and the irrigation bay slope and soil type. The configuration of bay size/slope in relation to the flow rate impacted on the eventual percentage of pumped water used by the crop. The research found that between 25% and 59% of the pumped water is used by the lucerne seed crop, the remainder mostly lost through drainage and only minimal losses due to evaporation. The findings indicate that there is substantial room for improvement in the efficiency of flood irrigated lucerne seed production.
Hay production from flood irrigation and pivot irrigation required nearly twice as much water to be pumped which is directly related to the increased number of required irrigations. In the case of the flood irrigated crop the percentage of pumped water used by the lucerne for hay production was no different than the seed crop the previous year. Since the same percentage of pumped water is required by lucerne in hay and seed production, hay production is not necessarily any less or more efficient but by virtue of the number of irrigations requires significantly more water than seed production.
Aside from developing a method for evaluating irrigation efficiency, the research achieved its aim of determining the water balance for irrigated lucerne seed production. The benefits of this research are that the results will support irrigator’s requests for defined water quantities in the licence conversion process. The conversion process should not be a system to reduce or increase licence allocations but rather a process of simply converting the current licence format into another, more useful format.
Any changes to allocations must be on the basis of confirmed knowledge of aquifer sustainability – particularly in respect of salinity and recharge.
The research determined that for flood irrigation in the defined region an irrigator requires between 7 and 14 ML of water for every hectare irrigated for lucerne seed production. The quantity required depends on both the existing infrastructure of the irrigated area and environment being irrigated.
There are many factors influencing the quantity required, including soil type, channel design, pump flow rate, bay size and slope. Some factors, such as soil type and possibly slope, can not be altered but bay size, channel design, delivery system and flow rate may be modified to improve efficiency and hence reduce the quantity required per hectare of seed producing area.
The defined quantity of water
required by the irrigated lucerne seed producer to continue their current
practice is now established. The conversion of licences should be relatively
straight forward, at least on the mathematical level. The contentious issue
will arise if and when the conversion process results in cuts to the volumes
required to maintain the current status quo, especially if the cuts are
not based on quantified and qualified reasoning.
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