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Rural Industries Research & Development Corporation
Improved Irrigation practice and crop diversification
for the irrigated pasture seed industry
Compiled by Bill Binks, PIRSA RURAL SOLUTIONSApril 2003
RIRDC Web Publication No W03/003 RIRDC Project No SAR-11A
Increasing groundwater salinity in the upper South East of South Australia is threatening the future of a valuable and nationally significant lucerne seed industry based in the region. In the Hundred of Stirling area in particular, salinity levels are predicted to have a major impact on seed yields if the trend continues. Excessive water application and subsequent drainage, resulting from flood irrigation on sandy soils has been attributed as the likely cause of rising salinity and a lowering water table.
This project was initiated to address this situation, by examining current irrigation practises and water use of lucerne seed and investigating potential alternative crops that may use less water. A literature review was conducted as the initial step in assessing alternative crops. A field trial was conducted over three irrigation seasons, assessing water use of lucerne seed and implications for deep drainage, and evaluating a range of moisture monitoring devices as potential irrigation scheduling tools. An objective of the project was to refine irrigation management practises in order to reduce water use and improve gross margins.
Of the seven crops researched in the literature review, canola, linola and safflower were recommended for establishing field trials. When examining these options in more detail, it was found that while canola is already grown successfully in the area, no widely suitable and economically comparable alternative to lucerne seed exists. Five other higher value horticultural crops were considered, but also eventually were dismissed, mainly due to poor tolerance to saline water and isolation from markets. Olives are considered to be viable alternative crop with a developing market, but further investigation of this long term perennial crop was considered outside the scope of the project. No field trials of alternative crops were established for making direct comparisons with lucerne seed. Project work shifted emphasis to exploring lucerne seed irrigation management.
Crop water use rates corresponded to pan evaporation for 5 days following irrigations, then progressively slowed as soil moisture was depleted. As a result of irrigations generally being applied when the soil was quite dry, the rate of water consumption fluctuated through the season and estimated growth potential was quite low rates in several periods. Cumulative crop water use over a standard 13- week irrigation period was estimated at 456 mm. Under a different irrigation schedule, with crop growth rates less inhibited by dry soil, this total water use figure might have been higher.
It was found that significant drainage below the rootzone following irrigations was unlikely, under the irrigation regime examined. Neutron probe readings and reasonable estimates of plant water use accounted for all the irrigation applied. If drainage did occur then the amount would be no more than 10 mm on the first and last irrigations. This finding was unexpected given the general perceptions of flood irrigation. It was noted that these results are only applicable to the particular conditions and irrigation regime used for this trial and may not represent the situation at other sites. Crop water use, and to a greater extent drainage, is likely to vary significantly according to the particular irrigation schedule applied.
Any differences between the three dormancy classes of lucerne were minimal, and became less apparent as the trial progressed. There were no consistent significant differences in terms of production of seed or hay or water use. The evidence for minimal drainage also suggested salt might be accumulating in the soil during crop production, until flushed out by winter rainfall. A salinity profile would be useful in confirming the movement of salt in this situation.
Capacitance probes were found to be relatively easy to use and provide information likely to be very useful for irrigation scheduling. A far less expensive and simplified option suggested is manually read gypsum blocks, used to indicate the onset of crop water stress based on soil suction of 100 kPa. Monitoring soil moisture levels using a neutron probe is an expensive and specialised exercise, and probably not the most suitable scheduling method for an independent irrigator. Data gathered from the range of devices in the trial suggests how the use of such technology has potential to enable refined irrigation scheduling and reduced water applications.
This project provides some initial information on irrigated lucerne seed under a particular set of circumstances. The data collected is a significant part of the project outcomes. Not all of this data has been analysed but is included in this final package, so that it may be accessed and used if required. There is considerable scope and a need for further research to occur under a range of conditions in the area, aimed at resolving the impact of current irrigation practises.
The region is reliant on
the lucerne seed industry and it appears at least for now, that no alternative
industries using less water, are able
to substitute for the economic base
this provides. However if current trends
of deteriorating groundwater continue, the
lucerne seed industry appears destined for
eventual collapse. This project could not definitively explain how flood
irrigation of lucerne impacts on groundwater. The industry still
must continue to review the practises used, define the impact on the
environment and discover how production can continue sustainably.
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