Irrigation Annual Volume – Prove it or lose it!
Over the next ten years a huge number of new groundwater take consents will need to be obtained to replace expiring consents. Is yours one of them?
The annual volume limit on existing consents will be reviewed by the Regional Council as part of this process, with pressure on them to reduce the total allocated from each aquifer. Current policy is to base new annual volumes on the consent holders water meter data.
There are fishhooks in this simple approach for setting new annual volumes which are likely to lead to unfair outcomes.
These include:
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Water meter data may not include a “high demand” year that the annual volume is based on — the longer the data record the better.
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Irrigation system limitations and/or management may have limited actual water use.
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Water supply restrictions may have affected actual water use.
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Part of the property may have been out of production due to redevelopment.
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Equipment breakdowns.
Periodically I get asked by other Councils to provide an independent assessment of the annual volume reasonably required to irrigate a property efficiently. The first things I ask for are a copy of all the water meter and soil moisture time-series data, preferably hourly data. I also get a copy of the climate data for the area for the time-period covered by the water meter data.
The annual volume limit is generally set to fully meet reasonable irrigation requirements nine years in ten, on average, and meet a significant proportion of the requirement in the “10th year”. “Reasonable requirements” usually implies Good Irrigation Management Practice is followed.
Any review of water meter data should establish whether any “9th or 10th year” seasons occurred during the time water meter data was collected. Analysis of the climate data will tell us this. Actual (metered) water use should be close to the annual volume limit in these years, if the assumptions made in setting the existing consent limit are correct.
Comparing the water meter and soil moisture data reveals a lot about the irrigation system and how it is being managed. These are important clues as to why measured annual volume differs from the consented annual volume in the high-demand years. These often point to changes the farmer or grower could make to improve production, such as increasing irrigation system capacity or changing irrigation trigger levels. If changes are to be made that are expected to increase actual water use, then these changes should be taken into consideration when setting the new annual volume limits. It makes no sense to prevent, through water use consent conditions, a farmer or grower achieving greater value per litre of water used.
A pre-requisite for a robust and comprehensive analysis of future irrigation water needs is, of course, good quality monitoring data. Data is needed to prove what’s happening on farm is appropriate. It’s no longer sufficient to be a good irrigation manager; you also need to demonstrate it. “Show me the numbers!” Without them, how will you justify your water allocation?
If you’re to prove what you need, and not lose ground, you need to be keeping the following data for your property:
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Accurate water meter data, with no missing data.
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Continuous soil moisture monitoring records — at least once-a-day measurement.
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Good quality rainfall records, if your farm isn’t close to an official rainfall station.
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Good records on the area of land actually irrigated.
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Secure and reliable data storage. Good numbers are no use if you can’t find them later!
The message is clear; collect and keep irrigation data now, you will need it!
