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Norwester season has arrived! Irrigation is a relatively simple task in hot dry weather, you turn it on, check the flow rate and system pressure are correct on a daily basis (this can be automated) and keep going until the weather forecast provides you certainty significant rainfall is coming. From late November through to mid-December Lincoln and Winchmore climate station data shows ET rates averaged over 4.5 mm/day with a few days well over 5 mm/day (the highest at 6.3 mm). You should have been irrigating flat-out during this period to meet plant water needs. The long-term weather forecast is now indicating we should get some rain prior to Christmas, but it will be a few more days before this becomes certain!

During the last few weeks of hot dry weather, we’ve had clients saying to us they’re having trouble keeping up.

For some this was a valid comment because their irrigation systems have low system capacity. System capacity is the maximum flow of water per unit of irrigated area that can be delivered by an irrigation system in a day. It is expressed as millimetres per day (mm/day) or litres per second per hectare (l/s/ha). There are some irrigation systems in Canterbury that have been designed to deliver between 3.5 and 4 mm/ day (0.40 and 0.46 l/s/ha). This is a legacy from the early irrigation schemes delivery flow rates and some older resource consents. During prolonged dry periods irrigation systems with low system capacity will start to ‘fall behind’ as irrigation can’t keep up with plant water use.

Despite this, having an Irrigation system with a low system capacity is manageable, but you need a different strategy for scheduling irrigation. The irrigation trigger point during the peak of the season must be set closer to the soils field capacity (this provides a buffer to get through periods of hot dry weather) and the long-term weather forecast needs to be very closely monitored. The downside with these irrigation systems is there’s no safety net if you get it wrong.

For clients that said they had trouble keeping up but had ample system capacity (5 mm/day or 0.58 l/s/ha) we’ve noted their issues are due to the irrigation scheduling strategy they’ve adopted. Running soil water down or very near to the stress point and then applying little amounts of irrigation often (5 mm of irrigation daily for example) is not good irrigation practice as plants can become stressed very easily. Soil water evaporation rates are also greater near the surface, and when combined with the plant root mass becoming concentrated in the top of the soil profile this leads to a very un-resilient farming system.

Typically, good irrigation practice for Canterbury soils during the peak of the irrigation season is to set the irrigation trigger point above the stress point (between 60 – 65% of Profile Available Water) and apply between 10 – 20mm depth of irrigation every 2-5 days depending upon your soil type.

If you would like specific support or advice for your irrigation scheduling, an assessment of your scheduling practice (including practical suggestions around how your irrigation practice could be improved), or you have issues with your irrigation equipment call Water Strategies to book in your Irrigation WOF.

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