WATER
August 2011
9
Irrigation Issues Dr Tony Daveron
We might be still waiting for winter and what might still come. Unfortunately, it is that time of the year where there are some unusual decisions regarding irrigation and where I could delve into my drawer of previous articles to meet the deadline this month. While Christchurch might have received a touch of winter in late July — July 25 to be precise, with snow boarders and skiers enjoying the slopes around the city, the rest of Canterbury was mostly bypassed. Certainly in our area of Christchurch snow boarders enjoyed 35cm of powder on the 25th and created an ice slope for the following day. But not in the next few days according to the weather forecasters. As I write this en route to Auckland (Saturday 13th) the forecast is grim for the next week. It will fortunately, one would hope put paid to those who considered the few balmy 17°C days necessitated irrigation. It never ceases to amaze me why irrigation starts unnecessarily. Some of those irrigating were amid a constant discharge pump test — fair enough, the water has to go somewhere. However, I know the consultants carrying out pump tests and two of those companies were carrying out tests for us. But the number of pump tests and those irrigating just did not equate. I seem to write the same thing every year and have the same reasons for irrigating prematurely every year: • ‘Really freshened the grass up’, and I agree but for different reasons. Almost certainly the grass looks better, all the dirt and dust has been washed off. • ‘There was a response’, except the response is inevitably visual with no pasture measurements to back up the response claims. There is no disputing there is a soil moisture deficit at the moment — measurements will confirm that fact. That deficit is about 5-8mm, not enough
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to justify irrigation. As usual at this time of the year the soil temperatures are just too low for there to be any response in growth. The plot of temperatures (see graph) over the last few days shows that the 9am temperatures are simply far too low for any growth response. In the last six days the 9am temperature was below 6.2°C and was as low as 3.6°C. There is little else to write about really — would you irrigate given these conditions? I wish you well in the snow and inclement weather forecast. While it will be of great benefit for groundwater recharge, there are better times for it to arrive for the stock farmers.
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