I've worked a lot and with many people on irrigation efficiency and on application of effluent to land. We keep coming up against the question, “What actually constitutes surface ponding?”
I’d love to get some agreement on this.
In the last year or so:
- I’ve heard that ponding only applies to durations lasting 4 hours or more, and I've heard it applies to anything from a few minutes duration.
- I’ve heard it has to be at least a pretty big area and I’ve heard anything at all counts.
- I've been told the Environment Court determined any duration mattered. A regional council had applied a four hour minimum when assessing ponding, but the court said the consent said 'no surface ponding' and that meant no surface ponding.
- I've seen lots of it and I've seen evidence that excessive application rates are problematic - both for irrigation and for land applied effluent.
There will almost always be some surface ponding; even drip systems micro-pond. So applying a concept of excessive surface ponding seems better. But what is excessive surface ponding?
Excessive surface ponding - defined
In the interests of sparking debate, I propose a definition of excessive surface ponding:
“Excessive surface ponding means the presence of surface water pooled in contiguous areas of greater than 0.04m2 found, one hour after application starts, at more than four of forty sample points selected at random over at least 25% of the application area, with each point being more than two metres apart.”
- Surface water pooled may need definition, but I’d say means clearly visible puddling - i.e. not just wet soil
- contiguous areas means connected
- 0.04m2 (about 20x20cm) is bigger than a hoof print (my first intent) but is maybe too small – the auction starts now, your bids please . . . .
- one hour after the start of application covers both travellers and stationary nozzles but is the time right?
- Four of forty sample points means 10% and hopefully is a big enough sample that is not too onerous to take
- Random is random – let's specify a method e.g. tossing a 0.25 m2 quadrat or ring backwards over your head with your eyes shut having spun twice clockwise
- 25% of the application area means you have to look around a bit
- More than 2 m apart spreads it out a bit and avoids sampling landing in the same contiguous, but relatively small, pond. Pick a number (and justify it!)
I don't know that I agree with my proposed definition.
For a start, I've watched high application rate water (and effluent too) disappear very rapidly, and upon digging significant holes, couldn't find sign of it in the rootzone. So maybe, if you can't see ponding for more than 10 minutes, that's when you should get worried!?
Please, have a think, then post a comment below. Let's try to have a reasoned and enlightening debate!