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!
Regards
Dan
For my 10 cents worth I would like to remind people that there are two mechanisms that cause surface ponding and I believe the risk of ponding is related to the causal mechanism and topography/risk of preferential flow paths. I have copied and paste the following commentary out of a previous report that I have written:
Ponding can be generated by two different processes. The first process is termed ‘infiltration excess’. Such conditions imply that rainfall (or irrigation) intensity exceeds the soil’s surface infiltration rate. On flat land this condition will result in surface ponding. A suitable lag time is required post rainfall and/or FDE irrigation for all of the ponded surface water to infiltrate the soil body. However, on sloping land ponded water will move downslope, hence creating surface runoff or overland flow. Natural soil properties can influence soil infiltration rate, as can animal grazing-induced soil physical damage. Soils with massive or platy soil structure are prone to infiltration excess conditions. The second process that results in ponding is known as ‘saturation excess’. This condition requires a fully saturated soil, often as a result of a high water table or a slowly permeable subsoil layer that restricts drainage. Saturated soils are filled beyond field capacity to the point that all large and typically air-filled pores are filled with water. Once all pores are storing water, the soil has no capacity to infiltrate further water and it therefore ponds or flows downslope. Overland flow will stop once the water source is removed. However, saturated soil profiles can only be alleviated by drainage or evapotranspiration.
Ponding itself is not the contamination point for direct loss of FDE but an indicator that preferential flow conditions are to be expected. Where infiltration excess ponding occurs on flat land with no direct pathway to a nearby water source, the environmental impact will be low. Such ponding will reflect a discrepancy between FDE application rate and soil infiltration rate. So long as an appropriate depth of FDE has been applied (i.e. < 20 mm) then all ponded FDE should be absorbed within one to two hours of application. The creation of saturation excess conditions following FDE application is an indicator of more serious ponding and suggests that direct losses of applied FDE will eventuate. The provision of suitable pond storage and adherence to a deferred irrigation strategy will eliminate the occurrence of saturation excess ponding. Well drained soils will not often achieve a state of saturation as they lack the drainage impediment usually required.
Hi all, my thoughts on the issue.
Firstly, it really doesn’t matter how we as an industry define surface ponding if it isn’t also accepted by the regulators that enforce it. If the Council has a requirement for NO ponding and they interpret that as an absolute bottom line then what we call it is irrelevant.
So, in setting an acceptable level of ponding for any system we need to first be asking ‘What is the Councils bottom line for this region and this farm?’ and then asking ‘Is that sufficient in terms of industry best practice and future proofing, or do we need to be better than that?’ (Remembering that in some regions such as Canterbury there are consents with various limits from ‘no ponding’ through to no ponding after 12 or 24 hours).
The wider and more long-term issue is that ponding is currently used as a really blunt tool to monitor applicator performance. In Canterbury it is overly harsh on flat land that has low infiltration rates while being overly permissive on free draining soils where no matter how much effluent is put on it drains away quickly.
Councils are working together to create greater consistency in the way that they assess effluent compliance across NZ, it would be great to see some debate about better and consistent ways to regulate applicator performance instead of solely relying on ponding as an indicator. Maybe a NES would be able to deliver this?
Here’s my two penneth worth……
Should we be taking a step back and looking at whether a surface ponding condition is the best thing to have on the consent to start with? Are there better measures that could be used instead? A no ponding scenario doesn’t neccesarily mean no losses – still vividly have pictures in my head of an irrigation event with no ponding but when you dug down a foot there was considerable sub surface flow resulting in a ‘swimming pool’ forming in the hole! Also in some scenarios ponding will not result in losses as above – this indicates to me that although it’s a useful indicator there may be better ways of managing losses from FDE?
It’s about creating decision making / self learning systems for farmers – first around the design – aligning application rate with infiltration rate and storage, and then around application – if soil saturated then this….. if it’s not then that…..as David has indicated above.
The environment is dynamic so the sooner we get away from a prescriptive approach the sooner we will end up dealing with the problem.
If we have to have surface ponding conditions my first question would be what is acceptable in the way of losses to the environment and relate this to the ponding definition? But again I expect this will differ muchly between the various environments!
Thanks for the question “What actually constitutes surface ponding?”
Being a scientist who has worried about this, I have the clear and unequivocal answer. It is when incipient ponding occurs and that the pressure potential first reaches zero (ie free-water) on the soil’s surface matrix. The so-called time-to-ponding, tp. After that, the free-water film is available to access macropores and is then capable of being redirected by the soil’s micro-topography to redistribute and run-off – the length-scale of which is determined by the micro-topography. Straight forward and easily done. But …
That’s being anal.
The question should be “What are the characteristics of surface ponding that will have deleterious consequences on the soil’s ability to buffer and filter contaminants?” Now that’s a toughie! For:
· Does the macropore flow lead to rapid and far-reaching transport of contaminants through the root zone to receiving waters?
· Does the length scale of the surface redistribution lead to convergence of pathways to surface water bodies that leads to preferential transport of contaminants.
These two latter points merit further investigation. We know about tp, we have DEM, and in principle we could do it in spatial models.
It’s not going to be easy though. Seriously, if we tried hard, we do have the tools and techniques to stretch out to my second question: viz
“What are the characteristics of surface ponding that will have deleterious consequences on the soil’s ability to buffer and filter contaminants?”
Ponding: Ponding is not an issue for me as long as the pond “average” depth (how is this measured?) does not exceed the amount of effluent being applied. That is, if the farmer was applying 20 mm, and immediately after the irrigator had passed there was a 20 mm pond (within the run length and width) which lingered for two hours without dispersing too far sideways then – no problem. It just shows that infiltration is slow and this is a “good” thing. (Besides the soil surface has “capacitance” – that is, the small indentations such as hoof prints are small holding ponds in themselves. They will accumulate liquid in miniature ponds.) We must remember that the farmer and the compliance officer should be given an easy way to measure pond depth and then agree as to what they see in front of them. If the measurement process is too laborious, the pond may have vanished by the time the measurements have been completed. The measurement technique has to be by way of a rapid but acceptable “rule-of-thumb” assessment. And yet it may have to stand up in court.
With respect to matters legal (Michael Hide – above), the intention behind our writing of a new standard/code is to give dairy farmers, council staff and others, a national standard for reference. Thus its adoption by regional councils (which is desired) would lead to the same examination/audit criteria being applied in all districts.
I agree to a point Stuart but the reality is that the surface redistribution does happen, and the majority of effluent puddles you see in Canterbury are as a result of that redistribution. You then get into the arguement of ponding causing macropore flow through the soil instead being absorbed into the soil where it can be used by pasture. (Although I would have thought that if the macropore drainage was that great there wouldn’t be ponding, and that brings me back to the point that ponding is blunt tool to measure applicator performance)
With regard to the legal matters, Regional Councils have already worked together to be more consistent, but there are still big differences between the regions. I suspect this will always be the case to some extent due to different landscapes, climates and political wills.