The below article has been written and provided by Mick and Angus Faulkner.
The result of more spray drift in recent times is due to three factors. The first is simply that spray capacity has increased (more hectares covered in the same amount of time) resulting in a huge increase in the amount of chemical in the air. The second factor is that farmers simply do not understand the amount of chemical they are putting into the air. The third is that many farmers either don’t understand or they have little regard for their neighbours, their communities, their own families, other land users, the environment etc by spraying in hazardous inversions. It isn’t just a little bit of spray loss that they think is inconsequential – it is very dangerous. Spray drift caused by spraying in hazardous inversions will likely lead to more restrictions and loss of valuable farm pesticides (herbicides, insecticides and fungicides).
1. Chemical labels are law.
Where it says on a label with the words ‘DO NOT……’ then that is a legal requirement. The reason is that applying in the conditions covered by the restriction will lead to drift. It is unambiguous – spraying in these conditions will lead to drift. The very best operation I’ve seen of all sprayers has about 10% of the total volume drifting (ie driftable fines, nominally less than around 150/180 micron). This ‘best case’ scenario is when the best nozzles (eg TTI, Hardi Injet (conventional), Wilger DR110 (PWM)) are used to produce Extra Coarse or Ultra Coarse droplet. It is also when boom heights are at 50cm above the target (50cm spacing) or 37cm above the target (25cm spacing) and the speed is lower than 18/20 km/hr. All these tests were done using straight water with no chemicals or adjuvants. As soon as these are added the number of fines increases dramatically, as they work by making droplets smaller.
2. What do the spray droplet sizes mean?
Droplet size for a particular spray quality is quoted as a range of Volume Mean Diameter. Let’s say that a coarse nozzle is quoted as producing droplets with a VMD of 326-400 micron. This sounds ok but reality is that it means 50% of the volume is composed of droplets less than 367 micron and 50% of the volume is composed of droplets greater than 367 micron. The problem with this is that there are many more of the smaller droplets (less than 180 micron) to make up the same volume, so there is still plenty of them to drift. Simply going to a nozzle that produces larger droplets is not the answer to this problem – because the 50% of the volume that is larger droplets become so large that they bounce off targets or their distribution is so low that there isn’t good coverage. The only way to satisfactorily reduce the number of small droplets being emitted from nozzles is to use nozzles that reduce the proportion of fine droplets, rather than increase the VMD. The nozzle types quoted in point 1 will achieve that. Hopefully in the future we start using DV0.1 (ie the droplet size of the smallest 10% of droplets) and DV0.9 (droplet size of largest 10% of droplets) and RS (relative span, which indicates the variation in droplet size which satisfies to reduce drift while also providing sufficient coverage).
3. The measurement of spray droplet size is at the target.
A lot of people think that they are ok if the droplet size once the spray pattern breaks away from the sheet is what is stated on the label. That is simply not true. Droplet size is determined by how big it is at 50cm from the nozzle. A droplet specification requirement of very coarse would likely not be met with a nozzle specification of very coarse. In just about all circumstances, to have very coarse droplets at the target (50cm) will require a minimum nozzle specification of extra coarse.
4. Droplets only move about 20cm by the force of the pressure in the nozzle.
Once they go that distance, they either fall further (if they are heavy enough) or they go wherever the air movement takes them. Anything less than 180 micron will definitely not descend. If the boom is raised higher, then even a larger droplet gets smaller rapidly by evaporation – at 80cm (or so) something that exits the nozzle at, say, 250 micron is already down to 200 micron by the time it reaches 20cm and still too small to reach the target by the time it reaches 50cm. It still has another 30cm to go before it theoretically reaches the target, and simply won’t make it as it has become too small. A lot of people struggle with the fact that droplets get smaller so quickly after they exit the spray unit. A simple demonstration is to ask them to kneel in the shower at home and have the water at a comfortable temperature. Then ask them to stand up and note the water temperature. Now, showers produce very large droplets of water (way bigger than any spray unit), yet they get very cold in such a short distance.
5. Nozzle spacing.
Using a shorter distance between nozzles produces a double overlap of the spray pattern at a lower height above ground level than when they are spaced at 50cm. When spraying with nozzles at 25cm spacing the double overlap occurs at 37cm. Booms should be maintained at 37cm above the target to get the benefit. Having 25cm nozzle spacing and having a boom height of 50cm negates the benefit. This results in considerable drift reduction as droplets have 13cm less distance to travel ie they are not getting smaller and smaller.
6. Small droplets will only drift sideways with wind during the day.
This is because there is almost always some sort of turbulence in the air that either sends them further skywards or drives them into the ground. What does go skywards is most likely caught in subsequent turbulent movement and returned to ground level. They can also go sideways in the wind and drift over the fence into an adjacent paddock. That is direct drift, and there are rules for that as well. Direct drift can almost be completely eliminated by using nozzles stated in point 1, keeping booms at 50cm, keep speed to less than 20 km/hr, and not spraying in windy conditions as per label restrictions.
7. Inversions and hazardous inversions.
The term inversion means exactly that – conditions are inverted from the norm. During the day the warmest temperature is at ground level and reduces with height. When an inversion occurs, it is referring to a temperature inversion, where the air at ground level is lower than the air above it. It is caused by the surface radiating heat and that’s why it generally happens at night. Sometimes it can be caused by rain directly cooling the surface and it isn’t unusual to see short term inversions during rain events, showers or thunderstorms. At the top of the inversion there is a boundary layer, above which the normal pattern of temperature reduction with height still exists. Below the boundary layer air movement is unpredictable in speed and direction and while it appears calm to us at ground level, there is inevitably something happening above us. But not all inversions are dangerous. Where there is turbulence, which means there is vertical as well as horizontal air movement and air from near the boundary layer is forced downwards and air closer to the surface moves upwards. This upwards, downwards and sideways air movement accurately describes turbulence. Hazardous inversions do not have any or sufficient turbulence and can only be measured with very accurate, sophisticated, three-dimensional sensors.
8. What’s so bad about a hazardous inversion?
When there is a hazardous inversion there is no turbulence and spray droplets stay suspended in the air trapped under the crown of the inversion, the boundary layer. This air moves sideways and the layers don’t mix. So, a layer of air drifting sideways is always produced. This layer will move wherever the air movement takes it. A lot of people think that on a still night there is no air movement. Nothing could be further from the truth – there is always air moving, it’s just that it moves sideways (in laminar layers) at slow to moderate speed. It is not uncommon to have sideways air movement of 5-12 km/hr of this layer, so how far it goes depends entirely on how long the inversion conditions last. We have been able to verify that sometimes a ‘wind’ of 20 km/hr can exist under the boundary layer of a hazardous inversion. A 6 hour inversion at 5 km/hr means the chemical is 30 km away unless it has been intercepted or pools somewhere. When the hazardous inversion conditions dissipate turbulence recommences and the air containing the chemicals is forced downwards. Hazardous inversion conditions dissipate when the sun rises and starts to heat the land surfaces, and sometimes during the night when wind patterns alter over the broader landscape. The air mixed with chemical that has accumulated for the duration of the hazardous inversion will now be forced downward by the turbulence and intercepted by whatever solid or semi solid surface is in that area. One of the sad things is that this could be directly over a town (because they generally are in the valleys), the house of another farmer, a sensitive crop (vines, tomatoes, cucumbers, trees etc), water courses etc. Whatever is in the boom spray does not discriminate as to where it goes. If Overwatch damage is evident then whatever else was in the tank is also where the Overwatch symptoms are. So, when everyone talks about 24-D they really are saying that everything in the tank has drifted – it’s just that 24-D leaves obvious symptoms on things like vines and tomatoes.
9. How much chemical is in the air and why is it so much more damaging now than years ago?
In an example of a very good spray operator doing the right thing, spraying at 50 ha an hour with the perfect boom and the nozzles mentioned in point 1, operating at no more than 20 km/hr and the boom at 50cm then 10% of the spray ‘brew’ is ending up in a layer under the inversion boundary and will come down somewhere. If that person sprays for 3 hours in an inversion, and the inversion dissipates soon after they spray, then at 5 km/hr their brew is up to 15 km away. If they spray, finish and the inversion continues for the rest of the night (say another 6 hours) then that ‘brew’ is between 30 and 45 km away unless it has pooled somewhere in the landscape. If they are using 2 l/ha glyphosate + 1 l/ha 24-D + 160 ml/ha Garlon then after 3 hours of spraying that’s 30 litres of glyphosate + 15 litres 24-D and 2.4 litres that is going up (to just under the inversion boundary) and will come down somewhere when turbulence recommences. Imagine how much is up there if 3 farmers are out spraying in a hazardous inversion using something like an Agrifac that is spraying 100 ha per hour. Then imagine how much is there when they are spraying at 30 km/hr, a boom height of 80cm or 1m, products with lots of adjuvant and nozzles that are only rated as coarse or very coarse. As much as 40% of that chemical is not having any impact on weed control and pumping 360 litres of glyphosate, 180 litres of 24-D and 28.8 litres of Garlon into the air over that 3 hour period. That amount of 24-D even diluted over a large area will belt every grapevine in the whole of the Clare Valley and everyone’s tomatoes, glory vines etc in just about every town on Yorke Peninsula.
10. Where does the spray go?
For as long as these hazardous inversion conditions exist, the air moves sideways and, if it has chemicals in it, they will be deposited somewhere. Deposition happens if they are intercepted, eg by a rise (locally or many kilometres away), or when the sun comes up and starts to reheat the land surfaces. When this happens the turbulence starts again and whatever is in the air will be brought down to ground level.
11. Farmers spraying have 3 choices to be law abiding.
Additionally they have duty of care in not harming others. This means there are three types of farmers and spray contractors:
- a. They are a farmer who doesn’t measure anything: so legally must stop 90 minutes before sunset and not start again until 90 minutes after sunrise.
- b. They measure a surface temperature inversion, often just called an inversion, such as with 10 metre towers with instruments at the top and near (within 3m) of the bottom: These operators legally must stop as soon as a temperature inversion is present. While it should be at the actual spray site itself, legal opinion is that it is ok if the weather station is located relatively nearby, or the operator can prove a consistent algorithm between the paddock being sprayed and a weather station measuring temperature inversions some distance away.
- c. They measure a hazardous temperature inversion: These operators legally must stop when a hazardous temperature inversion is present. They are able to spray when a surface temperature inversion is present, as long as it isn’t hazardous. The equipment to measure a hazardous inversion is expensive and the algorithms are covered by IP and licence arrangements. That’s why the Mesonet was established, so farmers didn’t have to spend huge amounts of money to get Hazardous Inversion data. If each farmer had to put in the technology they legally need for hazardous inversion detection it would cost them a fortune.
In reality there are, and will be, differences between times individual farmers can spray depending on the sort of technology they have invested in (Options a, b, and c above). It is not ok to do anything they feel they want to do.
