Golfers with a bad swing may often try to fix it not by getting at the source, but by making a wrong adjustment to the original problem. A golfer who slices the ball may turn the clubface in to compensate instead of adjusting his swing.
The same could be said for irrigation water management long after the contractors have left. Projects will often require some form of manipulation, such as resetting of budgets, reducing the irrigated area, changing planting type, reducing the imported soil depth and reducing sprinkler heads and relying purely on the manufacturer technical data.
Sooner rather than later, unintended consequences emerge, such as overwatering in some areas, dry spots in others, or a higher than expected water bill. Instead of reversing the original mistake, the landscaper or water manager will conduct another manipulation to fix the new problem, only to create further problems down the road. Increasing the run times just once is a common solution to ensure the dry patches or flaws are removed. Landscapers through no initial control themselves adapt to only what has been inherited, and soon resemble bad golfers, ending up twisted into a pretzel shape because of all the ‘adjustments’.
When we look at larger turf areas to be irrigated, there is a common irrigation standard that has always been a rule-of-thumb mentality from years gone past to inform users on how we set-out sprinklers. Look at the manufacturer data, decide what nozzle and operating pressure you wish to use and base your calculations on how far the sprinkler throws water in metres. The rule-of-thumb method simply tells us how far the user should space the sprinklers. If the tech data shows a 16.6-metre radius, we space the sprinklers in a typical square spacing grid configuration 16.6 metres apart. Rule-of-thumb, nothing can wrong! We can measure the sprinkler uniformity after the design, installation and commissioning are long gone to work out why the system is not performing via on-site irrigation audits using catch-cans and analysis. This is costly, time consuming, and if the initial design did not have a theoretical starting point basis, the audit is only a measure of confirming what we already know. I am using too much water and still have dry patches.
Let’s explore this myth a bit further with how a gear drive or rotor sprinkler operates. Water enters the base of the sprinkler. From there, it passes through a filter screen and then through a turbine. The water turns the turbine, which powers a set of gears, which rotate the sprinkler nozzle. The water then passes up through the body and exits through the nozzle. Generally, the nozzles can be removed and there are numerous nozzle sizes available.
What happens to the water when it throws out over different distances? Gravity takes over and the droplets fall to the ground, but just because we can we can see these droplets does not mean it is uniform across the entire area. Every reputable manufacturer tests these results with all nozzles in the range, at all pressures in their range and catches the water into cups along the distance to see exactly where the water drops and at what rate.
The first tests are done in a large shed with no wind. For the second tests, the documented results on all tech sheets do not often show the results but simplify by stating the absolute furthest distance at which a cup caught a single droplet of water and marked it as 16.6 metres maximum distance. It didn’t say there were over 150 drops of water at the two-metre mark, then 90 drops at eight-metre mark, 20 drops at the 12-metre and for some inexplicable reason back up to 50 drops at the 15-metre mark.
Here are two typical sprinkler profiles showing similar distances operating at the same pressure but quite different results on where the droplets fall and at what rate.
Both sprinklers are from reputable manufacturers, reliable and offer good performance when used in the correct circumstances either when installing or pre-prepared by certified irrigation designers. The best way to determine the true effectiveness is to enter the preferred sprinkler spacing and row spacing to suit the area into a software program with all the raw data that the manufacturers provide as well as any independent tests to see how a single sprinkler profile (as above) performs when the overlap is applied of the sprinklers from four directions. The below is a snapshot with three performance indicators showing Coefficient of Uniformity (CU), Distribution of Uniformity (DU), and Scheduling Coefficient (SC).
DU and SC are particularly good indicators to show how we can save tremendous amounts of water, associated water costs and ensure the landscape can be managed effectively.
DU compares the average of the lowest 25 per cent of test can readings to the average of all readings. A DU of 100 per cent would indicate that the application was perfectly even. In practice, this does not happen. It is generally accepted that sprinkler systems for turf should have a minimum DU of 75 per cent.
SC is used to provide a time adjustment factor to ensure that the dry or under watered areas receive an adequate depth of application. An efficient irrigation system should aim to achieve a scheduling coefficient less than 1.3.
The same sprinkler manufacturer, same nozzle and operating at the same pressure has been modified from the first example to a new sprinkler spacing at the designer’s discretion to improve uniformity. The DU has gone from 74 per cent to 87 per cent and SC has gone from 1.4 to 1.2. Yes, reduced sprinkler spacing will result in slightly more sprinklers, adding around 10 to 15 per cent to the capital cost, but this decision will save thousands of dollars over the lifespan of the system, typically 20 to 25 years. Lets see how.
A recent Sourceable article showed water requirements using simple time based irrigation versus weather based. What it did not look at was the effectiveness and uniformity of the irrigation system. Let us look at the above example when irrigating 14,000 square metres of turf. The table below simply applies this same water requirement but adjustments should be made based on the percentages received from the data on the uniformity of the system primarily on the SC, using actual efficiency based on real data.
As a quick explanation for both options we can see:
Option 1: DU = 74 per cent equals (1/0.74) = 1.35 (rounded up to 1.4)
Option 2: DU = 87 per cent (1/0.87) = 1.15 (rounded up to 1.2)
We can safely say that all major irrigation manufacturers across the world have their own internal data as well as independent testing of their sprinkler models that provide the sprinkler profiles as outlined above. Whilst these manufacturers have points of difference that distinguish them from their competitors, when it comes to rotors, a designer can make any sprinkler high in uniformity and look good provided they have the correct data to work with.
The key is knowing how to apply this to the specifics of the design, asking for preferences of the customer on the manufacturer they are most comfortable, discussing some of the ancillary benefits of operation of that sprinkler and how the cost measures up to perform at high uniformity. If the sprinkler can perform at higher uniformity with wider spacing, therefore using less sprinklers, this will be the best point-of-difference as it can reduce initial capital cost and save water and associated costs.