Gone are the days when water was poured on to keep the whole surface dark green,
without any other consideration. Today, we no longer have that luxury and we must apply just the right amount of water, but no more, to keep the playing or recreational area looking lush and healthy. This is where the notion of the efficiency of the irrigation system comes into play.
Irrigation professionals and specialised consultants will, in the course of their study, take a number of technical elements into account in order to lay out the sprinklers in such a way that the irrigation system is fully optimised and the minimum amount of water is used to achieve maximum efficiency.
Do not confuse Uniformity with Efficiency
A good irrigation system must strictly apply the exact amount of water required by the plants and distribute the water uniformly across the whole irrigated area.
Uniformity: The water must be distributed as uniformly as possible over the whole irrigated surface to be irrigated in order to avoid areas which are too dry or too wet. The uniformity of distribution is expressed by a statistical value known as DU. This value depends on the spacing of the sprinklers and their distribution curve. This value is also affected by the wind (speed and direction) and by the pressure of the system.
Efficiency: The efficiency of an irrigation is measured by the ratio between the water that filtrates through the soil and is stored around the root area and the amount of water actually applied. A system could have an excellent DU but a very poor efficiency caused by having too lengthy irrigation run times, leading to water losses. Efficiency is above all affected by the level of irrigation management.
A good irrigation installation would be a system that applies exactly the same amount of water and distributes it over the whole irrigated area, just as far as the rooting system, and this would thus have 100% uniformity. This would be impossible to achieve under real conditions.
A bit of theory
A sprinkler, whatever the make, with a given nozzle and pressure, has a distance/ precipitation rate curve of the following type.
In this example, the radius of throw is 23 metres and the hourly precipitation rate varies from 0 to 19.5 mm/hour. We note that there is less water (applied) the further we go from the sprinkler.
This is due to the fact that the surface area covered by the stream of water becomes larger the further you go from the sprinkler and so it receives less and less water.
In order to ensure that the water is applied more or less evenly over the whole surface area then several sprinklers will have to overlap.
Each manufacturer offers a performance data sheet for its sprinklers. These performances are determined under optimum conditions that would never be found in the field. That is why it is important to take great care when choosing the nozzles and radius of throw, with the knowledge that a sprinkler indicating, for example, a range of throw of 20 metres will never reach that distance in the field.
Layout for the sprinklers
The positioning of the sprinklers in the field is very important and will have an important part to play in the quality of the irrigation.
There are a number of options for laying out the sprinklers. The most common patterns are square or triangular. It is also possible to position the sprinklers in a rectangular or staggered form or in a straight line.
The shape of the field to be irrigated and wind direction will have an influence over the layout pattern.
Providing that the ground is suitable, a triangular pattern is more efficient than a square one. The straight-line configuration is the least efficient.
Indicators of a good irrigation system
How to know whether an irrigation system is performing properly?
A number of uniformity indicators are used:
- CU or Christiansen Coefficient of Uniformity
- DU or Distribution Uniformity
- SC or Scheduling Coefficient
It should be noted that the CU is mainly used in agriculture and the DU and SC are mainly used in Landscaping.
The higher the CU and DU percentage, the better the distribution of the water
SC (Scheduling Coefficient) is an irrigation run time multiplier. It gives an indication of what should be the maximum run time for applying, on the zone that receives the least water, the equivalent of the zone’s average precipitation rate. We therefore aim to have an SC as close to 1 as possible.
The theory is all well and good, but in practice…
The CIT (Center for Irrigation Technology), an independent US organisation, has developed a software (Space Pro), that enables the user to simulate the quality of the irrigation system with respect to the type of sprinkler, its nozzle, pressure and layout. They also tested all the main makes of sprinklers, thus providing us with a good basis for comparison.
Let us take the following example:
- Toro DT (Flex) 35 sprinkler with 35 nozzle
- Flow rate 8.6 m3/h at 5.5 bar – Theoretical radius of throw of 22.9 metres.
- Influence of the spacing overlap
- 22-metre square spacing
- 22-metre equilateral spacing
The blue square represents the zone that receives the most water and the red square represents that which receives the least water.
So let us compare the calculated coefficients.
Even if the results are better with a triangular pattern, the CU and DU are not bad with a square layout. However, the SC is quite different. With a square layout, in order to apply the zone’s average precipitation rate (33.2 mm/h) to the drier zone (red square), then the irrigation run time will have to be increased by a factor of 1.3 compared with 1.1 for a triangular pattern. This, of course, would lead to overirrigation of the zone represented by the blue square.
For an ETP of 5 mm, the irrigation run time and volume of water applied, would be:
A square layout pattern will, therefore, consume 355 litres more per sprinkler in applying the average precipitation rate over the whole surface area. And that is without taking over-irrigation into account, on the zones which are already receiving more than the average.
- Choice of the best overlap
Space Pro also gives us information about the overlap that would give the best results in terms of CU, DU and SC.
In the above example, the best spacing, for this type of sprinkler with a well-defined nozzle and pressure, would be around 23 by 20 metres (equilateral triangle).
Having good irrigation uniformity and limiting irrigation to the precise amount of water needed, requires a knowledge of hydraulics and a certain technical know-how, which can only be found with the specialised contractors and consultants.
The importance of the choice of nozzles, the pressure, layout pattern and spacing of the sprinklers cannot be stressed enough when striving to obtain irrigation installations of quality.