“Why are there so many sprinklers in that little area?” If I had a nickel for everytime I was asked that question, well let’s just say I’d have a lot of nickels! For some reason nothing brings into question an irrigation designer’s skill more than his or her proposed layout of small, isolated landscape areas. It’s a legitimate question really, especially when a client knows that an entire football field for example can technically be irrigated with as few as a dozen sprinklers. So let’s spend some time answering that all-too-common question.
How does it WOrk?
To understand why small or narrow landscapes are a problem, we have to look at how irrigation products actually work. It’s easy to assume that when water comes out of a sprinkler, it covers everything it gets wet evenly but nothing could be further from the truth. The reality is that the amount of water making it to the landscape varies the further away it gets from the sprinkler. This is true regardless of the type (fixed sprays or rotors for example) or manufacturer. Let’s look at some examples:
Here we can see some typical large irrigation rotors. What we’re seeing in each case is that the sprinkler is sending bigger stream of water with bigger droplets out to the furthest reaches while there is comparatively less water with smaller droplets in close to the head. It’s tempting to think that this would lead to more water the further away we get from the sprinkler but the reverse is actually true. Why? It has to do with geometry:
Or looking at a densogram showing darker blue where there is more water, it looks like this:
The area to be watered gets larger as the distance from the sprinkler increases so there is less water for more landscape. If we were able to look at a cross section of the soil with one sprinkler operating, this is what we would see:
Add more sprinklers with improper spacing and we would see this:
Because the goal of automatic irrigation is to apply water as efficiently and evenly as possible, this is less than ideal but it’s more or less fixable by designing with what irrigation people call a “head to head” layout. That means we locate sprinklers so that the spray from one reaches the next and vice versa. In section, it looks like this:
Although it’s still not perfect, this is just about as good as it gets with automatic irrigation because head to head design will be anywhere from 55%-75% efficient (and believe it or not, even rain isn’t 100% efficient!). In densogram form, good head to head coverage looks like this:
So what does all this have to do with irrigating small areas? Well, in order to achieve head to head coverage for the best efficiency, we have to space sprinklers no further apart than they can spray water and sometimes that’s very close. Think about a narrow area like a parking lot island for example:
In this case, the heads can only spray as far as the distance from the sidewalk to the curb. Any more and they will overspray the parking lot or sidewalk and that not only wastes water but also exposes the property owner to legal issues should anyone slip and fall. As you can see, the long and narrow area requires many heads to achieve proper coverage without overspray.
Notice in this layout how the large, open area in the middle has fewer heads than the small, narrow area at the right? This is why irrigation designers love wide open spaces!
There’s a Rhyme and a Reason
The next question I usually get is “can’t we just spray one way?” and that seems like it would make sense. After all, that would be half as many heads. Think back to the densogram of the single sprinkler though and you’ll remember that there is less and less water as you get away from the sprinkler. If we had a single row of sprinklers spraying one way, the densogram would look like this:
Here it’s easy to see how dry the edge without sprinklers will be. Add to this the heat island effect caused by nearby pavement and it’s clear that grass at the far edge will suffer (this option would possibly work for other plant material). The only way for a single row of sprinklers to effectively water a turf area this shape would be to overspray the edge by 50%, meaning that for a 5’ wide area, we would have to water 2.5’ over the edge which is bad news if that edge is a curb or sidewalk.
Small or narrow areas offer one less obvious drawback as well: they’re expensive! Not only does the larger number of sprinklers add to the cost but more sprinklers require more pipe, wire, valves, valve boxes, and so on. Worst of all, labor costs increase because all of this material has to be installed in a confined space where it is often difficult to use a machine. Hand digging is one of the fastest ways to increase the cost of a job.
In the field, the smaller the space, the harder the installation:
While bigger may not always be better, in the irrigation world it’s almost always less expensive!