Practice adjusted matched precipitation to eliminate dry spots

Matched precipitation is the science of providing a complete and uniform water application to a landscaped area. We attempt to achieve it because we’re very water conscious and want to have the best performance possible from an irrigation system, as well as happy customers with lower water bills.

Pre-design onsite evaluation is a must, and post-installation assessment and correction are equally important.

Let’s assume we have reached perfect matched precipitation on the plan or in the field. Now we must take it one step further and achieve what I call adjusted matched precipitation (AMP). It’s a phrase that I coined, and a practice that I (and probably many others) have been performing for years without even realizing it. It resulted from experience with real in-the-field problems that we’ve dealt with over the years.

But what do we mean by AMP? If matched precipitation is the science, then AMP is the art and the practice of applying that science in real-world situations. Unfortunately, we have found that many irrigation systems are designed from an office and without onsite inspections that take actual site conditions into account.

Pre-design evaluation onsite is a must, and post-installation assessment and correction are equally important. There are times when a system is installed according to plan and even adjusted right after installation, but there’s no follow up evaluation of performance under actual operating conditions. Often we come in long after installation and only know that the system doesn’t perform.

Here are some situations where AMP should be applied:

1. Varying soil conditions. Soils can sometimes vary from one sprinkler zone to another, which could result in dry spots. Soil that’s been excavated and backfilled often creates soils requiring AMP. This could be the result of prior construction activities or, for example, after installation of a septic system on a property. Even undisturbed soil can have different rates of percolation within a fairly small area and within the same sprinkler zone due to various conditions. Soils in some areas may be mostly fast draining sand and in other areas slowly draining clay.

2. Adjacent tree roots taking up water

3. Uneven terrain. Turfgrass and ornamentals on elevated areas and lower areas of the same property require different amounts of irrigation. This is another example of variable irrigation needs requiring AMP.

4. Shady versus sunny areas. We often see shady areas combined with sunny areas that require more moisture instead of having the areas on appropriately separated zones.

Even in perfect matched precipitation irrigation design and installation you will get dry spots that repeatedly occur in the same area every time. Often, at the first sign of a dry area people adjust their irrigation controllers by adding additional days on the schedule, or additional time or start times. This isn’t a good practice to compensate for that one small area. Rarely do they only address that specific zone, let alone address just that head or specific area within that zone. It’s too easy just to adjust the clock

With today’s technology we can easily change out the nozzles or use other methods to increase (or decrease) the volume at each individual head. Here’s how we correct these types of problems in the field:

  • Shady/sunny areas on a single zone: You can reduce the size of the nozzle (gpm) in the shady area to apply less volume of water if the zone cannot be separated. By doing so, you are now intentionally straying away from matched precipitation to achieve AMP.
  • Dry spots: You may have continual dry spots on an older system or even on a perfectly designed new installation. If you’re evaluating a new installation, check this after the turfgrass is well-established and during a mild warm up. (You can’t use a period of extreme heat, when it is triple-digit temperatures, for an accurate assessment.) Look for the areas that consistently dry out first. On an established lawn this will be apparent by the brown spots.

Every landscape irrigation system needs on-site followup to make sure it’s operating efficiently.

Hand-water that particular dry area one day and, if on the following day many more dry spots appear elsewhere, then you may not need to make any changes other than to the watering schedule. However, if you hand-water that area, and after more than two days the rest of the turf is not showing significant stress, then make adjustments to that area. Remember, you cannot do this in triple-digit heat to get an accurate assessment.

You must increase the gpm of the head serving the dry area to compensate. In rotor systems this is done very easily by changing out the nozzle to a higher flow (gpm) nozzle. With fixed spray nozzles it’s a little more difficult, but we do this quite often with Toro 570 series nozzles by removing the pressure compensating device (PCD) or by using a larger PCD designed for the next size up trajectory. According to Toro, this can increase the gpm by up to 30 percent.

These methods are simple to incorporate, and we prefer to do this first in order to try and preserve the integrity of the original irrigation design and layout, before we start adding more heads to a line. Increasing the amount of water to the one area that dries out sooner than all the rest may save a day, or more, per week on a water cycle. Just think how much water that can save.

Note that this in-the-field practice of AMP should be taken into consideration at the design phase. If a design utilizes 100 percent of the flow/fps available, then the only way to achieve AMP is to reduce the size (flow/gpm) of all the rest of the nozzles. This is neither desirable nor practical and can complicate things. So when we design a system we try and leave some reserve flow. That way, if we need to change out a nozzle to increase flow in a particular area, we have the flexibility to do so.

In a time when water conservation is an extremely important part of designing and installing an irrigation system, the industry as a whole is constantly looking for new ways to adequately irrigate, with the least amount of wasted water.

Our colleges and universities teach that matched precipitation is one of those solutions to help achieve that goal. Actually, we should take it a step further and incorporate adjusted matched precipitation as a fundamental part of its training in design, installation and practical field techniques. After all, the goal is a system that conserves water resources, and works to keep the landscape adequately irrigated and looking great.

Eric Whipp is the owner of Landscape Technologies, San Dimas, Calif.