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MMWD Blog: Think Blue Marin

Welcome to our blog! Written by staff at MMWD, “Think Blue Marin” explores all things water in south and central Marin—water supplies, conservation, new projects, watershed management, and more.

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Aug 03

Proof

Posted on August 3, 2018 at 10:37 AM by Ann Vallee

by Charlene Burgi

The proverbial saying that “the proof is in the pudding” comes to mind as we monitor our irrigation systems during the heat of summer. During this time of year, dry spots may appear in our lawns or other groundcovers, or we may see tender plants wither or wilt due to lack of water. 

Without uniform distribution, some parts of the garden cannot get adequate water from our irrigation systems. And in trying to reach those dry areas, we end up flooding other areas and wasting water to compensate for a poorly designed system.

 Using ag risers for unblocked head to head coverage
Using ag risers for unblocked
head-to-head coverage
What does a good irrigation design look like? Appearance is easy to describe. Irrigation heads on each station or valve need to be installed so the water from one sprinkler head will reach out touching surrounding sprinkler heads. However, the calculations of friction loss through pipes, and numbers such as gallons per minute, water pressure and elevation changes are but a few of the hidden secrets that equate to good irrigation design.

For example, the aforementioned head-to-head coverage may look adequate from above the ground, but if the water pressure is lacking to push the water the distance needed to reach the other heads, the design will fail. Or, there may be adequate pressure but too many heads on one valve. As a result, the valves need more gallons per minute than the pipes can deliver, causing the water to fall short of the other targeted sprinkler heads.

Another fatal flaw in irrigation design is mixing different types of irrigation on the same valve. One common mistake I see is drip spray heads on the same valve as drip emitters. Drip sprays apply water measured in gallons per minute (GPM) while drip emitters discharge water measured in gallons per hour (GPH). By mixing the two methods of irrigation on the same valve, some plants will be flooded with GPM while other plants will be starved for water dribbling out in GPH.

The same holds true for overhead spray systems. The goal is to have the overhead irrigation sprays provide an equal gallons per minute to the area they are covering. We call this having matched precipitation rates. This can only be achieved by using the same model nozzles on the same valve. For example, mixing an impact head with a rotor on the same valve would be a disaster for the plants: Given the same pressure, a rotor will emit roughly three gallons of water per minute and an impact head will throw out over five gallons of water per minute. You can see how under this scenario some plants will lack water while others get too much. This mix-and-match problem can even occur by using the same type of spray heads but from different manufacturers.

In essence, poor irrigation system designs equate to water waste, not to mention the stress placed on your plants. Take a walk in the garden with your sprinklers on to see how well your system is working. While you are at it, check for broken lines, missing emitters and nozzles that are spraying in unintended areas. Correct the problems and you will find your efforts reflected in lower water bills and happier plants!

I will be off line next week casting a fly into the beautiful lakes above Kamloops. I promise to share more about the beauty of native plants in British Columbia upon my return. Until then, ciao!

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