January 16, 2012

According to John: Busting the Water Pressure Myth

While we prepare to kick off the 2015 gardening season, it’s good to step back and take stock of our techniques and protocols.  We believe there is nothing more important for successful gardening than healthy soil. However right up there in importance is proper watering. Toward that end, many gardeners around the country apply water to their gardens with irrigation systems.  Several years ago one of my neighbors, who is an avid Master Gardener, was depressed because her pressure in her irrigation system was suddenly terrible and many of her plants were dying and her turf had dry spots. She asked me to help and after we fixed the problem, she said, “John, I always thought diamonds were the most valuable thing in my life, but I was mistaken; now I know it is water pressure.”

Since it is so important to a good irrigation system and to my neighbor, let’s clear up a common misconception. Do you think we get more pressure to a sprinkler if we sized the final 40 feet of pipe as ½ inch or 2 inch?   Most people (and many landscape and horticultural professionals) answer ½ inch, because they think the smaller size creates pressure. The correct answer is 2 inch pipe because we lose pressure due to friction as water runs through pipe, and the smaller the pipe, the more friction there is. Velocity is increased with smaller pipe sizes, but not pressure. To maximize water pressure: the bigger the pipe, the better.   Many are disbelievers and get very argumentative and demand to know, if this is so, why we always use smaller pipe at the end of lines? The answer is the same as the answer to many questions one might ask: Cost. Good irrigation designers calculate the smallest pipe that can be used without losing unacceptable amounts of pressure due to friction. Smaller pipe is much less expensive than big pipe.   So, remember: for better water pressure, go for the bigger pipe. And the next time there is a special occasion in your life, forget the diamonds and install a bigger pipe instead!