Irrigation Audit

What is an irrigation audit? Unlike the dreaded IRS audit, this is one you want in order to see whether your garden and landscape are getting the right amount of water.

Paul James works with gardener Kelley Moritz to measure how much moisture her plants are getting each time she waters. Kelley’s approach to watering her lawn and landscape is identical to that of millions of gardeners, with different types of manually placed sprinklers, and for the most part it works well.

The only way to know if plants are getting the water they need, however, is to conduct an irrigation audit. First, Paul and Kelley measure how much water the plants are getting; second, they determine how much water is being absorbed by the soil.

The general rule of thumb for watering is approximately one inch per week for the entire landscape, maybe a bit more in the summer or during prolonged periods of drought. You want the water to percolate to a depth of at least three to four inches.

Figure A

An irrigation audit is low-tech and easy. To measure the amount of water a sprinkler delivers, Paul and Kelley place plastic cups in various places within the range of each sprinkler, in every section of lawn and/or bed. If you try this yourself, be sure to place a cup right by the sprinkler (figure A); some sprinklers deliver either the least or most amount of water at this spot.

Figure B

Now turn on your sprinkler and let it run. Kelley uses a conventional oscillating sprinkler for 45 minutes to an hour for her lawn. After letting the sprinkler run the typical amount of time, check the cups and see if the amount of water in the cups from the various areas is roughly the same (figure B). In Kelley’s case, in slightly less than one hour’s time, the sprinkler delivered approximately one inch of water to Kelley’s lawn. Record the information from one section, then move on to the next section of lawn or bed until you've measured the results of your entire lawn and landscape.

Step two involves a little digging to measure how deeply into the soil the water is being absorbed. How much water is actually absorbed can vary tremendously with soil type.

Figure C

To measure that, Paul and Kelley use trowels to examine cross-sections of the soil in the various areas of lawn and beds that have just been watered (figure C).

The results? It turns out Kelley’s lawn is definitely getting enough water. The water perked three inches deep, which is roughly the depth of the root zone. Kelley’s patio area is getting a little more water than it actually needs, so she can shorten the amount of time she runs the sprinkler there.

Figure D

After you perform your own irrigation audit there are a couple of easy, inexpensive things you can do to maximize your watering: replace the washers at the faucet and on sprinklers regularly, and install water timers on the faucet (figure D) so you can perform other gardening tasks without having to keep track of the time.