Dry-climate gardens depend on you, the gardener, for sustenance. To calculate supplemental water provided in drip irrigation, begin with this formula:

Drip Intake =  Emitter size  X  Number of emitters  X  Run time  X  Run Frequency

• Emitters, regardless of design type, disperse water at a controlled rate from drip lines to plant material. Sizes range from 0.5, 1, 2, 4, 6, and even 9 gallons per hour (gph). Some gardeners prefer small ones so their flow is fully absorbed; others like large ones that never clog.

• Number of emitters can vary. While one is fine for some plants, roots can seek water better from two. Trees and large shrubs can take three or more encircled under their canopy.

• Run time is how long the irrigation is set to run on the controller. Each zone, or grouping of plants interconnected via drip lines, is watered simultaneously. Changing run times affects all plants in that zone.

• Run frequency is how often the water is on and, depending on the controller, can be set for particular days, number of times a week, number of days between watering, or manual. Experts recommend deep watering with longer run times and less frequency when water is scarce.

To literally see your existing setup’s impact, zero in on a few plants, apply the formula, and fill a bucket with that water volume. For instance, if one plant has two 2gph emitters and is in a zone that runs 45 minutes twice weekly, it gets 6 gallons each week (2 x 2 x 45/60 x 2). If you hand watered, would you give it that much, or more, or less?

Examine species’ standard consumption in listings such as SanMarcosGrowers.com, Sunset Western Garden Book, and WUCOL (Water Use Classification of Landscape Species). Since actual water use can vary with on-site conditions, factor in your soil composition, sun/shade exposure, and slopes. Drought-tolerant plants are thirstier in hot sun or sandy soil; moderate-moisture ones need less water in partial shade; water can flow right by any plant on a steep clay slope.

Although a wealth of irrigation calculators exist for crop farmers and commercial installations, residential applications are few and far between. Two exceptions worth exploring are the city of Austin, Texas’s Irrigation Runtime Calculator and Metropolitan Water District of Southern California’s Watering Calculator. Although their watering schedules are area specific, their interplay of emitters, plant types, soil, exposure, and degree of slope is of universal relevance.

After going through this exercise, I cut run times and frequencies for every single zone in our garden. In parallel, I downsized almost all emitters from 2 gph to 1 gph or 0.5 gph, and depending on the plant and location, selectively added a 2nd emitter. Total water use plummeted, and to my surprise, the garden became more robust and lush than before.

To think one small step of calculating drip intake became one giant leap for sustainability…