How Many Gallons of Water in an Inch?

Q. You posted a graph on your blog of inches of water plants use each day during each month during the year. How many gallons is an inch of water?

Potential evapotranspiration for the Las Vegas Valley over 12 months in average inches of water used per day. Numbers across the bottom stand for months; 1 = January, 12 = December.

A. I bet you want to know in minutes! That’s one problem when talking about irrigation. Irrigation clocks measure the volume of water in minutes. We apply water as a depth or in gallons, not minutes.

            An acre-foot of water is roughly 325,900 gallons. An acre inch is roughly 1/12 of that which is 27,158 gallons. One inch of water in a one cubic foot container is 7.48/12 = 0.62 gallons.

 Taken from my manual, Basic Horticulture

            One inch of water applied to pure sand penetrates to a depth of about 20 inches. Fine sand, 14 inches deep. Fine sandy loam, 10 inches. Silt loam, 7 inches and clay loam, 6 inches.

            The amount of water to apply is determined by the depth of its roots. The shallowest rooted plants are lawns, annual flowers and annual vegetables. We assume the depth of their roots is less than a foot. The next deepest-rooted plants are 2 to 4 foot tall perennials with a rooting depth of 12 to 18 inches. And finally, trees and large shrubs are the deepest with an effective route depth of about 24 inches.

          

  Larger plants are given more water but watered less often because their "gas tank" (water held in the soil available to the roots) is much bigger. Plants that are shallow rooted like lawns, annual flowers and vegetables are watered more often because their "gas tank" is much smaller.

            It is very important to group these categories of plants (lawns/flowers/vegetables, medium sized plants, trees and large shrubs) on separate irrigation valves. In this way they can be watered separately and at different times. Fourth and fifth categories, desert plants and cacti, could also be argued.