# January 25, 2014

## 1 - Irrigation is more complicated than just supplying water to plants.

Yard and Garden January 25, 2014

Q.

Where can I find a formula to convert gallons per minute into inches of water; as in gallons per minute from a lawn sprinkler head covering 100 sq. ft.?

-Bob D.

Via NMSU University Wide Extension

A.

A local irrigation supply company may be a good place to find a specific formula for this conversion. Some municipalities may also have information for their utility customers to help them conserve water.

You can calculate the information you need by starting with the fact that a cubic inch of water is 0.00432900433 gallon. A square foot is 12 inches times 12 inches, or 144 square inches. So a square foot of water one inch deep is 144 cubic inches. If you multiply this by 0.00433 gallons per cubic inch the result is 0.624 gallons. You can then multiply this by the area you are irrigating. Since you said 100 square feet it will take 62.4 gallons to cover 100 square feet with one inch of water. With this information you calculate the time various sprinkler heads must operate to supply the required water based on the gallons per minute supplied by the sprinkler heads.

When you are irrigating plants there are other factors that you should consider. Spray or sprinkler irrigation allows for considerable loss of water by evaporation. The smaller the droplets, the greater the evaporation and the more impact wind will have on the placement of the irrigation water. This is the reason that drip irrigation is more efficient than sprinkler irrigation in situations where it is appropriate. Another important fact is that the deposition of water in the zone of irrigation is non-uniform, so proper design of the irrigation system and placement of sprinklers is important and testing to measure uniformity is important.

Temperature and relative humidity also affect evaporative water loss. The amount of water needed to irrigate plants depends on the type of plants being irrigated, the characteristics of the plants root systems and the type of soil in which the plants are growing. Season also affects plants' water needs. Dormant plants need water, but much less water than plants that are actively growing.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.