Q1: Why are weekly crop ET rates reported as inches per week instead of gallons per tree per week or per day when drip emitters, microsprinklers, and minisprinklers are rated in gallons per hour or gallons per minute?
We recognize the issue but need to report the weekly crop ET rates in inches per week for two basic reasons:
- A variety of orchard designs and tree spacing are used in irrigated agriculture so the number of trees per acre will vary. This affects tree size, orchards with more trees per acre tend to have smaller trees which use less water per tree per day and orchards with fewer trees per acre tend to grow larger and will use more water per tree per day. We cannot account for this variation in a concise, one-page weekly report for multiple crops and locations if we attempt to report crop ET rates in gallons per tree per week or per day. When we report crop ET in inches per week, variations in orchard design do not affect the values because it is based on all of the orchard vegetation (tree canopy and orchard middles) that uses water per acre.
- In the spring, like in 2019, rainfall in our region can be significant during the irrigation season. When crop ET is reported in inches per week, it is a straightforward comparison between ET and rainfall.
Q2: How can the weekly crop ET rates be converted to other units involving gallons?
- To convert from inches ET per week to gallons per acre per week?
1.0 inch of ET per acre equals 27,154 gallons of crop water use. So, weekly ET rates of 0.5, 1.0, 1.5, and 2.0 inches per acre converts to 13,577 (0.5” x 27,154), 27, 154 (1.0” x 27,154), 40,731 (1.5” x 27,154), and 54,308 (2.0“ x 27,154) gallons of water per week, respectively.
- To convert from gallons per acre per week to gallons per day?
Since there are seven days in a week, weekly ET rates expressed as gallons per acre per week can simply be converted to daily rates by dividing by 7.0. So weekly ET rates of 13,577, 27,154, 40,731, and 54,308 inches per week per acre converts to 1,940, 3,879, 5,819, and 7,758 gallons per acre per day, respectively.
- To convert from gallons per acre per day to gallons per tree per day?
Divide the gallons per acre per day by the number of trees per acre. As an example, assume the weekly ET is 2.0 inches per week for walnut, almond or pistachio, and prune. This equates to 7,758 gallons of water use per acre per day. For a walnut orchard with larger trees and wider spacing, trees per acre may be on the order of 90 trees per acre. In this case, daily water use would be 86 gallons per tree per day (7,758 ÷ 90 trees). For an almond or pistachio orchard with mid-size trees, the number of trees per acre may be on the order of 150 trees per acre. In this instance, daily water use per tree would be 52 gallons per tree per day (7,758 ÷ 150 trees). For a prune orchard with even smaller trees, the number of trees per acre may be about 200 trees and the daily water use per tree would be 38 gallons per tree per day (7,758 ÷ 200 trees).