Scheduling irrigation's using reference evapotranspiration and an irrigation controller

Ted Sammis

Reference evaporation also called potential evapotranaspiration (Pet) is calculated from climate data and is the water used by a low growing grass usually fescue. The calculation reference evapotranspiration can be acquired from most state climatology internet home pages or can be calculated using temperature data and the Potential Crop Evapotranspiration Wizard on the New Mexico climate center home page. Potential evapotranspiration in inches/day needs to be scaled down to represent the grass or type of ground cover grown. Crop scaling factors (kc) for selected vegetation can be acquired on the internet. The calculated evapotranspiration changes daily according to the changing climate. Landscape can be irrigated using sprinklers or trickle irrigation systems controlled by an irrigation controller. The irrigation controller turns on the different valves for programmed start and stop times. Different plant material have different irrigation requirements and each type of plant material should have a separate valve to control the amount of water delivered to that plant material.The amount of water that needs to be applied is equal to:

Water application time= reference evapotranspiration *number of days from last irrigation * scaling factor (kc) / application rate of the irrigation system


reference evapotranspiration = 0.20 inches/day
number of days since last irrigation = 2 day
scaling factor for warm season grass =0.75
application rate of sprinkler system =0.4 inches/hr

water application time= 0.2*2*0.75 /0.4=0.75hr=45minutes

Consequently, the program on the irrigation controller should have a start time of 6:00 am and a stop time of 6:45am. However, if the reference evapotranspiration changes the next week and the value is 0.35 inches/day then the stop time must be set at 7:18 am. The time adjustment must be made for all stations in proportion to the change in reference evapotranspiration. If the controller has a lot of stations, the time required to reprogram the controller becomes excessive and most individuals will not reprogram the controlling as the evapotranspiration requirement of the ground cover changes with changing climate. Rain Bird and other manufactures of irrigation controllers sell controllers that have a system water budgeting option that varies from 10 to 200% of the programmed time in increments of 10%. Consequently, if the controller was programmed for 45 minutes based on a reference evapotranspiration of 0.2 inches/day and the reference evapotranspiration changed to 0.35 inches/day then the system water budget option would be changed from 100% to (0.35/0.20) 175% or 180% because of the 10% increment adjustment. Consequently, irrigation controllers should be programmed and then the system water budget option percentage adjusted as the water requirement of plants change due to changes in climate. The suggestion is to reprogram the controller each week based on the change in average reference evapotranspiration for the previous week compared to the value used when calculating the watering time in the initial programming of the controller.


If you have any questions please contact
Updated: Aug. 18 1997
Copyright 1996 New Mexico State University
Department of Agronomy and Horticulture
Box 30001 / Dept.3Q / Las Cruces, N M 88003-8003
Telephone: (505)646-3405
FAX: (505)646-6041