by James W. Bauder, Extension Soil and Water Quality Specialist

More than two decades ago, researchers at Montana State University worked to develop simplified methods to help irrigators make decisions about irrigation water management. The outcome of that effort was referred to as "Evaporation Pan" or washtub irrigation scheduling. The following information reviews some of the findings and recommendations of those studies.      


A Simple Approach
A wide variety of methods can be used to measure or estimate water use by plants, which is valuable when deciding when and how much water to apply. The evaporation pan or washtub method is a simple, inexpensive, and readily understandable way to estimate irrigated crop water use. It also requires little attention and can provide relatively reliable estimates of plant water use on a daily or weekly basis.   


        A 20" diameter or larger washtub is used as a reservoir, which is visually monitored for amount of water stored. The pan should be covered with a one-inch mesh chicken wire and placed on a couple of concrete or wooden blocks in or near the irrigated field. You also will need a rain gauge, a ruler, and a five-gallon water jug to periodically refill the pan (sort of like when you irrigate).

      How it All Works
The approach or ‘trick' to using an evaporation pan or wash tub is to fill the pan with about eight inches of water. Then, monitor it on a daily or fairly frequent schedule to see the rate at which water evaporates from the pan. The change in depth of water in the pan is then related by a "crop factor" to water use by the irrigated crop.

To use the pan, measure water use for a day or short period of time, keeping track of the value or depth at the start of the season or when the pan is refilled following heavy rain or irrigation. A checkbook approach is used to schedule irrigations. The value for daily water use or for a few days is subtracted from the stored soil water, and when the depletable water is exhausted, it is time to irrigate      

Fluctuations in Seasonal Water Use
Actual water use by plants early in the growing season is substantially less than evaporation from the pan, mainly because the plant is small with shallow roots, and weather conditions are relatively mild. At mid-season, plant water use may be about the same as pan evaporation, and as the crop approaches maturity, water use decreases again. Despite these fluctuations, evaporation from the pan provides a good approximation. If you want a more accurate value for crop water use, adjust the crop factor values. Simply multiply the inches of water evaporated (the change in depth of water in the pan since the last reading) by the crop factor. The net result is the inches of water used by the plant. When this value approaches the amount of water you can apply with your irrigation system or the amount of water that can be stored in the soil, it is time to irrigate again.      

The entire process of setting up an evaporation pan is explained in detail in the Montana State University Extension Service publication "Using Evaporation Tubs to Schedule Irrigations (1986) MT198343AG." Contact Extension Publications or your local County Agent for more information.