How to successfully use regional evapotranspiration (ET) estimates

When using ET estimates from the UNL Weather Data Network, the data provided may not seem to match actual field conditions. Sometimes the problem may be with the weather station, however, more often it's due to the user not properly adjusting the data for a particular field situation. Following is a list of possible problems and remedial actions that can help ensure successful use of the ET information for irrigation scheduling.

1. The weather station site has improper exposure. ET estimates can be higher or lower than ET of the irrigated field. Watch ET from surrounding weather stations — the closest station is not always the most representative. Variations in temperature, humidity, and wind occur over short distances as the topography changes. If the irrigated field has an unusual exposure, a fixed percentage adjustment to the data may be required.

2. The estimate of crop growth stage does not match the actual growth stage for the irrigated field. Many hybrids are grown in the state and it is not possible to specifically represent each one in the ET estimates. Check the growth stage in the field and choose the maturity class(es) that result in the closest estimate(s) of growth stage. Watch several classes to see hybrid differences. The hybrid in question will fit somewhere in between.

3. Prior to full canopy, rain or irrigation has made the soil surface wet. Actual crop ET rates may be slightly higher than the estimates. If there is a higher than usual frequency of days with precipitation or irrigation make a check of the actual soil moisture conditions.

4. Prior to irrigation, there is a dry root zone in the field. Actual crop ET rates may be lower than the estimated ET. Check the root zone for soil moisture and irrigate if conditions warrant.

5. There are differences in emergence dates on irrigated fields. Obtain a separate ET estimate for each irrigated field by selecting the crop and emergence date for all scheduled fields.

6. Field observations of soil moisture are consistently higher than estimates from scheduling programs. The scheduling programs may not properly represent drainage in the soils. For slow draining soils, the water (above field capacity) is actually available to the crop for a longer time than in a faster draining soil. The 'effective' field capacity is higher after rainfall or irrigation.

7. The plant population in the irrigated field is higher or lower than average. Recognize that a higher population will have higher ET and a lower population will have lower ET in the early and late season. Differences during mid-season disappear as both population densities have sufficient leaf area.

Darrell Watts, Extension Water Quality Specialist
Kenneth Hubbard, Director, High Plains Climate Center
DeLynn Hay, Extension Specialist for Water Resources and Irrigation

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