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Predicting the last irrigation of the year


This growing season started with a wet, cool spring that delayed planting in some areas. The net result is that crops may mature in different areas of the state over a longer time than usual this fall.

Fortunately, the procedure for making end-of-season irrigation decisions is based on crop maturity rather than the calendar date.

The last few irrigations of the season require some of the most important water management decisions of the year. Applying an extra, unneeded irrigation may waste 1 to 3 inches of water and 2 to 5 gallons of diesel fuel per acre, according to Steve Melvin, Extension irrigation engineer in Frontier County.

Furrow irrigators need to decide soon due to the difference in application amounts, while pivot irrigators can delay last irrigation decisions and take advantage of any rainfall.

Criteria needed

The irrigation management objective near the end of the season for fully watered crops should be to provide enough soil water in the root zone to carry the crop to maturity and produce top yields while leaving the soil fairly dry.

The following information is necessary to predict the amount of water needed to take the crop through to maturity:

predicted crop maturity date

predicted water use by the crop to maturity

remaining available water in the soil

predicted rainfall before crop maturity

The crop is using less water each day because it is getting more mature, and the days are getting shorter and cooler, Melvin says. The long-term, daily crop water use records show that the water use rate drops from around 2.1 inches per week at silking to only about 1.2 inches per week by the full dent stage. This 40% reduction requires irrigators to adjust their thinking about how much water needs to be applied each week.

Predicting rainfall

Often irrigators give up on any chance of getting rain late in the growing season. But historical data proves there is a good chance of receiving some rain in late August and into September. The maps above show the average weekly rainfall across Nebraska in August and September. The maps illustrate the likelihood of getting 0.5 to 1 inch of rain in August or September.

For best efficiency, rainfall plus the water applied during the last few irrigations of the season should not exceed the calculated irrigation requirement. Start using the stored soil water earlier in the season rather than just keeping the soil wet through the last irrigation and then letting it dry down all at once, Melvin says. By starting to delay irrigation toward the end of the season, the probability of receiving enough rainfall to meet crop water requirements increases.

For calculation procedures and charts, see NebGuide, “Predicting the Last Irrigation of the Season,” (G1871). It includes a worksheet to help you make the calculations for corn, sorghum, soybeans and dry beans.

Source: University of Nebraska-Lincoln NebGuide

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This article published in the August, 2011 edition of NEBRASKA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.