It's Time to Determine Timing, Amount of Final Irrigation

It's most important water management decision you can make.

Published on: Aug 24, 2010
The 2010 growing season started with a wet, cool spring that delayed planting in some areas and has now moved into a relatively hot summer that is helping crops mature fairly quickly. 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, says Steve Melvin, Extension educator in Frontier County.

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, he says.

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.

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, according to Melvin.

To predict the amount of water needed to take the crop through to maturity, you need to know predicted crop maturity date, predicted water use by crop to maturity, remaining available water in the soil and 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. 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.

"Often irrigators give up on any chance of getting rain late in the growing season. However, historical data proves there is a good chance of receiving some rain in late August and into September," Melvin says.

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. By starting to delay irrigation toward the end of the season, the probability of receiving enough rainfall to meet crop water requirements increases.