UNL CropWatch Aug. 13, 2010: Predicting End of Season Irrigation Needs

UNL CropWatch Aug. 13, 2010: Predicting End of Season Irrigation Needs

August 13, 2010

Planning Irrigation
By the Numbers

See Predicting the Last Irrigation of the Season (NebGuide G1871) for information and a worksheet to predict the last irrigation for corn, grain sorghum, soybeans, and dry beans.

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.
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.

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 for Estimating the Last Irrigation of the Season

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. The long-term, daily crop water use chart shows 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.

Nebraska map of average precip in August

Figure 1. Average weekly precipitation in inches for August.

Nebraska map of average precipitation in September

Figure 2. Average weekly precipitation in inches for September.

Predicting Rainfall

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. Figures 1-2 show the average weekly rainfall across Nebraska in August and September. Figures 3-6 (below) illustrate the likelihood of getting 0.5 to 1 inch of rain in August or September.

Predicting End of Season Irrigation

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.

For calculation procedures and charts see NebGuide Predicting the Last Irrigation of the Season (G1871).

Steve Melvin
Extension Educator, Frontier County


Nebraska map


Figure 3. Percent chance of receiving up to 1/2 inch of rain during a week in August.

Nebraska map

Figure 4. Percent chance of receiving up to 1 inch of rain during a week in August..

Nebraska map

Figure 5. Percent chance of reciving up to 1/2 inch of rain during a week in September.

 Nebraska map

Figure 6. Percent chance of receiving up to 1 inch of rain during a week in September.