August 22, 2013

What a difference a year makes. I was looking through last year’s CropWatch and noticed we had an article about scheduling the last irrigation on August 3. This year it’s August 20 and we’re just thinking about it.

In contrast to last year, water use for the first three weeks of August 2013 has been about 50% of normal. Many irrigators, remembering last season, have irrigated to keep a fairly full profile as we near the season end. Depending on soil type, some fields may have enough stored soil water to get the crop to maturity without additional irrigation or rainfall. Let’s look at the calculations.

During the heart of the irrigation season, we recommend depleting the available soil water level to the 50% level. As we near the end of the season, we can push the threshold to 60% depletion. The water-holding capacity of a soil varies with its texture and needs to be considered when determining how much soil water is available. For example, a loamy sand will hold about 1.1 inches of water per foot or 4.4 inches in top 4 feet while a silt loam soil can hold 2.2 inches per foot or 8.8 inches in the top 4 feet. If the silt loam is at field capacity and we draw down the available soil water to 40% (60% depletion), we would have about 5.3 inches of useable water in the top four feet of soil.

Corn at the beginning dent stage needs 5 inches of water to reach maturity. This example field would have enough water to reach maturity and have an estimated 0.3 inches to spare if the corn is beginning to dent. The loamy sand, at field capacity, would have 2.6 inches available above the 40% (60% depletion) level. This field would need an additional 2.4 inches of water to reach maturity.

Management Resources

So how do you determine how much water is left in your soil? If you are using a soil probe and the hand feel method, check out the NRCS publication, Estimating Soil Moisture by Feel and Appearance.

If you have Watermark sensors, you can use the UNL Crop Water App to determine how much useable water is left in the top three feet of soil. Information about the app is available on UNL's Water website at http://water.unl.edu/cropswater/nawmdn. Based on Watermark sensors installed at depths of 1, 2, and 3 feet, it will estimate the water used and water still available for a range of Nebraska soils. You also can see historic sensor readings, graph the data, and pin your GPS locations. Download the app from the:

• iOS Apple Store
• Android Store Link

For more information on soils and water use for different crops see Predicting the Last Irrigation of the Season (UNL NebGuide G1781).

Chuck Burr
Extension Educator, West Central REC
Gary Zoubeck
Extension Educator, York County
William Kranz
Extension Irrigation Specialist, Haskell Ag Lab