Scheduling the Last Irrigation

Scheduling the Last Irrigation

August 3, 2012

Center pivot irrigation

It’s been a long irrigation season, but for some growers, harvest will be several weeks early and the end to the irrigation season is in sight.

Water demand so far this summer has been high and some systems have had difficulty keeping up. Again last week, ETgage® readings were near or above 2.0 inches. This has already occurred six times this summer in the York area, when typically it may only happen twice all year.

As I mentioned earlier, the ETgages that I’ve been monitoring dropped about 2.0 inches in the last couple of weeks. Much of the corn that I’m monitoring in south central Nebraska is in the late dough stage and just beginning to dent and soybeans are in the late R4 stage. The crop coefficient for these stages of corn and soybeans continues to be 1.1. To estimate crop water use we multiply 2.1 (average ETgage® reading) times 1.1 for a total crop ET of 2.31 inches a week or about 0.33 inches a day.

Table 1. Normal water reqauirements for corn and soybeans at various stages of maturity in Nebraska. (Source:  Predicting the Last Irrigation of the Season, NebGuide G1871)
Stage of growth Approximate days
to maturity
Water use
to maturity
R4 or Dough 34 7.5
R4.7 or Beginning dent 24 5.0
R5        ¼ milk line 19 3.75

½ milk line

13 2.25

¾ milk line

7 1.0
R6 Physiological maturity 0 0.0
R4 End of pod elongation  37  9.0
R5 Beginning seed enlargement  29  6.5
R6 End of seed enlargement  18  3.5
R6.5 Leaves begin to yellow  10  1.9
R7 Beginning maturity  0  0.0

The last few years we’ve encouraged producers to leave room for rainfall and not finish the season with a full profile. That should be easy this year.  We just have not had many breaks where we could shut those systems off. However, as the crop begins to reach maturity, the crop water use will begin to slow (Table 1).

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 while a silt loam soil can hold 2.2 inches per foot or 8.8 inches in the top 4 feet. Let’s look at an example using a silt loam soil. If we draw down the available soil water to 60%, we would have 0.88 inch of water per foot of soil or 3.5 inches remaining in the top four feet of soil. Even though you have 3.5 inches remaining, if you draw the soil water past 60%, it can affect yields; therefore, our target is to have 60% depleted at the end of the season. This will leave room to capture 5.28 inches of water during the off season from rain or snowfall.

It's important to remember that corn in the R5 stage at ½ milk line needs only 2.25 inches of soil water or rainfall to complete the season. The days to maturity will probably be less than those shown in Table 1 because temperatures at this time in August are warmer than they typically are in late August or September, however, the water needs to maturity should be close.

During the heart of the season, we recommend depleting the available soil water level only to the 50% level. As we near the end of the season, we can push the threshold to 60% depletion.

It’s also important to remember that if your Watermark© readings are 33 or less and you hae a silt loam soil, in this example you wpi;d have 1.1 inches or more of soil water available per foot of soil or 4.4 inches for your crops to use.


We have charts available on the Nebraska Ag Water Management Network website to help you calculate the amount of soil water you have based on your Watermark® sensor readings. We also are developing an app for IPhones and I Pads to do the same.  Other resources which may be helpful in managing your irrigation program include:

  • Predicting the Last Irrigation of the Season (NebGuide G1781)
  • Nebraska Agricultural Water Management Network, including
    — weekly ETgage data and  local reference ET weather station data,
    — crop water use by growth stage for various crops,
    — Watermark© sensor calculator
  • CropWatch Weather, including maps and data of potential evapotranspiration rates

Gary Zoubek
Extension Educator




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A field of corn.