Irrigation Management: A Nice Rain for Some – Continued Drought for Others

James Adkins, Agricultural Engineer;

Wednesday evening brought thunderstorms and significant rain to parts of Delaware while other areas only received wind and lightening. The storms followed a track roughly parallel to Rt 9 with farms from Laurel to Lewes receiving from ¾” – 2”. Zero to a mere trace of rain fell in a band from Seaford to Primehook to the south and Harrington to Bowers to the north. Another band of rain averaging ½” passed through Woodside and Dover with folks to the north receiving shattered showers with significant rains north of the C & D canal.

24 hour rainfall from DEOS for rainfall on July 17. (

Both corn and full season soybeans are predicted to use over 0.26” per day next week. With sporadic and limited thunderstorms in the forecast, farmers are continuing to face the challenge of when to start back after a rain event. For the sandy loam soils present at UD’s Warrington farm and the high predicted ET rates, corn will need irrigation on day 4 after a profile filling rain and soybeans will need irrigating 3 days after a big rain. The shallower root zone of soybean versus corn is the cause of the reduced irrigation break. Coarser soils like loamy sands should initiate irrigation 2-3 days after rain while heavier loams, silt loams and clay can take a 5 to 6-day irrigation lapse.

Keep in mind that just because a farm received an 1” of rain there is no guarantee that all of it infiltrated and was stored in the soil. Thunderstorms tend to dump water faster than the soil can absorb and thus runoff from the high parts of the field to the low creates soil moisture variability and will require irrigation sooner to prevent yield loss in the high areas.

The information presented below is an example of the soil moisture status at University of Delaware’s Warrington Irrigation Research Farm. Actual field values will vary greatly depending on crop stage, soil type and local rainfall. There are many tools available that provide field by field values to assist farmers in making irrigation scheduling decisions including paid services through local crop consultants, irrigation equipment manufacturer’s, Climate Corp, etc and free tools like KanSched and the Delaware Irrigation Management System (DIMS)

Field Corn
Daily corn evapotranspiration (ET) rates for April 25th planted 114 day corn at R1 averaged 0.25”/day for the past week. While the temperature has been very high with plenty of sunlight, the high humidity has kept crop ET rates under the 0.3”/day we typically see at this stage. Soil moisture sensors in the field are in line with the model predictions and have been a good confirmation that our estimated soil moisture values are correct. This field received 0.47” of rain on July 11 and 1.05” last night (July 17) in addition to 1.5” of irrigation in 5 events since last Thursday. This same field is predicted to use 0.24”, 0.26”, 0.32”, 0.30”, 0.26” for Thursday 7-18 – Monday 7/22 for an estimated daily usage of 0.27” per day for the upcoming week. These are estimated values and are no substitute for daily ET use models and field level soil moisture data.

NOTE: Despite a profile filling rain on Wednesday night, this field will require irrigation on or before Sunday.

At this point in the growing season most corn fields are at least into the VT – R2 stage; Crop water usage will be nearly the same from V14 until R2/blister stage. Farmers should continue to intensively irrigate through the R2 stage and gradually taper off through R3 until black layer.

Irrigated Corn Soil Moisture Report for the UD Warrington Farm Stage R1 – DIMS Report

Full Season Soybeans
May 2nd planted soybeans at the UD Warrington Irrigation Research Farm are into the R3 stage as of July 11th. We received 1.52” of rain in 2 events and applied a total of 0.9” in 3 irrigation events over the past week. The average daily crop water use was 0.24” per day and the predicted daily ET for next week is 0.26” per day (the same amount as corn). Remember to irrigate in small but frequent doses to avoid pushing water beyond the root zone. Multiple years of soil moisture sensor data show soybeans to use water primarily from the shallow (0-8”) soil profile.

Double Crop/Late Season Soybeans
Continue to irrigate in small amounts of around 0.2-0.3 inches to maximize canopy development. Keep in mind that irrigation that infiltrates beyond 6” will be of little benefit to the crop. Barley and early wheat beans should be using an estimated 0.18” per day.

Irrigated Soybean Soil Moisture Report for the UD Warrington Farm Stage R3 – DIMS Report

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