Irrigation Scheduling – When to Resume After Rain

James Adkins, Agricultural Engineer;

Continued scattered thunderstorms across the region brought significant relief to a few but most fields received less than 0.2” of rain last week. Both corn and full season soybeans are predicted to use 0.25” per day next week. With sporadic and limited thunderstorms in the forecast, farmers are facing the challenge of when to start back after a rain event.

Coarse loamy sand soils only hold 1.6” in the entire 18” root zone and need to maintain a minimum of 50% of the available water to avoid yield loss. This means a farmer with sandy loam soils needs to initiate irrigation before 0.8” of crop water use. With daily evapotranspiration averaging 0.2”-0.25” per day irrigation should start no more than 3 days after a major (1 inch plus) rain. Sandy loams average 2.34” of holding capacity meaning a 50% managed allowable depletion of 1.17”, or a 4-day break after a profile filling rain. High organic matter and clay soils can hold additional 1-2 days of moisture before needing irrigation. Keep in mind that just because a farm received and 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 0.2”/day for the past week. The cloudy weather on Sunday 7/7 and Monday 7/8 limited transpiration rates to 0.15” bringing the weekly average down from last week. This field received 0.62” of rain in addition to 0.9’ of irrigation in 3 events since last Thursday. This same field is predicted to use 0.24”, 0.22”, 0.28”, 0.30”, 0.24” for Friday 7-5 – Tuesday 7/9 for an estimated daily usage of ¼” 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.

At this point in the growing season most corn fields are at least into the VT 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 VT – DIMS Report

Full Season Soybeans
May 2nd planted soybeans at the UD Warrington Irrigation Research Farm are approaching the R3 stage as of July 11th. We received 0.62” of rain and applied a total of 0.6” in 2 irrigation events last week. The average daily crop water use was 0.19” per day and the predicted daily ET for next week is 0.25” 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. Soybeans that have reached 70+% canopy are using and estimated 0.12” per day.

Irrigated Soybean Soil Moisture Report for the UD Warrington Farm Stage R2 – DIMS Report