Irrigation Management: Cloudy Forecast = Lower ET

James Adkins, Agricultural Engineer;

Weather predictions for the next 5 days are calling for lower temperatures and cloudy skies. This, combined with the reduced crop water demands as corn begins to enter the dent stage, makes for a significant reduction in irrigation demand for the upcoming week. Full season soybeans are still using considerable water which will continue until the leaves start to yellow. If the weather forecast holds, both corn and full season soybeans are predicted to use around 0.18” per day next week. The moderate chance of rainfall over the next 5 days will hopefully allow irrigators a chance to catch up.

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 R3/R4 averaged 0.23”/day for the past week. 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 had not received any rain over the past week and 6 irrigation events totaling 1.8” applied. Wednesday evening thunderstorms yielded 1.06” of rain bringing this field to 100% available water. Many fields did not receive much or any rain and will need to continue irrigating. This field is predicted to use 0.18”, 0.18”, 0.16”, 0.20”, 0.23” for Thursday 8-1 – Monday 8-5 for an estimated daily usage of 0.19” 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 R2+ stage. UD research on when irrigation can be stopped has been largely inconclusive. Farmers should continue to intensively irrigate through the R2 stage and gradually taper off through R3 until black layer. Historically, crop water use after R4 has been light (less than ½”) to get to the black layer stage. With pumping costs averaging $4.5 per acre inch, the cost of applying water up to black layer is worthwhile insurance.

Irrigated Corn Soil Moisture Report for the UD Warrington Farm Stage R3/R4 – DIMS Report

Full Season Soybeans
May 2nd planted soybeans at the UD Warrington Irrigation Research Farm are into the R4 stage as of Aug 1st. The average daily crop water use for the past week was 0.22” per day and the predicted daily ET for next week is 0.18” per day (the same amount as corn). We have observed high rate of water use from the shallow profile; Remember to irrigate in small but frequent doses to avoid pushing water beyond the root zone. Multiple years of soil moisture sensor data show so use water primarily from the shallow (0-8”) soil profile.

Double Crop/Late Season Soybeans
At this point, most double crop soybeans should be close to full canopy. Once full canopy is achieved, late soybeans will use the same amount of water as the full season beans above.

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