Drought conditions spread to more counties in Iowa this past week, and Iowa State University Extension climatologist Elwynn Taylor predicts Iowans may see little relief from the hot dry weather this week—the week of July 2. "We've had warm, dry conditions through May and June this year, and that is usually the best predictor for the rest of the summer," says Taylor.

Concerns continue to grow that dry, hot conditions will reduce Iowa's nation-leading corn and soybean crops this year. The weekly statewide survey by USDA shows less than adequate moisture in the topsoil and subsoil for 73% of the state, as of July 1. That compares to just 2% at the same time a year ago.

Will Your Corn Have Enough Water To Survive?

An updated drought monitor map identifies "moderate drought" conditions in the eastern Iowa counties of Scott, Clinton, Jackson, Dubuque, Delaware and Jones. An S-shaped path from the northern border of Iowa down through central Iowa and into Missouri already has been in "drought" status for a week. However, two tiers of counties on the Iowa side of the Missouri River have been removed from "abnormally dry" status after rainfall last week and over the weekend.

Soil type, tillage practices, crop residue cover, drainage and weed control all play a significant role in soil moisture availability.

If you are in an area of Iowa that has little or no subsoil moisture in reserve for the crop to use this growing season, how long can your crops survive if they don't get the needed rain each week? It depends on a number of factors—some of them are management factors that farmers can control. Taylor, along with his ISU colleagues, agronomists Mahdi Al-Kaisi and Roger Elmore, explain the factors that affect soil moisture availability and use by crops. Their explanation is as follows.

Soil moisture during the growing season is essential to obtain optimal yield. Optimal yield is affected by the availability of moisture as stored water in the soil profile or timely recharge during rain events. Soil texture, tillage practices, residue cover, drainage and weed control can play a significant role in soil moisture availability. Generally, the no-till system is the most effective practice in conserving soil moisture among other tillage systems, especially during dry periods in rain-fed agricultural areas.

Crops need water to take up nutrients and most importantly to reduce heat stress

Crops need water to uptake nutrients and most importantly to reduce heat stress through the transpiration process. That is where the plant releases water through the leaf openings called stomata. This is the plant's main cooling mechanism.

Evaporation of water from soil and plant surfaces is another source of water loss during the growing season. The two processes together are called evapotranspiration (ET) or crop water use. Crop water use represents both soil evaporation and plant transpiration used for crop growth and cooling purposes.

* Potential evapotranspiration vs. actual evaporation.  Actual Evapotranspiration (ETa) or actual crop water use can be estimated or determined if we know potential evapotranspiration (PET) and the correct crop coefficient (Kc) at a certain growth stage for a particular crop. Potential ET (sometimes called reference ETr) is defined as the rate of readily available water removal from wet soil and plant surface under well-watered conditions when moisture availability is not a limiting factor.

To calculate crop water use or Eta, use this formula: ETa =Kc x PET. The Kc is the crop coefficient provided for each crop at different growth stages and PET is potential evapotranspiration provided from weather stations at different growth stages.

The first step in figuring crop water use is to determine PET for the locations of interest. Obtain PET in this way:

Or to get PET from yesterday, select: http://mesonet.agron.iastate.edu/agclimate/display.php?prod=6. Once you have PET, you can use the table on the website to estimate corn water use based on the current corn growth stage matched with the present PET. For more information on "Crop Water Use" see: http://www.ipm.iastate.edu/ipm/icm/2000/5-29-2000/wateruse.html.