June 19, 2012
High Temperatures to Increase Soil Evaporation Rate in U.S.
The sun in the Northern Hemisphere is at its maximum intensity on June 21st and during the third week of June, the average amount of evaporation and transpiration from a typical field of corn in the central Corn Belt is approximately 1.4 inches of water per week. That number can vary of course depending on the air temperature, relative humidity, cloud cover, color of the soil, soil type, and the size of the corn or soybean crops. This week the temperatures in the Midwest are going to be extremely high with strong gusty winds and sunny skies and the resulting evaporation rate this week will probably exceed 1.4 inches.
Therefore, just to stay even on soil moisture we would need 1.4 inches of rainfall per week. It is unusual to average that much rain per week at the end of June. As a result, the corn crop relies on the subsoil moisture to make up the difference, but the problem this year is that the subsoil moisture has been depleted as well, so the normal amount of subsoil moisture is not available this year.
As the corn plant progresses through its growth phases, its demand for water increases and reaches its maximum during the grain filling phase. A corn crop can still have acceptable yields with less than the normal amount of rainfall if the timing of the rainfall is good and the temperatures are not excessive. If the temperatures are excessive, then the timing and the amount of rainfall become even more critical and the margin for error becomes much smaller. Where the soils are the driest in the central and eastern Corn Belt, the corn crop cannot afford to miss any rainfall opportunity.
The two most important weeks for the U.S. corn crop this year will probably be the last week of June and the first week of July when a majority of the corn will be in pre-pollination and pollination.