The three driest water years, October 1 thru September 30, since 1950, has been in 1956, 1988, and 2012 in the Central Iowa Crop District. With these important faces in mind, Dr. Elwynn Taylor, ISU Extension Climatologist, points out that the subsequent years also received below normal rainfall and experienced below trend line corn yields in Iowa.
During a normal mid-summer, July and August, stored moisture is taken from the top 5 foot of soil. This sub-soil moisture helps the plant meet its daily moisture requirements. Generally, a foot of silt loam soil will contain 2 inches of plant available water. In eastern Iowa, the 20 year plant available moisture has generally been 9 to 10 inches of water on 15 April. In early December 2011, we received about three inches of rain just prior to the soil freezing. So far, for November and early December 2012, little rain has fallen. Iowa State University (ISU) has reported in eastern Iowa, sub-soil moisture availability to be from 2.8 to 8.8 inches per 5 foot of soil.