click image to zoomFigure 3. Normal subsoil moisture as of April 15 during the period 1961-1980. Soil moisture capacity is between 10 and 11 inches for the top 5 feet of soil for moist agricultural soils of Iowa. November moisture in 2012 exceeded observations for November 2011 in southeast Iowa and will likely result in normal values when augmented by spring rain. In northwest Iowa, the November 2012 observations were less than 2011 in locations sampled. Measurements of soil moisture are made at 1-foot intervals to 5 feet at several locations. Figure 3 is a map of the “normal April 15” soil moisture. Observations of the moisture at selected locations were made in the fall of 2011 and 2012.
The fall observations are useful in that November water is retained throughout the winter and is indicative (to the extent they are short of normal) of the moisture deficit yet to be corrected.
click image to zoomFigure 4. Precipitation received in central Iowa during the water year (Oct. 1 through Sep. 30). The three historically driest years since 1950 are 1956, 1988 and 2012. In both cases, the subsequent year also received below normal precipitation and experienced below trend yields in Iowa. Historically, severely deficit precipitation years of the magnitude of 2012 do not recover to normal annual precipitation in a single year (Figure 3).
Accordingly, an additional year of significant moisture stress is considered to be not unlikely and a fourth consecutive year of below trend U.S. corn yield a distinct possibility. The probabilities will become more definitive in the early weeks of 2013 as the likely phase of the El Niño/La Niña for the growing season becomes manifest.