The drought of 2012 in the Midwest was a continuation of a weather anomaly that began in 2010. The historical indicator of El Niño and La Niña is the Southern Oscillation Index (SOI).
The index is based on the 90-day standardized deviation of atmospheric pressure between Tahiti and Darwin, Australia. The standardized difference reached 0.8 on July 22, 2010, signifying the beginning of a La Niña event (Figure 1).
click image to zoomFigure1. The La Nina event that initiated July 22, 2010, reached peak strength on Oct. 23, 2010, second only to the mid-1950s event. The SOI reached El Nino threshold on Aug. 16, 2012, but did not establish. On Oct. 30, 2012, the SOI returned to the La Nina side of a neutral condition. Data from http://www.longpaddock.qld.gov.au/. By Oct. 23, 2010, the event was clearly the second strongest event in the 100+ years of record keeping. The young but potent La Nina resulted in an abrupt change in weather on a planetary basis that included record flooding in Montana, N.D. and adjacent Canada, and enormous amounts of water to drain into the Missouri river during 2011. The event ended a several year drought in the northwest United States and ended (with drought) a six year continuous string of above trend U.S. corn yields (figure 2).
click image to zoomFigure 2. U.S. corn yield 1982-2012. The U.S. corn yield exceeded the trend for six consecutive years (2004-2009) and fell below trend in 2010. The “trend yield” for 2013 is near 160BPA (indicated by star). USDA graphic: NASS.USDA.gov. Drought in 2011 was much like the drought in the mid-1950s: developing in the south central United States with rain becoming scant in the Corn Belt after early July. Midwest crops depleted the subsoil moisture to the extent of rooting depth, and over winter precipitation did not bring full recharge to western Corn Belt soils.
Rooting conditions in 2012 were near ideal and observed corn and soybean roots to depths greater than 8 feet were reported in numerous locations. Deep rooting provided sufficient water to enable a greater than anticipated crop yield in numerous Corn Belt locations, but resulted in about 8 feet of moisture depleted soil and a resultant requirement of 16+ inches of moisture needed to replenish subsoil moisture.
It is not likely that subsoil moisture will be fully replenished by the beginning of the 2013 planting season.
Moisture deficit in the subsoil increases the risk of crop yields being below trend and prevents the recovery of river, pond and well water to normal levels.