Serious drought lingers
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/. The drought of 2012 in the Midwest was a continuation of a weather anomaly that began in 2010. The historical indicator of El Nino and La Nina 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 Nina event (Figure 1). 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.
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. 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.