Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
11/5/98
Contacts:
Elwynn Taylor, Extension Climatology, (515) 294-1923
Treva Blumenshine, Extension Communication Systems, (515)
294-5672
Crop Weather Outlook For 1999
AMES, Iowa -- The most likely corn yield for the United States in 1999 is 126 bu/A, according to Elwynn Taylor, Iowa State University Extension climatologist. The 1999 outlook considers subsoil moisture as marginal at this time because it is mixed from dry to wet and so does not increase the chance of a good crop. The Benner 18.6-Year Cycle is in the low risk mode. La Nina may or may not persist into the 1999 growing season. Accordingly the odds are calculated as 1-in-5 chance of drought with 112 bu/A or less and 22 percent chance of yields exceeding 136 bu/A.
Weather cannot be managed, but risk associated with the weather can be. Historically there is a 1-in-6 chance that drought will reduce Midwest corn yields to 10 percent or more below the trend line any randomly selected year, according to Taylor. "In recent years, we have been able to improve the detail associated with the 1-in-6 risk for specific years. Overall, the 2 percent chance of yield-reducing flood and 17 percent chance of yield-reducing drought remain true."
The improved analysis comes from understanding El Nino/La Nina events. The chance of severe drought during El Nino drops from 17 to 5 percent, Taylor continued. The outlook given in November 1997 for the 1998 season was 133 bu/A based on a continuing El Nino. The actual yield did exceed the trend as was anticipated. Correct above/below trend forecasts based on the El Nino/La Nina reached 13 consecutive years. These statistics would not be expected by chance, but the El Nino/La Nina signal is not that reliable, so there was some element of chance in the prediction record.
The 1999 outlook is based on La Nina and on subsoil moisture, according to Taylor. La Nina was weak but consistent through October with the strength index (SOI) at 1.1. If La Nina continues through the 1999 growing season, the chance of severe drought increases to 40 percent. If La Nina fades before May 1999 the risk shrinks to 14 percent. La Nina is typically shorter than El Nino so the event may very well be ended by spring. Subsoil moisture increased substantially in most of the Corn Belt during October 1998, but was reported to vary greatly from place to place. Farms with low moisture will have greater risk than estimated, and farms with moist soils may have reduced drought risk for 1999. Running tiles are taken as indicating wet soils, but dry tiles do not necessarily mean dry soils.
"Some have noted that the Midwest is over due for a yield reducing drought," he continued. The 1995 drought was late in the season and was not considered a strong drought. The 1988 drought was the most recent wide-spread drought of consequence. The Benner 18.6-Years Cycle would indicate that it is not likely to have more than one serious drought between 1992 and 2004. The 1995 drought
may or may not qualify as that one drought for the 12 year period, according to Taylor. The chance of a drought does not increase because a drought is overdue. History shows that being overdue for drought slightly diminishes the chance.
Should La Nina end or the SOI fall below 0.4 by May 1999, the expected yield would improve considerably. Should La Nina strengthen or the SOI go above 1.2 by May 1999, the chance of yields exceeding 126 bu/A shrinks. Taylor said, "This outlook is nothing but an analysis of what has happened over the past 100 years. A wise manager plans ahead but also plans to adjust as conditions develop."
ml: isufarm
Extension programs are available to all without regard to race, color, national origin, religion, sex, age, or disability.