Issued by Scott Irwin and Darrel Good, University of Illinois
Drought conditions that impacted corn and soybean production in many areas of the U.S. in 2012 generally began in June. By the end of the growing season, large precipitation deficits developed in some areas. Some of those areas remain very dry, while more normal levels of precipitation have been received in other areas since August.
That diverse pattern is illustrated in Table 1, which shows cumulative precipitation deficits for four major corn producing states for the period June through November 2012. Average statewide precipitation for this six-month period in Illinois, Indiana, Iowa, and Nebraska is compared to the average precipitation for the same six-month period since 1895.
All four states received less than the average amount of precipitation since June, but the deficits are smaller in Illinois and Indiana, larger in Iowa, and much larger in Nebraska.
The Dec. 11 U.S. Drought Monitor Map also indicates that extreme to exceptional drought conditions exist in much of the Great Plains. These current drought conditions lead to concerns about moisture deficits continuing into 2013 and the possibility of sub-par corn and soybean yields again next year.
While soil moisture deficits going into the 2013 planting and growing season are of concern, it appears that some observers and analysts are forgetting a basic fact about the relationship of crop yields and weather. A large body of research shows that corn and soybean yields are overwhelmingly determined by summer weather conditions, with July weather being the most important.
Yes, preseason moisture deficits can impact yield for the upcoming crop but this impact is typically quite small relative to the impact of precipitation and temperature during the reproductive periods for corn and soybeans, a fact that should be all too obvious after the "flash drought" of summer 2012.
In this light, the relevant question is NOT the potential impact of current moisture deficits on yields in 2013, but instead do the deficits signal anything about weather patterns next summer? More specifically, is there any historical relationship between precipitation levels experienced in one year and precipitation levels the following summer? We investigate this issue using historical precipitation data over 1895-2012 for Illinois, Indiana, Iowa and Nebraska.
This is the longest available dataset available from the National Weather Service and it should provide the most accurate estimates of the correlations going forward, assuming that climate change has not substantially altered weather patterns. We calculate the following statistical correlations:
- July precipitation in one year (t) and July precipitation the following year (t+1),
- Total summer (June, July, and August) precipitation in one year and summer precipitation in the following year,
- Total June through November precipitation in one year and July precipitation in the following year,
- Total June through November precipitation in one year and summer precipitation in the following year.
Those correlations, where 1 indicates perfect positive correlation in precipitation levels and -1 indicates perfect negative correlation, are presented in Table 2. On average, all correlations are very small, with some modest positive correlation between both summer and total June through November precipitation in one year and summer precipitation the following year in Nebraska.