By Elwynn Taylor and Roger Elmore, Department of Agronomy
"It has been hot, but at least the soil moisture is good," northwest Iowa farmer this week.
"Last year it was so dry I thought we would lose the crop, but temperatures were not so bad and it turned out great," central Iowa farmer in 1995.
Most everyone knows that "dry and hot" is worse than "cool and moist" when it comes to the impact of summer weather on Midwest corn yield. Naturally it is not all that simple, still the generalization is meaningful.
Years with better corn yields across Iowa tend to be a bit warmer than normal before the crop tassels and a bit on the cooler and wetter than usual side of usual afterward. The late Dr. Louis Thompson of Iowa State University Department of Agronomy based a corn yield forecast model on these well-known effects. The Thompson Model achieved international acclaim because it provided a useful assessment of the impact that temperature and rain have on Midwest corn production. Although the crop yield cannot be accurately forecast while it is still developing, it is of value to know the crop yield risk by week and by crop reporting district as the season progresses. A week-by-week index showing relative crop risk according to the interaction of temperature and precipitation has proven to give a realistic picture of localities with poor and areas with better than usual crop yield. The Aridity Index does this for you; the index is directly related to the probability of having a district yield that exceeds the historical trend line.
Figure 1- Aridity Index by crop reporting districts. Locations with an index higher than -4 are on track to likely have above trend yield (yellow through green and blue colors). Districts with an AI below -4 will have below trend line yields if weather conditions persist. The nature of the aridity index is such that major changes can be observed week to week. The individual history by week for each reporting district is available by clicking the original image on the website. View depicted is for the week ending July 18, 2011.
The Aridity Index is based on the observed temperature and precipitation relationships for the central United States. When rain is diminished to one standard deviation below normal, the impact on the crop is much the same as if the temperature had been elevated by one standard deviation. Daily observations of temperature and precipitation are available for every county. Accordingly, the AI can be updated for each crop reporting district on a weekly basis (or conceivably by county on a daily basis). This index treats just the temperature/precipitation interaction. It does not consider the Growing Degree Days, the impacts of excess water, the fate of nitrogen in the soil of the farm, or any number of other factors impacting crop yield. Still many find it of real value when knowledge of district level crop response is important. The AI analysis is especially valuable when used in conjunction with USDA-NASS reports of state-by-state crop condition (if more than 50 percent of the crop acres are in good and/or excellent condition an above trend line yield is likely).
The AI computer program and display was developed by Agricultural Meteorologist Darren Miller while he was a graduate student.
Elwynn Taylor is extension climatologist and can be reached at firstname.lastname@example.org or by calling (515) 294-1923. Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by email at email@example.com or (515) 294-6655.