Fundamentally Speaking

By Joel Karlin
Contributing Analyst

In a prior piece, we discussed the impact of a major drought on crop yields the following year.

The end of December U.S. Palmer Drought Severity Index (PDSI) was -5.47 and anything less than -4.00 is classified as extreme drought.

The end of December PDSI was not only the worst of 2012 indicating a worsening situation as the year went in but is the lowest national PDSI since April 1981.

We went back and for the top 18 corn producing states those instances where their individual PDSI was -4.00 or lower at the end of December.

There were 30 such cases and only 10 times were final corn yields in those states above the 1960-2011 trend indicating that severe drought the prior year puts the crop in a precarious state the following year.

There was, however, eleven times when despite a very low PDSI reading the prior December, final corn yields came in at trend or higher suggesting that good summer conditions offset the ongoing drought conditions.

This particular piece looks the percent deviation of final corn yields from the 1960-2011 trend compared that year’s average July-August temperature and combined July-August precipitation as a percent of that states 1960-2011 average July-Aug temperature and the average combined July-August rainfall.

In all but two of the years, average July-Aug precipitation was above normal and in all but three cases average July-Aug temperatures were below normal.

The oddest case was North Dakota that in December 1990 had a PDSI reading of -4.16 and only had improved to -3.86 by August 1991.

Yet that states corn yield in 1991 was 4.1% above trend despite the average July-Aug temperature that year 2.5% above average and rainfall those two months averaging 84% of normal.

There were instances were corn yields were below trend despite having cooler than normal temperatures and above normal rainfall suggesting that the lack of subsoil moisture reserves proved a hindrance even with decent summer growing conditions.

(KA)

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