The biggest story so far this season has been the excessive wetness across much of the central U.S., leading to major disruptions in planting and a corn crop that has been well behind normal. Indeed almost every Ag Blog I wrote from April through June focused on the remarkably persistent high Soil Moisture Index (SMI) values in the central U.S. An example from late May is given below in Figure 1.
Figure 1. Soil Moisture Index (SMI) from the 30 May 2019 Ag Blog.
As I have alluded in other recent Ag Blogs, the ceiling for corn yield this year is assuredly lower than in recent years. Over the last week to ten days the precipitation has been more isolated in the Corn Belt and most of the region has been a bit on the warm side. While it is way too early to talk about flash drought, one could be in the developing stages IF widespread precipitation doesn’t materialize in the next few weeks and the heat returns in August. As of now, this week looks abnormally hot (though not record hot) in the Corn Belt, followed by more seasonal temperatures next week. If the return to seasonal temperatures is coupled with rain, then this discussion of flash drought is moot. However, given the warm temperatures and problems with excessive moisture earlier this year, especially in Illinois, Indiana, and Ohio, crops may show signs of water stress much more quickly than usual. This alone does not indicate a flash drought, as we argued in a Washington Post article rebuttal last year about conditions in the D.C. area, but that stress is real and could lead to further reductions in yield.
There was, however, another year where the spring was on the cool side and wet (not to this year’s extreme for moisture) and thousands of acres were left unplanted. Then after the 4th of July, the precipitation ceased and was coupled with above normal temperatures. Unfortunately this was not a modest dry spell but one of the most rapidly developing flash droughts we have seen in the Corn Belt. The year was 1983. Anecdotal evidence and climate data would suggest something quite similar also happened in 1947. While efforts for monitoring drought in real-time were crude in 1983 compared to today, the beauty of the NASA Land Information System (LIS) is it allows for a retrospective analysis of the soil moisture index.
Figure 2 shows the evolution of the SMI over the eastern 2/3 of the U.S. from early July to early September in 1983. Notice that in early July much of the prime corn and soybean area was quite moist, if not too wet. A large ridge of high pressure started to dominate the central U.S. in mid-July and by the end of the month, the SMI had shifted dramatically to negative values. This pattern more or less continued into early September and many locations from eastern Nebraska into northern Illinois had a top-3 hottest August. Places like Omaha, Des Moines, and Moline had multiple periods of consecutive days over 100F during this flash drought, capped off by Des Moines hitting a post-Dust Bowl era record of 108F in the middle of August.
Figure 3 shows the result of the combination of a wet spring and a flash drought on corn yield. Of the 60 districts used for analysis, only 3 were above trend. For the sake of comparison, the historic flash droughts of 1988 and 2012 had more districts above trend than 1983, and for some places in the Eastern and Western Corn Belt, it was the worst year off trend in the period from 1974-present day.
At this point, I am not projecting that conditions for the remainder of the season will be similar to 1983 (or 1947, 1995). Other factors (e.g., genetics, management) have also improved substantially since the early 1980’s so it’s impossible to make a true apples to apples comparison from today to then. In other words, this post should be viewed merely a worst case scenario and not an actual forecast. For additional insights on the seasonal forecast and our current projections of yield, we do have commercial products available.
Figure 2. Soil Moisture Index (SMI) for the week ending 5 July, 26 July, 16 August, and 6 September 1983.
Figure 3. Corn trend in 1983 by crop reporting district. Data courtesy of USDA NASS.
Please direct any feedback or questions my way ( Eric.Hunt@aer.com ).