Ag Blog Update 31 Aug

Ag Blog Update 31 Aug


During the 2020 growing season, Eric Hunt of Atmospheric and Environmental Research, Inc. will be providing weekly updates of the soil moisture index (SMI) from the Noah-MP land surface model in the NASA LIS framework for the entire U.S. and regional analysis of the SMI over the four regions of U.S. where the majority of corn, soybean, wheat, and cotton production occurs. The analysis is intended to provide the larger agricultural and meteorological communities insight as to areas where soil moisture is excessive or deficient compared to average for that location and what that may mean for impacts. It is my goal that these maps can be an early warning signal for flash drought development or where flash flooding could be likely in the coming week if heavy precipitation materializes. Please be advised that the SMI should be viewed as complementary, not a substitute, to the U.S. Drought Monitor (USDM) and that declarations of drought or flash flood potential for a particular location should never be based on the SMI alone. Remote sensing based products such as The Evaporative Stress Index (ESI) are also included in our analysis (when available) as are various other maps that help give insight into current conditions across the U.S.

This blog post was partially supported by NASA grant 80NSSC19K1266.


Today’s Ag Blog will be dedicated to the drought that has been ongoing and expanding over parts of the Corn Belt. At the end of the blog is the final corn yield estimate for the season. This is also a good time to remind readers that my background is more in meteorology and applied climatology with a significant research emphasis on drought. I have some basic background in agronomy but suffice to say I do not have the same expertise that many full-time agronomists who have spent their careers in the field (quite literally) studying the effect of water stress on crops and who often still scout fields. In other words what you are getting out of this blog is a viewpoint of an agricultural climatologist that loves data, as evidenced by a recent publication on changes in the U.S. Corn Belt. Also, it is more of a goal of mine to get it right than to be right.

The month of August 2020 will not go down as one of the best Augusts for corn in U.S. history. This is particularly true in Iowa where the two D’s (derecho and drought) were unwanted and destructive guests. Ironically enough, the only time some places in Iowa received any decent precipitation this month came with the added “treat” of 70-110 mph winds. As the month went on, the drought that had been mostly confined to west central Iowa and northeast Nebraska started to expand further south and east. This will undoubtedly have an effect on crops. The big question is how much.

At the beginning of August when the drought was mostly confined to western Iowa and everywhere else was mostly in very good shape, I was quite bullish on corn yields this season. However, the month ended up being drier than expected over a large area and there’s just no way that hasn’t had some detrimental impact. But a closer look at history would suggest this is not going to lead significant yield losses so much as bring the crop closer to at-trend. Mark Twain is reported to have said “History doesn’t repeat itself, but it often rhymes.” That is a fairly good description to my approach when it comes to evaluating potential impacts from this season.

When looking at the evolution of the median SMI of the Corn Belt over the course of the weeks in the summer months (June, July, and August), this season has one reasonable proxy (2014) and one that is pretty close (2003). First though, I need to explain the “Goldilocks range”. In the story of Goldilocks, she found one bowl of porridge too hot, one too cold, and one just right. In the case of soil moisture, the best seasons in the past 40 (e.g., 1994) are those when the SMI stayed in this zone between -1.0 and 1.5 the entire season (Figure 1). Any length of time above 1.5 often implies excessive moisture and multiple weeks in a row below -1.0 is a good indication that too much of the region is short on moisture and in drought or headed toward one. In the case of 2003 and 2014, the SMI declined for multiple weeks in a row following silking. The difference is in 2014 the decline was more gradual because of cooler temperatures and was followed by improvement later in August. In 2003 the decline is a classic flash drought signal in soil moisture and was not followed by improvement. Unfortunately, the 2020 season is following the 2003 season much more closely than the 2014 season.

Figure 1. The median SMI of the Corn Belt in 2020 (black), 2014 (blue), 2003 (gray), 1994 (dark blue), and 1988 (dark red). The “Goldilocks zone” is between -1.0 and 1.5 and denoted by the thin dark lines. The vertical lines at weeks 28-29 indicates the most common time for a crop to enter silking (as was the case this season). Results are based on output from the 0-1 m (surface to 3.23 feet) layers in the Noah-Multiparameterization (Noah-MP) land surface model. Noah-MP is run in the NASA Land Information System (LIS) framework with the North American Land Data Assimilation Version 2 (NLDAS-2) forcing dataset. The SMI calculation is based on the soil moisture index created in Hunt et al. (2009) such that ‘5’(dark green) is the wettest and ‘-5’ (dark red) the driest for the period of record.  The period of record used calculate the SMI for the current map is 1979-present.

Now let’s take a look at how the SMI evolved spatially between late July and late August in 2003 and 2020 (Figure 2). There are clear differences, not least of which is how much wetter the eastern U.S. was in the summer of 2003. By late July of 2003 SMI values were strongly negative over much of the Great Plains, much of which dated to the drought that developed in the spring and summer of 2002. Over the next few weeks the SMI remained negative in the Plains and western Corn Belt and began to drop sharply in central and northern Iowa, much of Minnesota and Wisconsin, and into northwestern Illinois. This season the most negative SMI values were confined to the western Corn Belt, most notably from northeastern Nebraska into western Iowa. As I showed in Figure 3 from the Ag Blog back on 21 July, the flash drought in that region has its origins from mid-June when a week of abnormally warm temperatures and windy weather encompassed the middle of the country under the “Omega block”. Just as an aside, it was that same week that remains of Tropical Storm Cristobal moved into parts of the central Corn Belt. It is highly possible that all of Iowa and possibly more of the Corn Belt would be in much worse shape today were it not for that moisture. But I digress. Over the past month, the dry conditions have led to a gradual decline in the SMI across the eastern half of Iowa extending into Illinois. As of last Thursday, the entire Corn Belt along I-80 from far eastern Nebraska into Ohio had negative SMI values. The good news for northern sections of Indiana and Ohio is significant rain fell last Friday.

Figure 2. Comparison of the soil moisture index between 2003 (left) and 2020 (right).

In 2003 the epicenter of the flash drought was over eastern Nebraska, southwestern Iowa, northeastern Kansas, and northwest Missouri. This led to corn that was more than 10 points below trend in those districts. This year the epicenter has been shifted to the north about 100 miles and the highly negative SMI values have remain confined to the western half of Iowa and northeast Nebraska. While the area of strongly negative SMI values has been less this season compared to 2003, the area affected contains three of the top 5 corn producing districts in the U.S. My best estimate is west central Iowa will be between 10 and 20 points below trend with below trend corn likely in the other western Iowa districts and central Iowa as well. In 2003 the epicenter was over an area that historically is more marginal and less important to total U.S. corn production.

However, it should also be noted that most of the Corn Belt had reasonable moisture in the first several weeks after silking. While the recent dryness is probably taking the top end off of corn yield in say western Illinois, it is not likely to take it below trend without some other major catastrophe. Given the lower water use requirements of corn in the later stage of the season, this expansion of drought isn’t as ill-timed as say had this been the situation a month ago. Also, up until last week there had been a remarkable lack of heat in the month following silking, with almost all of the Corn Belt having no days over 95F in the 30-days ending 20 August (Figure 3). That lack of heat and several cool nights is the only reason that this season hasn’t taken a harder turn for the worse. The heat last week wasn’t particularly special by historical standards and was less significant in intensity and in duration than in August of 2003 when 100F heat went into central Iowa. But it was certainly hot enough to accelerate water stress issues that were already there.

Figure 3. Number of days over 95F between July 18th and August 20th. Credit to Eric Snodgrass of Nutrien for the figure generated from PRISM data. 

Speaking of water stress, avid blog readers know that I put a lot of value into the Quick Drought Response Index (QuickDRI) as a way of “truthing” water stress of vegetation. There’s good news and bad news here. Let’s start with the good news. QuickDRI has been less bullish on water stress than the SMI for most of the summer. It also was showing significantly less stress over the majority of the Corn Belt at comparable dates to 2003 (Figure 4).

Figure 4. QuickDRI comparison between 2003 and 2020. 

The bad news is QuickDRI has shown pretty rapid deterioration across much of Iowa and the more severe end of water stress has become more visible. But likely due to the lack of a long stretch of heat, the stress is less than in 2003 and certainly less than 2012.

The Final Corn Forecast:

When it comes to the final corn forecast for the year, I must admit that this is a tougher call than last year. Last season had a horrible start and a decent (but not great) rest of the season to finish below trend. There was, in my opinion anyway, a more narrow range of yields that were likely. This year had much better planting conditions overall and conditions in the month after pollination were generally very good. Those things would point to very strong yields, possibly bringing corn into the 190-195 bpa range if improvement had occurred in western Iowa. But that clearly didn’t happen and the recent dryness and severe drought conditions over some of the most productive areas of Iowa makes that high-end yield very improbable. The forecast of 189.4 bpa from August 3rd, when conditions for August looked better, is not likely to verify.

However, it is also my expectation that corn in the other top producing states will still have solid yields and corn in more marginal areas such as northern Missouri and southern Illinois will be up. The wild cards for me are the north central and northeast Iowa districts, which have mostly been on the periphery of the flash drought and have caught some rain over the past ten days. They won’t offset all the loss from the western part of the state, but they may help push Iowa closer to the national average. Another potential wild card is Nebraska, which could be looking at record rainfed production in the southeast coupled with very strong yields in the prime irrigated country from York to Holdrege. In a way, this could be the inverse of 2005 when the prime corn growing region of Illinois was affected by drought and most of Iowa was up. While we’re on the topic of Iowa corn, I will make two other forecasts before offering up the final number.

  1. Iowa’s corn yield will be below the national corn yield for the first time since 1993.
  2. Illinois will produce more corn than Iowa for the first time since 1993.

It is my expectation for the current corn crop to finish at 181.7 bpa with a range of 177.0 to 185.0 (Figure 5). Corn production is expected to be 14.89 billion bushels.

Figure 5. Corn yield forecast for 2020. Red and blue arrows represent the lowest and high possible yields respectively.


About the author:


Eric Hunt is an agricultural climatologist from Lincoln, NE and has several members of his extended family actively farming in Illinois and Nebraska. Eric has been with AER since 2012 and received his Ph.D. from the University of Nebraska. Among other activities, he is currently working on NASA funded projects to study the evolution of flash drought. He routinely blogs about agriculture and weather on the AER website. He can be reached via email at and @DroughtLIS on Twitter.

About AER:

Founded in 1977, Atmospheric and Environmental Research is an award-winning environmental research, consulting and weather information services company with demonstrated expertise in numerical weather prediction, climate dynamics and radiation, circulation diagnostics, atmospheric chemistry, air quality and risk assessment, planetary sciences, remote sensing, satellite meteorology, and systems engineering. Consulting services are available. AER is a business unit of Verisk Analytics (VRSK). For more information, please visit our web site at

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