Historical Perspective of the 2021 Drought in Northern Illinois

Drought in northern Illinois this spring and summer caused impacts to agriculture, ecology, and water resources. However, reports from the region suggested the impacts from the ongoing 2021 drought in northern Illinois have not been as extreme as past severe drought events, such as those in 1988 and 2012. Here we provide a summary of the 2021 drought through October and compare weather, climate, and soil conditions from this drought to those in prior years.   

Very Dry Spring

The 2021 drought followed an exceptionally wet period in northern Illinois. The 18-month period between December 2018 and May 2020 was the wettest on record in the northeast Illinois climate division, and 2020 was the third consecutive year in which the May total precipitation record was broken in Chicago.

Following a cold and snowy end to winter, March 2021 was much milder by comparison and 1 to 2 inches drier than normal in northeast Illinois. The dry start to spring was followed by 1-to-2-inch precipitation deficits in April and May in northern Illinois, with the largest deficits in the central and northern Chicagoland area (Figure 1). Spring 2021 ended as the second driest in Lake County, fourth driest in McHenry County, and fifth driest in DuPage County. Spring 2021 was between 0.5 inches and 1.5 inches drier than in 2012 and 1988 in northeast Illinois.

Figure 1. Total precipitation in March, April, May, and spring 2021, expressed as a departure from the 1991-2020 normals.

Timely Summer Rain

The first half of June started warm and dry across the state. The average temperature in the first 20 days of June was the fifth highest on record in both the northeast and northwest Illinois climate divisions, and precipitation over the same time was the fourth lowest on record in both divisions. The drought in northeast Illinois began to spread west as most areas between Rockford and the Quad Cities began to show signs of drought.

A large atmospheric ridge established over the Pacific Northwest in late June, and most of Illinois found itself on the edge of a stationary front that produced several rounds of heavy rain across the state. Chicago received nearly 6 inches of rain in the last 10 days of June, almost twice what the city had received for the entire spring.

Following the wet end to June, July precipitation was hard to come by. Although July ended 1 to 2 inches drier than normal across northern Illinois, the month was also 1 to 2 degrees cooler than normal, which reduced the effects of evaporation and evaporative demand on crops and other vegetation. Meanwhile, high pressure over the north-central Atlantic Ocean kept moisture movement out of the Gulf of Mexico into the Midwest throughout the months of July and August. This persistent pattern caused unusually high humidity in northern Illinois despite very little soil moisture. In general, dry summers in Illinois tend to have lower humidity, but July and August average dewpoint temperatures, a common measure of humidity, were both above normal across northern Illinois this year.

Several rounds of rain moved across northern Illinois in early August, bringing an additional relief to parched soils, crops, and ecosystems. The rain was especially timely for agriculture, giving corn and soybean crops much needed water during reproduction. The latter two-thirds of August was very dry across the region and vegetation stress set in again. The re-emergence of drought was especially noticeable in northwest Illinois, as most areas west of Rockford had received considerably less rain in early August than farther east. Overall, summer 2021 was 3 to 4 inches drier than normal in northwest Illinois, and near to slightly wetter than normal in parts of northeast Illinois (Figure 2).

Figure 2. Total precipitation in June, July, August, and summer 2021, expressed as a departure from the 1991-2020 normals.

It is still too early to fully account the impacts of the 2021 drought in northern Illinois, and indeed most of the region remains in drought as of late October. Reports and observations from the area suggest unirrigated corn and soybean crops and specialty crops, including apples, did suffer from moisture stress and likely caused some hit to yields. Likewise, the drought caused a myriad of ecological impacts, including young and mature trees prematurely dropping leaves, lawns going dormant early in the summer season, and less vigorous blooms in native flowering perennials.

Additionally, many residents in the region reported very low levels in area ponds, lakes, and rivers. The unusually dry spring and start to summer caused concerns about near-record low flow on the Fox River in northeast Illinois. Several municipalities in McHenry County were under mandatory watering restrictions throughout the summer, and most counties and municipalities strongly encouraged water conservation due to the dry conditions. With that said, the known impacts of the 2021 drought in northern Illinois are not as severe or extensive as those from droughts in 1988 and 2012. In the next section we explore meteorological differences between these droughts and discuss some of the unique characteristics of this year’s drought event that may have contributed to overall less severe impacts.

Differences and Similarities to 1988 and 2012

In essence, drought is defined as a period of below normal precipitation sufficient to cause impacts. However, the severity of drought impacts is not solely dependent on the magnitude of precipitation deficit, but is also affected by precipitation timing, evaporative demand, and human demand on water resources. Because of these complex interactions, each drought has a unique set of characteristics and impacts.

The 2021 growing season drought in northern Illinois was, by some measures, as severe as those in 2012 and 1988. For example, the six-month period from March to September in 2021 was the driest on record in Rockford, half an inch drier than the same period in 1988 and over 2 inches drier than in 2012. However, as Figure 3 shows, the timing of spring and summer precipitation in the Rockford area this year differed quite a bit from those previous drought years.

Figure 3. Monthly total precipitation at Rockford between March and September in the drought years of 1988, 2012, and 2021.

March and April were drier this year than in 1988 and 2012. Ironically, dry weather in March and April can help reduce ecological vulnerability to drought later in the growing season. For agriculture, relatively dry soils in spring facilitate strong root growth, which means that plants can reach deeper layer moisture in the soil if the top layer dries out. Additionally, some species of deciduous trees will produce smaller leaves in response to spring drought, which reduces water loss through transpiration later in the growing season.

Although June was climatologically drier than normal this year, the greatly beneficial late June rain in northern Illinois this year separated it from the extremely dry months of June in 1988 and 2012. July precipitation this year was nearly equal to that in 1988 and 2012, but August was 1 to 1.5 inches wetter this year than in both previous drought events. Precipitation in early August was crucial to preventing widespread drought damage to crops and ecosystems and potential impacts to water resources in the region akin to impacts that occurred in 1988 and 2012.

Although precipitation is the primary driver of drought, the amount of atmospheric demand for evaporation from soils and transpiration from plants, often called evaporative demand, also plays an important role in drought severity and persistence. Evaporative demand is largely driven by air temperature, humidity, radiation, and wind. Very high evaporative demand will exacerbate drought impacts to agriculture, water resources, and natural ecosystems. Along with evaporation from lakes and streams, drought in the growing season usually causes elevated human demand for watering drought-stressed lawns and trees. These factors together can tax water resources and quickly deplete lake and stream levels. In turn, low evaporative demand during drought, although rare, can help reduce drought impacts.

The spring and summer of 2021 were both warmer than normal across northern Illinois, but daily maximum temperatures were not nearly as high as in 1988 and 2012. For example, the summer average daily maximum temperature in Chicago in 2021 was 84.1 degrees, 3 degrees less than in 2012 and 1988. Meanwhile, summer humidity in 2021 was consistently higher than in the summers of 1988 and 2012. Despite similar average summer temperatures in Chicago in the three years, the average dew point temperature in 2021 was 3 to 4 degrees higher (i.e., higher humidity) than in 2012 and 1988 (Figure 4).

Figure 4. Summer average temperature (horizontal axis) versus the summer average dew point temperature (vertical axis) in Chicago.

As Figure 5 shows using observations from the Illinois Climate Network station in DeKalb, the combination of lower temperatures and higher humidity in 2021 reduced evaporative demand relative to 1988 and 2012. Cumulative evaporative demand between March 1 and September 1 in DeKalb was 27 inches in 2021 compared to 32 inches in 2012 and 35 inches in 1988. When factoring in differences in precipitation between the years, the growing season water balance, the precipitation minus evaporative demand, in DeKalb in 2021 was 6 to 10 inches less than in 2012 and 1988.  This difference in evaporative demand kept more moisture in the soil later into the growing season this year relative to prior droughts.

Figure 5. Plots show the evolution of the 1988 (blue), 2012 (red), and 2021 (yellow) droughts in DeKalb. The left panel shows the cumulative water balance (inches) from March 1 to September 1 in each year. Water balance is calculated as the daily difference between precipitation and evaporative demand. The right panel shows the cumulative daily evaporative demand from March 1 to September 1 in each year. Potential evapotranspiration is used to represent evaporative demand.

From a weather and climate perspective, the biggest difference between this year’s drought in northern Illinois and the droughts of record in 1988 and 2012 boil down to (1) small but beneficial rains in late June and early August this year that did not occur in the previous two droughts and (2) substantially lower evaporative demand this year due to lower maximum air temperature and higher humidity.

When Will the Drought End?

Northern Illinois saw more rain in the first three weeks of October than in the months of May, June, and September put together. Although a bit too late for agriculture, lawns, and perennials, the rain went a long way to improve soil moisture and streamflow conditions.

Figure 6 shows the total amount of moisture in the top 40-inch soil column at the Illinois Climate Network station in Freeport. As of October 25, the top 40-inch soil column had approximately 12 inches of water, compared to over 14 inches in 2020 and 17 inches in both 2019 and 2018. Although soils are still much drier than in past years at Freeport, they have 1 to 1.5 inches of additional water in the top 40-inch column than in mid-October in 2012.

Figure 6. Mid-October soil moisture (inches) in the top 40-inch soil column at Freeport.

So, the region has improved considerably over the past 3 to 4 weeks, and soil moisture and streamflow are in better shape than this time in 2012. However, much of northern Illinois remains in an 8-to-10-inch precipitation deficit going back to the start of the year. Winter can be an important season for drought recovery, despite winter months being among the climatologically driest in Illinois. Recent research has shown about a quarter of droughts in Illinois end in winter, thanks in part to very low atmospheric and vegetation demand for water.

Soil moisture recharge in winter depends on the amount of winter precipitation, but also precipitation type and whether that precipitation falls on frozen soils. Precipitation ahead of soil frost development this winter will be beneficial to continuing moisture recharge across northern Illinois. Recharge efficiency of winter rain declines sharply once the topsoil freezes. Meanwhile, abundant snow and below freezing temperatures in mid- to late winter can help prolong healthy snowpack and facilitate gradual snow melt and soil moisture recharge into spring.

The National Oceanic and Atmospheric Administration (NOAA) recently released its 2021–22 winter forecast, which covers December 2021 through February 2022 (https://www.noaa.gov/news-release/us-winter-outlook-drier-warmer-south-wetter-north-with-return-of-la-nina). The outlooks show elevated odds of warmer and wetter than normal conditions across the state (Figure 7). A wetter winter does not necessarily mean above average snowfall, and warmer winters tend to limit persistent snowpack. However, wet conditions through early winter along with a delay to soil frost would help continue the drought recovery we have seen in northern Illinois in October.