Concerns Rising as Flash Drought Conditions Intensify
Summer temperatures have been through the roof across a large portion of the corn belt. Coupled with low and rapidly depleting soil moisture, some regions are experiencing or anticipating flash drought occurrences that could lead to major agricultural losses.
The loose definition of flash drought is a drought that develops rapidly over a short timeframe. Conventional drought is mainly brought on by a lack of precipitation, where flash drought is also influenced by other anomalies like abnormally high temperatures, strong winds, low humidity, and/or sunny skies that lead to abnormally high evaporation, or evapotranspiration (ET) rates. This includes moisture evaporation from crops themselves, which most of us know as “sweat” on corn and soybeans especially.
Flash droughts have occurred several times in recent years, including 2012 when widespread areas in the central U.S. experienced a three-, four-, or even a five-category increase in drought severity over a 2-month period. Locations that generally had near-normal conditions at the end of May had fallen into extreme drought conditions only two months later. A study of the 2017 Northern Great Plains Drought showed that the rapid onset of the drought in the spring and summer of that year was mainly due to failed precipitation in the May-July period over Montana, North Dakota, and South Dakota, which was the third-lowest on record dating back to at least 1895.
I ran across a really interesting study, “Flash Drought Characteristics Based on U.S. Drought Monitor“, that examined some seasonal patterns. According to the study, unlike conventional drought, which can occur year-round and everywhere in the United States, flash drought has preferred seasons and locations to occur, mostly in the warm season and over the central United States. They also find a strong correlation with La Niña events. During the 2000–2017 period, five years had widespread fast development of drought (2000, 2003, 2006, 2007, and 2012), and four out of the five years (except 2003) occurred after or during a La Niña episode.
During the onset phase of a flash drought, the American Meteorological Society says soil moisture deficits often develop in the topsoil layer first and then move deeper into the soil column; however, large deficits can also develop over a deeper layer if the vegetation has a deep root structure that can access subsoil moisture. To cope with higher atmospheric demand, vegetation often accelerates flash drought development through a more rapid depletion of root-zone soil moisture due to enhanced ET. As flash drought conditions continue to intensify, large soil moisture deficits develop over a deep layer of the soil column.
Visible signs of moisture stress appear as conditions become more severe, such as yellow or curled leaves in vegetation. These visible signs of deterioration tend to occur after the initial decreases in soil moisture. The intensification rate and final severity of a flash drought is strongly influenced by the strength and persistence of the atmospheric anomalies forcing its evolution, the magnitude of the precipitation deficits, and the vulnerability of the crops or rangelands to drought. The magnitude of drought stress also greatly depends on the stage of the crop. After the period of rapid intensification ends, a flash drought could potentially develop into hydrological drought or simply end with a heavy precipitation event. Below are more details about drought stress on corn and soybeans specifically.
Corn: A primary signal of drought stress in corn is leaf rolling, followed by greying of leaf tissue under extreme drought conditions. Drought stress after pollination and fertilization in corn can result in aborted kernels or poor kernel fill, causing low test weight and reduced yield. According to Iowa State University Integrated Crop Management (ICM), kernel abortion near the ear tip will occur in the two weeks following pollination. Continued drought into the milk stage will result in further kernel abortion and smaller, lighter kernels. Drought that occurs in the mid to late grain filling period (milk, dough, and dent stages) results in decreased kernel weights and premature physiological maturity. Once physiological maturity occurs additional drought stress will have no impact on grain yield. Because drought stress typically coincides with higher than normal temperatures the grain fill period is often reduced. Drought stress may also predispose the plants to development of stalk rots.
Soybeans: Leaf flipping is generally the first sign of drought stress in soybean plants. Severe conditions will lead to leaf clamping, where the outer leaves fold in, sandwiching the center leaflet. Drought can reduce pod number by up to 20% as a result of flower and pod abortion, according to ICM. Drought stress often results in earlier maturity or shortening of the grain filling period resulting in lower seed weights and yields. Soybeans will ultimately suffer from limited or completely halted nitrogen fixation, which can be brought on by even moderate drought. Drought stress also promotes spider mite infestations. Drought effects on soybean are generally not as severe as corn. (Sources: USDA, NOAA, NWS, ISU Integrated Crop Management, MDPI)