September 3, 2021
Forty-five years-ago in September, Minnesota was amid its driest-ever autumn season.
The drought of 1976 had already inflicted serious damage to agricultural production, with many counties petitioning for federal disaster assistance. Hydrologic systems (rivers, streams, lakes) were devastated as well, dropping to dangerously low levels for aquatic habitat and river navigation.
Total precipitation for September to November of 1976 only averaged a little more than 1.5 inches statewide, about 5 to 6 inches below normal. The challenging and risky wildfire season, which started in the summer, carried over into the autumn as well.
The state agricultural economy and the outlook for crops and livestock to rebound in 1977 seemed dismal at best. Nobody would have guessed that 1977 would bring the wettest year in state history (up to that time), a 1-in-a-hundred chance. With such a weather about-face and some good management decisions, many Minnesota farmers were able to turn things around in 1977.
This example is an extreme rarity in climate terms. In Minnesota, recovery from severe drought is rarely achieved without the benefit of abundant precipitation in the autumn season. The autumn season is the primary time of year that agricultural soils are recharged with moisture in Minnesota.
Autumn precipitation ideal
University of Minnesota Agricultural Experiment Station studies have shown that 70% to 85% of the precipitation that falls during the autumn season infiltrates the soil and is stored for next year’s crop season. This storage efficiency is a significantly higher percentage than any other time of the year.
Many factors contribute to the higher efficiency of storage. By autumn, crops have matured, been harvested or are going into winter dormancy, and their roots are no longer taking up soil moisture. In addition, the shorter day length and cooler temperatures diminish daily evaporation rates, and the larger-scale storm fronts that move over the state bring moderate rainfall rates that can readily infiltrate the soil and not produce so much runoff.
Thus, for drought recovery purposes, this autumn season of 2021 is quite significant — more important than most years. More than 75% of the Minnesota landscape experienced at least severe drought at some point this growing season, while growing-season rainfall deficits reached 8 or more inches in many areas.
An abundantly wet and prolonged autumn season before the soil freezes up would obviously be highly beneficial to drought recovery. Let’s hope this type of pattern emerges.
Seeley is an Extension professor emeritus of meteorology and climatology at the University of Minnesota.
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