Storms in Iowa have been striking smaller areas with higher amounts of rain as the Earth has warmed, which has led to significant disparities in the soil moisture available for crops, according to Justin Glisan, the state climatologist.
“What we’re seeing now are pockets of very high rainfalls,” Glisan told an online gathering of the Iowa Farmers Union on Thursday.
Iowa’s average temperatures have warmed about 1.3 degrees since the late 1800s. Between 0.2 and 0.5 degree of that increase has happened in the past decade, and that has led to more available moisture in the air, Glisan said. Every degree increase results in about 4% more water vapor.
He said the state’s rainfalls followed a more predictable pattern decades ago, with the state being drier in the northwest and wetter in the southeast.
That has trended toward heavier rain storms striking narrower areas, increasing the potential for certain areas to get large amounts of rain and others to get far less.
An example: In 2018, when southeast Iowa had significant drought, the northern third of the state had its wettest year on record, Glisan said.
Plymouth County is unlucky this year
This year, the unlucky area with extreme drought is in northwest Iowa near Sioux City, which had some recent rainfall but not enough to substantially improve conditions.
“One storm’s not going to do it,” Joel DeJong, an Iowa State University Extension field agronomist who monitors counties in that corner of the state, told Iowa Capital Dispatch.
Plymouth County is almost fully engulfed in extreme drought, according to the latest U.S. Drought Monitor report on Thursday. That is the second-worst classification, next to exceptional drought. But travel two counties to the north to Lyon and there is ample water in the soil and no drought.
“You get along the Minnesota border and precipitation is pretty good,” DeJong said. “Parts of my nine counties actually are sitting in really good shape right now, but some of it really has some potentially significant stress coming.”
That’s because the rest of July is predicted to be warmer and drier than normal, which is also a trend that has emerged for the month in recent years.
Climate change creates volatility
As the climate has warmed there has been more water vapor in the air to rain down on Iowa, but July — a crucial month for crop development — has been drier, Glisan said. April and October are trending wetter, which can complicate planting and harvest.
That was true this year when planting was delayed in much of the state for weeks by persistent rains. That has the potential to limit corn yields because it will shorten the period of time for growth. But, unlike last year’s more widespread dry conditions, this year there is adequate topsoil and subsoil moisture in more than two-thirds of the state that can help crops withstand dry weeks.
Despite the increasing volatility of the state’s rainfall, Glisan predicts crop yields will continue to increase for decades because of the overall increase in rainfall and innovations in agriculture.
“The flip side of that is once we get out in the 2060s, 2070s, that’s where we start to see precipitation extremes catch up with a rise in temperature, so we’re expecting wetter wets and drier dries,” he said. “Those droughts could become more pervasive if we continue to see the temperatures rise.”
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