November 6, 2003

Spring months, not winter,
are key to moisture in Iowa soil

Virginia Wilber
digital Iowa staff reporter
Drake University

DES MOINES, IA -- April showers may not only mean May flowers, but may also mean healthy crops in the summer. For Iowa's rich farmland that depends on good soil moisture, precipitation this winter will not be as important as precipitation in March, April and May 2004.

"Precipitation in the winter months does not matter as much as precipitation in the spring." Jeff Wallenfang, meteorologist with the National Weather Service office in Des Moines, said. "It doesn't take a lot of moisture in the spring to give us good soil moisture."

According to Wallenfang, weather in April can go a long way to make up for moisture deficits such as those caused by the lack of rain this summer.

Of course, April showers are not always as plentiful as farmers would like and the month can bring very windy weather, having the opposite effect farmers want of drying out soil.

Currently, statewide precipitation levels are 4 1/2 inches below normal, but Harry Hillaker, state climatologist with the Iowa Department of Argiculture and Land Stewardship, said large amounts of precipitation are not needed to bring these levels back to normal.

"People sometimes assume we need to make up those 4 1/2 inches but in reality we don't need large amounts of precipitation, especially for subsoil," Hillaker said. "We just need normal amounts of rainfall."

Another important factor is distribution of precipitation at spaced-out, regular intervals, Wallenfang said.

"If rains come at crucial times and are spaced out evenly... we can make up moisture deficits," Wallenfang said.

Typically, the winter months of December, January and February are the three driest months, amounting to less than 10 percent of total precipitation in a year. Wallenfang said the normal snowfall of about 30 inches is not a large liquid equivalent.

Hillaker said with the ground frozen during the winter months, not much precipitation can make it into the ground to affect subsoil moisture.

Topsoil is the first 6 to 12 inches of soil and has moisture percentages that vary quite a bit during the growing season. After the topsoil, the next 4 to 5 feet are referred to as subsoil. Moisture percentages of subsoil do not vary as much as those of topsoil.

Besides the amount of precipitation received in the spring months, how quickly temperatures rise also affects soil moisture. The slower temperatures rise, the more moisture is absorbed by soil.

Based on past winters that have followed similar weather conditions like those currently in Iowa, the National Weather Service is predicting normal temperatures and precipitation for this winter.

"There have been no strong pattern influences to sway predictions from a normal winter," Wallenfang said.

One thing that has been somewhat uncharacteristic of Iowa weather this year, Wallenfang said, is that there have been no strong El Nino or La Nina conditions.

El Nino conditions refer to warm sea-surface temperatures in the Pacific Ocean that occur when warmer water drifts eastward toward the Americas. Warmer than normal winters, like that of 1999, are usually the result of El Nino.

On the other hand, people blame colder winters on La Nina. La Nina conditions occur when the waters of the Pacific Ocean are colder than normal. La Nina usually allows for much less variability in temperatures and can lead to conditions like that of the winter of 2001.

When conditions are neutral, such as the National Weather Service is predicting for this winter, they are referred to as La Nada, literally meaning "the nothing." Last winter Iowans also experienced such neutral conditions.

The Iowa Department of Agriculture and Land Stewardship's prediction is that midwinter will be a little drier and warmer than usual.

"We are experiencing La Nada conditions and possibly weak El Nino conditions for drier weather," Hillaker said. "However, we are predicting that the spring will bring normal temperatures."