The Ottumwa Courier

September 26, 2013

Rain's impact on drought muted

Courier staff writer

---- — OTTUMWA — It sounds hard to believe, but last week's rain didn't help most of the state's struggle with drought. In fact, it got worse in some areas.

The U.S. Drought Monitor released its latest report Thursday morning. For the first time since drought's resurgence this summer, part of the state entered extreme drought. It's a comparatively small area, a patch near Fort Dodge that encompasses only 1.76 percent of the state. But it's unwelcome.

Curiously, you don't need to go far from that patch to find an area of Iowa that has no drought rating whatever. A strip from approximately Dubuque to Waterloo is not considered to be drought-stricken at all. And the area has grown since the last report, expanding to just over 5 percent of Iowa.

Mark Svoboda of the National Drought Mitigation Center, said the week's findings aren't as counterintuitive as they seem. Different factors play into the map's creation, ranging from soil moisture levels to rainfall measurements. And it's important to remember drought doesn't arrive or leave in a single event.

“One thing is, when you're dealing with drought, it's kind of a cumulative impact,” Svoboda said. Instead, it's “a slow dwindling of the water supply.”

So, a given rain event might recharge streams or reservoirs while doing little to resolve drought. That's particularly likely if large amounts of rain fall in a short timespan, as it did last week.

That's when Svoboda said he center relies on contacts on the ground to make sense of the data. He cited State Climatologist Harry Hillaker as an important contact for Iowa. The center's experts can look at the data as recorded; Hillaker can tell them how the event unfolded.

The map involves a combination of objective measurements like rainfall and soil moisture and subjective assessments, like the impact on crops.

“Drought is really defined by its impact,” Svoboda said. And, in the case of rain, it's possible for those impacts to appear weeks after the event.

Southeast Iowa remained unchanged. Most of the region is in severe drought. Most of Appanoose County is a step lower, in moderate drought, but that's the only county in the area that finds itself primarily in a different category.

But what does severe drought mean? Svoboda said the categories relate to the probabilities of equivalent conditions occurring in a given year. Severe drought (category D2) indicates conditions in the 10th percentile. So these conditions would be expected to happen an average of one year in 10.

Svoboda compared the measure to the idea of a 100-year or 1,000-year flood. It's not a precise comparison, but it is one people tend to be more familiar with.

“This would be equivalent to a one-in-10-year dryness,” he said.

There is a good chance for rain in the forecast. The National Weather Service expects a 70 percent chance of rain for the Ottumwa area on Saturday. Totals could be up to a half-inch, not nearly enough to make up the area's nearly 3-inch deficit for the month.

But given that it was more than a month between the last two significant rainfalls in Ottumwa, it's not something the area is in position to spurn, either.