Ag Weather Forum

Important Week for Iowa Rain

Bryce Anderson
By  Bryce Anderson , DTN Senior Ag Meteorologist
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The July 14 Drought Monitor places almost the entire western half of Iowa in dryness and drought including Severe Drought in eight west-central counties. (National Drought Mitigation Center graphic)

For the first time since the widespread drought year 2012, part of the state of Iowa is in severe drought on the U.S. Drought Monitor. Eight counties in west-central Iowa -- Cass, Adair, Audubon, Guthrie, Dallas, Carroll, Greene and Boone -- are either entirely or mostly in this drought classification.

The following effects are noted when severe drought develops: Crop or pasture losses likely; water shortages common; water restrictions imposed. This discussion focuses on the first effect -- crop or pasture losses likely. We're looking at an approximate figure of around 935,000 planted corn acres, between 6-7% of the total Iowa corn acreage. This area of Iowa has had close to 4 inches below-normal precipitation since the first of June. Soil moisture from the heavy rain year 2019 has been used up, with that drawdown increased by the heat wave during July 16-18.

For these counties, DTN's rainfall forecast this week through July 23 has an average of around 1.25 inches for these dry counties. If we include approximate evapotranspiration rates for these upcoming days, that total is an average of 0.61 inches, giving a net estimate of 0.64 inches average from rain this week. When the temperatures increase again during the latter part of the week, the ET rate can easily get to around 0.30 inches per day, so the precipitation offers about two days' worth of moisture. That means that the rainfall offers a benefit, but not long-lasting. The inch or so of rain would get the crop through the tasseling and pollination phase; then, there is the issue of moisture for grain fill.

Another feature that will be important is the intensity of the rain. Heavy rain will soak in less-than-moderate or low-intensity rainfall. If the rain all comes at once, most of it will end up as runoff. In an area where crops can make use of every available drop, that would be a bad break. Fortunately, the forecast details do not have that heavy rain happening.

In addition to west-central Iowa, almost the entire western half of the state of Iowa is either rated abnormally dry or in moderate drought by the Drought Monitor. There are a lot of acres where rainfall would be welcome.

Bryce Anderson can be reached at bryce.anderson@dtn.com

Follow him on Twitter @BAndersonDTN

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