Drought Probability?

March 9, 2013 02:57 AM
Drought Probability?

New study uses soil moisture deficits for better analysis

The 2012 drought was an exceptional case, but drought dials up crop damage every year—about $8 billion worth in an average year, in fact. Researchers are now busy trying to find better ways to analyze and monitor drought.

For example, researchers at Oklahoma State University (OSU) are creating a calendar of seasonal drought patterns to help farmers synchronize the growth of their crops with those periods of time when drought is less likely.

The challenge lies with how drought is calculated. Researchers tend to use common atmospheric data such as temperature and rainfall. But these measurements do not factor an important component of drought.

"Atmospheric variables do not take into account soil moisture," says Tyson Ochsner, OSU assistant professor of applied soil physics and lead author of the study. "Soil moisture can provide an important buffer against short-term precipitation deficits."

Ochsner and his co-authors tapped into 15 years of soil-moisture measurements from eight locations across Oklahoma to calculate soil water deficits and analyze days on which dry conditions would be most likely. They found that soil-moisture deficits more successfully predicted water-stressed plants than atmospheric analysis.

Researchers calculated deficits by determining the difference between the soil’s water-holding capacity and actual water content. They determined a threshold when plants would experience stress from lack of water. Each threshold was individually calculated because of the specific soil characteristics at each site.

The researchers also found that previous attempts to use atmospheric data often underestimated drought conditions. They hope to use soil water data whenever it’s available to better paint a picture of the days on which drought conditions are most likely.

Where this data is not available, Ochsner is looking into changes to atmospheric predictions to make them more accurate. Further research is needed to optimize atmospheric calculations and provide more accurate analysis for fields without access to soil-water data, he says.

Stubborn soil-water deficits. Got a soil water deficit on your farm? It might not go away easily, even with multiple soaking rains. Iowa State University ag meteorologist Elwynn Taylor serves up the following hypothetical to illustrate the issue.

Assume that your field suffered severe droughts and there is no available water in the rooting zone, around 5' deep. If you assume the field has a water-carrying capacity of about 2" per foot, even a nice, soaking, 1" rainstorm isn’t going to penetrate 5' into the soil—not even close.

"Let’s say an inch of rain falls, none runs off, and it all soaks in," Taylor says. "The soil has mud in the top 3" for a few hours, but it soaks down about 6". After a day or so, half the water in inch-6 wicks into inch-7. In another day, half the water in inch-7 wicks into inch-8. That’s about as
far as it goes."

In other words, any rainfall on drought-stressed ground is good, but the road to total replenishment might be a longer process than some realize.

In fact, researchers at the University of Missouri say a two-year recovery is a more likely reality in many areas.

Randall Miles, associate professor of soil science at the University of Missouri School of Natural Resources, also has observed soil in the Midwest that is dry down to 8'. He says in 2012, corn roots often had to go down to this depth to extract water. Using that same 2"-per-1'-of-soil metric, Miles says that adds up to a 16" water deficit.

"Even if parts of the Midwest receive a lot of snowfall and rain this spring, it will take time for the moisture to move deeply into the soil where the driest conditions exist," he says.

In those areas that need to overcome a 16" shortfall in soil moisture, farmers don’t just need 16" of rainfall—they need 16" above normal conditions, Miles explains.

Soil moisture isn’t the only thing desperately seeking recovery from the drought, Miles adds. Two years is also the estimated time it will take to rebuild soil microbes. in the horizon.

Barge traffic on the Mississippi and Missouri rivers is also patiently waiting for the water levels to rise.

"Until we start seeing normal rainfall, the water levels on the Mississippi and Missouri rivers are going to be low, affecting the number of barges that can pass on the rivers at one time," the soil scientist says.

AgWeb Exclusive: Soil Moisture Maps

Maps that track rain, snow and other weather events are helpful planning tools. The Climate Corporation, a weather insurance company, has developed a similar map that tracks the depletion and replenishment of soil moisture throughout the season. The first of these maps tracks Illinois soil moisture through the 2012 season, is available on www.agweb.com/soil_moisture.

"We created a video visualization of how drought swept across the state of Illinois this past year, starting in April and going through September," says Jeff Hamlin, Climate Corporation director of agronomic research. "It gives the very granular view of how the soil moisture conditions varied across each 2.5x2.5-mile square of land in the state for each day from April 15 through Sept. 30."

The model assumes an April 15 planting date with 108-day corn, he adds.

Climate Corporation plans to develop similar maps for several additional Midwest states that will update weekly throughout the 2013 season. These maps will appear exclusively on www.agweb.com—check in after May 1 to track soil moisture progression in your state.

Soil Moisture Chart

You can e-mail Ben Potter at bpotter@farmjournal.com.

FJ Drought Probability p58

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