Drought Probability? Check Your Soil.

February 5, 2013 05:04 AM

Last year’s drought was an exceptional case, but drought dials up crop damage every year – about $8 billion worth in an average year, in fact. But researchers at Oklahoma State University hope to create a reliable calendar of seasonal drought patterns to help farmers synchronize the growth of their crops with those periods of time when drought might be less likely to occur.

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 take into consideration an important component of drought.

"Atmospheric variables do not take into account soil moisture," says Tyson Ochsner, assistant professor of applied soil physics and lead author of the study. "And 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 traditional atmospheric analysis.

The researchers calculated the deficits by determining the difference between the soil’s water-holding capacity and its actual water content. They then determined a threshold at which plants would experience water stress – and therefore, drought conditions. Each threshold was individually calculated because of the specific soil characteristics at each site.

"The soil-water contents differ across sites and depths depending on the sand, silt and clay contents," Ochsner says. "Readily available water is a site- and depth-specific parameter."

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. But where these data are 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 estimations for fields without access to soil-water data, he says. ready to plant 2

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 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, rain must fully wet the soil before it moves deeper down, he says. Any rainfall on drought-stressed ground is good, but the road to total replenishment might be a longer process than some realize. For full analysis on the 2012-13 drought, visit AgWeb’s drought news and coverage page

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