Ochsner called the prediction method utilizing only atmospheric water deficit the AWD and refers to the method that factors in the soil water deficit as the SWD.

Fifteen years of daily precipitation, air temperature, and soil moisture measurements for eight locations across Oklahoma were used to calculate the probability of water deficits sufficient to cause plant water stress for each day of the growing season.

For the SWD method, the drought threshold was set at 50 percent depletion of the soil’s total available water capacity. For the AWD method, the threshold was a 7-d cumulative AWD of 50 mm, which researchers say provides an inferior prediction model than when soil water deficits are factored into the production model. In other words, researchers found that soil water deficits more successfully identified periods during which plants were likely to be water stressed than did traditional atmospheric measurements alone.

Calculating soil water deficits and stress thresholds for the eight study sites over a 15-year span, the research team compared their assessment of drought probability to assessments made using atmospheric data and found that when using atmospheric data alone, it often underestimated drought conditions, but when soil water deficit measurements were factored in, it provided a more accurate prediction model that consistently assessed drought probabilities.

But researchers warned soil moistures vary greatly from one site to the next, and each site must use specific soil moisture assessments in order to accurately improve the prediction model.

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

In some cases, soil measurements may not always be available, in which case calculations used for atmospheric assessments must be reconfigured. In two such instances, researchers decreased the threshold at which plants became stressed, allowing for a smaller deficit to be considered a drought condition. In addition, they increased the number of days over which atmospheric deficits were summed. After the changes, estimates better reflected soil water deficit probabilities.