Researchers question reliability of Palmer Drought Index

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A popular drought prediction tool is at the center of a new study.  This tool, the Palmer Drought Severity Index (PDSI), is a standard measure of drought conditions, but according to a report from The Christian Science Monitor, the PSDSI’s simplistic design leaves room for scientific interpretation error that can cause some scientists to overstate drought trends.

The study, published in the journal Nature, demonstrates the need for scientists to use the right tool for the job. For projecting future global drought trends, however, that tool is not the PDSI.

The index’s original design makes it difficult for scientists to assess future drought risks and anticipate the extent, intensity and frequency of these future occurrences.

Eric Wood, a professor at Princeton University and a co-author of the study, points that some drought projections produce results where “you’d think it will never rain again.”

The research team behind the study found that the problem with the index lies in its design. Originally the PDSI was developed to track drought conditions on a short-term basis, such as from week-to-week, with a focus on key agricultural areas of the United States.

It was not designed to track global trends over the course of decades or even centuries, yet some researchers continue to utilize its data as a projection tool. 

"We've known for quite a long time that the PDSI calculation is prone to problems dealing with climate change," Richard Seager, a researcher who focuses on drought and climate change at Columbia University's Lamont-Doherty Earth Observatory, told The Christian Science Monitor. "Rising temperatures drive it haywire."

The issue with the index has been traced to how it determines evaporation rates. Currently it uses precipitation and temperature to calculate this rate, but other factors, including relative humidity, wind speed and the amount of the sun’s energy reaching the surface, also affect evaporation rates.

Researchers found significant differences between the evaporation rates when it was used to calculate drought trends. While both calculations noted to a drying trend, the rates of this increase varied considerably.

“When the team calculated the PDSI using the temperature-focused approach for evaporation and used the results to identify drought trends, the results indicated that between 1950 and 2008, dryness increased for 98 percent of the globe's land area,” researchers wrote in their study. “When the team used the more sophisticated approach to calculating evaporation, the picture was more mixed: 52 percent of the land area saw increased drying, while 42 percent saw a decrease.”

The results of the calculation adjustments were notable, though the researchers advised scientists not to depend on the index for predicting future trends.

Click here to read, “How reliable are drought predictions? Study finds flaw in popular tool.”



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