Call for standard indices to measure droughts

Can we tell if there are more or fewer droughts as a result of climate change? How does one compare the intensity, duration and spatial coverage of agricultural droughts in countries?

These were some of the questions raised at the recent International Workshop on Drought and Extreme Temperatures, organized jointly by the World Meteorological Organization (WMO) and the China Meteorological Administration (CMA), which pondered ways to cope with the unwelcome impact of droughts and extreme temperatures on agriculture, rangelands and forestry.

The problem is that there are at least 20 kinds of drought indices - numerical scales based on data such as rainfall and temperature - used to measure three types of droughts: meteorological, agricultural and hydrological.

"Many a time, appropriate indices are not used, as there are no standards or guidelines for the use of drought indices," said Robert Stefanski, a WMO scientific officer who works on weather and climate issues related to agriculture. The workshop has asked the WMO to develop standards for agricultural drought indices.

''The standards would address the appropriate use of an index for a particular type of drought (meteorological, agricultural, and hydrological)''

"Many agricultural drought indices are country- and sometimes even crop-specific, and are used as an early warning for decision-makers (governments and farmers)," said Stefanski. "The standards would address the appropriate use of an index for a particular type of drought (meteorological, agricultural, and hydrological)".

Prompted by the need to measure drought intensity and duration for climate-change studies, participants also called for a unified and standardized drought index.

Stefanski pointed out that the Intergovernmental Panel on Climate Change (IPCC) had used one drought index, the Palmer Index, "which was originally derived for the USA, and these kinds of indices need to be adapted and modified for the whole world."

"Standardizing and establishing guidelines for the use of indices will help such studies," he said, but developing a unified and standardized drought index would be "more difficult".

He said several ideas that combined two or three drought indices into a single one were presented at the workshop, "but this concept needs to be tested over different climates. This is why there needs to be further research into this issue."

For more information on the different types of drought see: