How the MBDI Compares
Funder & Affiliates
Rank of drought severity
To compare drought severity for a specified time frame, the Moisture Balance Drought Index (MBDI) uses a ranking system based on where that time frame falls in the historical line-up since 1895.
The driest years will fall into the lowest ranks, such as bottom quarter of values – the 25th percentile or below. The wettest years will fall within the upper ranks, the top 75th percentile or above. Because the historic record contains more than 100 years, the driest years can have values below 1.
The table below uses values for Payson, Arizona, for the months of June during the decade from 1997-2006 to provide a simplified illustration about how the MBDI ranking system works. In actuality, the record goes back to 1895, so even assessments of conditions from a single decade will be ranked against the full record.
Using real climate data from Payson, Arizona, this hypothetical example shows how the MBDI ranking system would work at the scale of a decade.
Credit: Table design by Jorge Arteaga
In this example, June of 1997 had the lowest value for P – PE in the record, while June of 1998 had the highest value. As a result, June of 1997 was ranked in the lowest percentile (10%) while June of 1998 was ranked in the highest percentile (100%). In practice, ranks can fall below 1 because there are more than 100 years in the record.
Rankings can be undertaken at a variety of scales, from one month – as in this case – to four years. This use of multiple time scales allows for comparisons of the multiple dimensions of drought.