The Hutchinson Drought Index

In the Hutchinson Drought Index the raw monthly rainfall totals are integrated to rolling 6-monthly totals which are then ranked into percentiles by month and this is rescaled to range between -4 and +4 in keeping with the range of the Palmer Index. Mild drought is below -1 in the Palmer index and so consecutive months below this threshold are counted. In the original method 5 or more consecutive months was defined as the beginning of a drought, which continued until the rescaled percentiles exceed -1 again. The enhanced method imposes a more conservative threshold of zero (the median) to break a drought.

There was also an alternative method devised by Hutchinson where the rescaled percentile values are integrated using conditional cumulative sums.