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How can we calculate PNI (Percent of Normal Index) and what are the inputs data and time scales?

The PNI was described by Willeke et al. (1994) as a percentage of normal precipitation. It can be calculated for different time scales (monthly, seasonally, and yearly). PNI has been found to be rather effective for describing drought for a single region or/and for a single season (Hayes, 2006). PNI is calculated as following:

PNI=(Pi/P)*100

where Pi is the precipitation in time increment i (mm), and P is the normal precipitation for the study period (mm).


Willeke G, Hosking J R M, Wallis J R, et al. 1994. The national drought atlas. In: Institute for Water Resources Report 94-NDS-4. U.S Army Corp of Engineers, CD-ROM. Norfolk, VA.

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