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

The PNI index is simple, by definition,easy to calculate,and is easy understood by a general audience (Smakhtin and Hughes, 2004). The index can be calculated for a variety of time scales. Usually these time scales range from a single month to a group of months representing particular season to an annual or water year (Willekeet al.1994). 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:


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

PN has a popular method that is quick and easy to calculate with basic mathematics.The PN is one of the most straightforward measures of rainfall deviation from its long-term mean. "Normal" may be and is usually set to a long-term mean precipitation value at a location. The value of "normal" may be calculated for a month, a season or a year and is considered to be 100%. The same PN may have different specific impacts at different locations and, therefore, it is a bit of a simplistic measure of precipitation deficit. Also, what is normal may be perceived differently in different regions.

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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