Online Drought Indicators and Indices


Palmer Z Index


Index name: Palmer Z Index.

Ease of use: Yellow.

Origins: The Palmer Z Index responds to short-term conditions better than PDSI and is typically calculated for much shorter timescales, enabling it to identify rapidly developing drought conditions. As part of the original work done by Palmer in the early 1960s, the Palmer Z Index is usually calculated on a monthly basis along with PDSI output as the moisture anomaly.

Characteristics: Sometimes referred to as the ‘Moisture Anomaly Index’, and the derived values provide a comparable measure of the relative anomalies of a region for both dryness and wetness when compared to the entire record for that location.

Input parameters: The Palmer Z Index is a derivative of PSDI and the Z values are part of the output.

Applications: Useful for comparing current periods to other known drought periods. It can also be used to determine the end of a drought period, when it is used to determine how much moisture is needed to reach the near normal category, as defined by Palmer.

Strengths: Same as for PDSI. The scientific literature contains a number of relevant papers. The use of soil data and a total water balance methodology makes the Palmer Z Index quite robust for identifying drought.

Weaknesses: Same as for PDSI, with the need for serially complete data possibly causing problems. It has a timescale of approximately nine months, which leads to a lag in identifying drought conditions based upon simplification of the soil moisture component within the calculations. This lag may be up to several months, which is a drawback when trying to identify a rapidly emerging drought situation. Seasonal issues also exist, as the Palmer Z Index does not handle frozen precipitation or frozen soils well.

Resource: Contact NDMC to access the code for the Palmer suite,

Reference: Palmer, W.C., 1965: Meteorological Drought. Research Paper No. 45, US Weather Bureau, Washington, DC.

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