Could you please tell me, What are the basic criteria's for a drought index?

Drought monitor is really essential for knowing drought events. In this regards, we can identify various indicators of drought, such as rainfall, snowpack, streamflow, and more, and track these indicators to monitor drought. A drought index value is typically a single number, which is interpreted on a scale of abnormally wet, average, and abnormally dry. The American Meteorological Society (1997) suggests that the time and space processes of supply and demand are the two basic processes that should be included in an objective definition of drought and, thus, in the derivation of a drought index. The World Meteorological Organization defines a drought index as "an index which is related to some of the cumulative effects of a prolonged and abnormal moisture deficiency" (World Meteorological Organization, 1992).

Many different indices have been developed over the past several decades to indicate the occurrence and drought severity. The simplest index relates precipitation amounts during a specific period of time to the historical average during that same time period. Several different indices of varying complexity have been developed to assess drought based on both water supply and demand using multiple environmental criteria. Friedman (1957) identified four basic criteria that any drought index should meet:

1) the timescale should be appropriate to the problem at hand;

2) the index should be a quantitative measure of large-scale, long-continuing drought conditions;

3) the index should be applicable to the problem being studied, and

4) a long accurate past record of the index should be available or computable.

Friedman, D. G., 1957: The prediction of long-continuing drought in the south and southwest Texas. Occasional Papers in Meteorology, No. 1, The Travelers Weather Research Center, Hartford, CT, 182 pp.

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