An evaluation of the Standardized Precipitation Index, the China‐Z Index and the statistical Z‐Score

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Abstract
The Standardized Precipitation Index (SPI) was developed to detect drought and wet periods at different time scales, an important characteristic that is not accomplished with typical drought indices. More and more users employ the SPI to monitor droughts. Although calculation of the SPI is easier than other drought indices, such as the Palmer Drought Index, it is still relatively complex. In China, an index called the China‐Z Index (CZI) has been used since 1995 by the National Climate Centre of China to monitor moisture conditions across the country. The calculation of this index is easier than the SPI. A third index, the statistical Z‐Score, can also be used to monitor droughts. This paper evaluates the SPI, CZI and Z‐Score on 1‐, 3‐, 6‐, 9‐ and 12‐month time scales using monthly precipitation totals for four locations in China from January 1951 to December 1998 representing humid and arid climates, and cases of drought and flood. Advantages and disadvantages for the application of each index are compared. Study results indicate that the CZI and Z‐Score can provide results similar to the SPI for all time scales, and that the calculations of the CZI and Z‐Score are relatively easy compared with the SPI, possibly offering better tools to monitor moisture conditions. Copyright © 2001 Royal Meteorological Society

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