Evaluating the sensitivity of precipitation-based drought indices to different lengths of record

Abstract

Nowadays, preparing long-term data is one of the main concerns of researchers in many countries. Many hydro-climatological studies have reported that at least 30-year data are required for such studies. Unfortunately, it is impossible to provide 30-year data in many countries (even developed countries). Even the recording the precipitation data in many of the regions varies from one station to another station. There are also regions which cannot be ignored due to data for less than 30 years. The objective of this research was to evaluate the sensitivity of the seven different precipitation-based drought indices, including the China Z Index (CZI), the Modified China Z Index (MCZI), Percent of Normal Precipitation Index (PNPI), Deciles Index (DI), the Z-score Index (ZSI), Effective Drought Index (EDI), and Standardized Precipitation Index (SPI) to different lengths of record at monthly, seasonal, and annual time scales. In this research, monthly, seasonal, and annual precipitation data of 8 meteorological stations representing eight different climate classes in Iran were used for a 55-year period (1960–2014). For each of the monthly, seasonal, and annual time scales, the lengths of record 55, 50, 45, 40, 35, and 30 years were extracted from the main period (1960–2014), which each of these lengths was lengths ending to 2014. First, the correlation coefficient of the indices was obtained for all lengths of record. Then, match and non-match of the indices obtained from the longer lengths of record with the values obtained from shorter lengths of record were investigated for all three-time scales. Finally, the unique drought and flood years of stations were compared for the time scales. The results revealed that better time stability was seen in the Effective Drought Index (EDI) and Z-score Index (ZSI) compared to other indices in the monthly time scale. It means that the sensitivity of these two indices to different lengths of record was less. Percentage of Normal Precipitation Index (PNPI) in two seasonal and annual scales also shows the lowest sensitivity was seen among the studied indices. However, Deciles Index (DI) and the Modified China Z Index (MCZI) were two indices, which had the highest sensitivity to different lengths of record among other indices. It means that these two indices have more sensitivity to lengths of record studied and they should be used carefully.