A comprehensive overview on applied drought indicators

One of the most important natural disasters is drought phenomenon. If it lasts longer than a season or a long period, it will have economic, social and environmental effects. Drought has no absolute definition and has a specific definition in each region. Each drought varies in severity, duration and extent. Moreover, Drought can be considered as the equivalent of a dry and unusual period or climate that lasts long enough to create a serious imbalance in the hydrological state of a region. Drought can also be regarded as an inevitable phenomenon, including disasters that cannot be prevented but can be managed and organized. Drought indices are an important tool for monitoring and evaluating drought and establish an accurate relationship between many climatic parameters. The information obtained from the indices can be useful for planners, designers and managers of water resources, which is also confirmed by comparative studies of indices.

This study focuses on studies of drought indices and how they function on different time and place scales. In order to conduct this research, the required materials were collected from domestic and foreign sources. In this study, 19 drought indices used in Iran and the world are evaluated, the results of different researchers are investigated, andlogical conclusions are made from different researches.

The results showed that each index can be used in different regions according to its nature and water resources indicators, plant-related indicators and risk indicators of agricultural, economic and social indices. All indicators in turn and in areas compatible with the nature of these indicators can have a good prediction of drought, and these indicators all should not be compared because the results of this comparison will not be appropriate, but they should be selected appropriately according to the study area. Meanwhile, time scale is very important in each of the indicators and should be considered in order to achieve a more reliable result. Another point that was obtained from the results of this study is that most of the indices used in Iran and the world have paid more attention to the severity of drought andless to the duration, extent and frequency of drought, which is one of the weaknesses of existing drought indices in Iran and the world. An index of drought can persist in the world to pay for the severity, duration, frequency and extent of drought simultaneously and accordingly, it can definitely have appropriate predictions.

The results showed that the indices of DI, SPI, PDSI, SPEI, BMDI index are the most widely used drought indices. The results also showed that the weakest drought index is the PNPI index and the strongest indeices of drought are SPEI and BMDI indices. In general, in order to use drought indices, the time and place scale related to those indices should be considered in their use. Since each index of drought has been created under certain conditions,  it is suggested that they should not be used in Iran or be used with caution. The overall results of this study suggest that it is better to conduct native statistical models of linear and nonlinear regression according to specific climates of Iran in relation to drought indices according to Iranian meteorological and hydrological data and be introduced multivariate Iranian drought index which definitely has more valuable results.