If we use one drought index, does it sufficient to make a decision for drought condition?

As WMO defined, four types of drought are defined: a) the meteorological drought which is defined as a lack of precipitation over a region for a period of time, b) the hydrological drought which is related to a period with inadequate surface and subsurface water resources to satisfy water needs, c) the agricultural drought, which, usually, refers to a period with declining soil moisture and consequent crop failure, d) the socio-economic drought which is associated to the failure of water resources systems to meet the water demands.

The drought indices try to quantify every kinds of drought. Drought indices are very important to monitor and to assess drought, because they simplify complex interrelationships between many climate parameters. In each drought indices, we need to different weather-climate data, such as precipitation, temperature, soil moisture, evapotranspiration, and etc., so the performance of each indices is different. Numerous specialized indices have been proposed to quantify the four types of drought events. Precipitation has been used to develop a variety of indices, because it is a key variable to study meteorological drought.

Although most of the drought indices are strongly cross-correlated and exhibit rather comparable seasonal and annual DDI trends, selecting a single most appropriate index of meteorological drought is still a dificult task and the difficulty arises from various sources including the spatial variability of climates and the temporal scales of the intended applications (Morid et al., 2006; Shahabfar and Eitzinger, 2013). Therefore, a variety of indices should always be examined to select the best or better drought indices for a specific case study (Salehnia et al., 2017).

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