Drought indicators

Drought can be considered as a multi-scalar phenomenon, a function of the response time of the basin. In fact, the hydrological responses related to the flow of watercourses and groundwater recharge, soil moisture, as well as the biological response of crops and vegetation are variable and occur at different times. Therefore, the different definitions of drought refer to different time scales: 1-3 months for meteorological drought, 3-6 months for agricultural drought and 12 months and more for hydrological drought (Tirivarombo et al., 2018).

There are a number of statistical indices that allow for drought monitoring.

One of these is the SPI (Standardized Precipitation Index), which is based on the use of rainfall data only.

However, drought is a phenomenon that does not depend on a single climatic variable, but on multiple: rain, temperature, evapotranspiration, wind speed or the ability of the soil to retain water. Therefore, it is also useful to refer to a different index, the SPEI (Standardized Precipitation and Evapotranspiration Index), which considers both the rain and evapotranspiration variables, which can be calculated through models that exploit the temperature data. (Vicente-Serrano et al., 2010).

These two indices are able to monitor the phenomenon at different time scales (1, 3, 6, 12, etc.), which implies the possibility of analyzing the various types of drought (Tirivarombo et al., 2018). Furthermore, being standardized indicators, they allow the comparison between regions characterized by different climatic regimes.

Furthermore, these indices are able to express the importance of the episode of surplus or deficit of the water resource. In fact, the severity of humidity and drought events can be classified into different levels, depending on the range in which the relative index falls.

Negative values ​​of the index (lower than -1) show drier periods than the climatological norm, thus characterizing a deficit of water availability (drought), while positive values ​​(greater than 1) correspond to wetter climatic conditions (humidity). The greater the distance from normal conditions, the greater the severity of dry or wet conditions.


Classification of the severity of humidity / drought events


Tirivarombo et al. (2018), Drought monitoring and analysis: Standardised Precipitation Evapotranspiration Index (SPEI) and Standardised Precipitation Index (SPI), Physics and Chemistry of the Earth 106, 1-10

M. Vicente-Serrano et al. (2010), A Multiscalar Drought Index Sensitive to Global Warming: The Standardized Precipitation Evapotranspiration Index, Journal of Climate 23, 1696-1718

Beguería et al. (2014), Standardized precipitation evapotranspiration index (SPEI) revisited: parameter tting, evapotranspiration models, tools, datasets and drought monitoring, International Journal of Climatology 34, 3001-3023