**2.2 Calculation of hydrological drought index (HDI) in data statistical analysis**

The drought index is a comparison of the deficit to the watershed area as indicated in the following Equation [8]:

$$\text{HDI} = \frac{\text{deficit}}{\text{Area}} \, \frac{m^3/\text{sec}}{km^2} \tag{1}$$

*HDI* is the hydrological drought index, the deficit is the difference between X0 and Xt (ten daily), X0 is the dry threshold, and Xt is the ten daily periods of discharge.

Drought severity involves the analysis of the duration and deficit in dry conditions.
