**2.5 Drought index parameter reliability analysis**

The parameter reliability analysis was conducted because the model describes the sample to be generalized to the population and this led to the application of a samplebased model suitability test. In mathematical analysis, reliability is the ratio of the

**Figure 3.** *Concept of research.*

total items to the total variance and was applied to the dry duration and deficit variables of the applied watershed. The general formula used is the Cronbach's alpha equation as follows [17]:

$$a\_r = \frac{k}{k-1} \left[ 1 - \frac{\Sigma \sigma\_i^2}{\sigma\_i^2 + 2 \left( \Sigma \sigma\_{\vec{\eta}} \right)} \right] \tag{9}$$

Where <sup>¼</sup> <sup>1</sup> 10 *i* <sup>2</sup> is the number of variances *i* (the number of diagonals), <sup>1</sup> <sup>10</sup> *ij* is the covariance of items i and j, *σ<sup>i</sup>* <sup>2</sup> <sup>þ</sup> <sup>2</sup> *Σσij* � � is the total variance, and <sup>α</sup><sup>r</sup> is the reliability of the model.
