*3.2. Trend Analysis*

To reveal the trend of the drought condition from 2001 to 2018, a temporal trend analysis based on the ordinary least squares (OLS) regression was conducted for each drought index (*DI*) pixel. Then, a linear equation of a *DI* was fit as a function of the variable "YEAR" to calculate the slope (Equation (4)). An image of the changing slope over the period 2001–2018 was thus obtained.

$$SLOPE = \frac{n \times \sum\_{i=1}^{n} i \times DI\_i - \sum\_{i=1}^{n} i \sum\_{i=1}^{n} DI\_i}{n \times \sum\_{i=1}^{n} i^2 - \left(\sum\_{i=1}^{n} i\right)^2} \tag{4}$$

In Equation (4), *n* represents the total number of observation years (*n* = 18). *DIi* represents the mean value of drought index for the *i*th year. *SLOPE* > 0 represents an increasing trend of *DI* from 2001 to 2018. Conversely, *SLOPE* < 0 represents a decreasing trend. F-statistics were conducted to determine the significance of the fitted linear regression model.
