Several meteorological drought indices are developed to describe the characteristics of climate change in multi-temporal scales, e.g., the Standardized Precipitation Index (SPI) [14] and SPEI [15]. SPEI was developed combining the sensitivity of the Palmer Drought Severity Index (PDSI) [16] to changes in evaporation demand and robustness of the multi-temporal nature of the SPI. It is defined as an index of water balance (precipitation minus PET). The Penman-Monteith equation [17] and the Thornthwaite equation [18] are two representative parameterizations using the underlying physical principles (changes in available energy, humidity, and wind speed) and air temperature, respectively, for PET calculation. Considering available climatic data in the YZR Basin, we employed the Thornthwaite equation based SPEI in this study. A time-series of the difference between precipitation and PET was fitted to a three parameter log-logistic probability distribution to take into account common negative values [19]. This is done because the log-logistic distribution shows a very close fit to the data series. SPEI employs the dry and wet categories as used by SPI in Table 1 [20].