Effects of Kernel Functions on SPEI and Drought Characteristics

Drought management and planning are based on recognizing drought characteristics and its spatial and ‎temporal extent. In the present research, SPEI12 was calculated using 4 kernels including rectangular, ‎triangular, circular and Gaussian at the 12 Zayandeh Roud watershed’s climatological stations. Four ‎important characteristics of drought including the time of occurrence, duration, magnitude and severity ‎were evaluated by applying the highest weight to the 5th month. It was revealed that kernels other than ‎rectangular were more sensitive in recognizing the four drought characteristics. While the rectangular ‎kernel showed the highest coefficient of variation (CV) in drought duration, the Gaussian kernel has the ‎least CV in duration and magnitude and the circular kernel has the least CV in severity. Moreover, it was ‎found that the Gaussian kernel was more successful in detecting the occurrence of drought in comparison ‎with others. Finally, the results of this research indicated that rectangular kernel i.e. using equal weights ‎for all months in deriving SPEI, may lead to overestimate or underestimate drought characteristics.‎