Spatial and Temporal Variations of Drought by Using Palmer Drought Severity Index (Case study: Ghareghom and part of Atrak Basins)

Drought spatio-temporal variations, for the dry-land farming season (March to June), were studied in Razavi and North Khorasan basins. Data were collected from five weather stations; namely: Mashhad, Sarakhs, Golmakan, Ghochan, and Bojnord; for a length of 34 year (1968 to 2001), as the base historic period. Palmer Drought Severity Index (PDSI) was adopted to analyze monthly wet and dry spells. Descriptive statistics for PDSIs’ were measured. Correlations for spatial dependencies (among the stations) and temporal dependencies were calculated and checked via paired t-test. The results showed high correlation coefficients for both. Moreover, a good fit of the PDSIs to normal distribution were found; when checked via Kolmogorov-Smirnov, Ryan-Joiner, and Anderson-Darling tests at 5% significance level. F-test revealed no significant difference between minimum and maximum standard deviations of the monthly indices. Positive skews of PDSIs for Sarakhs, Golmakan, and Bojnord represent higher frequencies of wet spells as well as more sever droughts; while for Mashhad and Ghochan the case is reverse.