Evaluation of drought duration risk using annual secular precipitation data in ancient stations of Iran

The aim of the study is to evaluate the return period and risk of drought duration in five ancient and unique stations of Iran, having more than one-hundred annual precipitation data. For determining the return period and risk of drought in the stationsof interest (including Jask, Bushehr, Isfahan, Mashhad and Tehran) the following stages were performed: a) Applying a homogeneity test (i.e., Run test) on annual precipitation series was indicated the Jask station unlike other stations which were homogenous, is non-homogenous, b) Using a goodness-of-fit test (), the most suitable distribution function for annual precipitation data was selected from among fourteen functions, c) Based on the best-fit function obtained for each station, the amount of precipitation for seven probability levels (Thresholds) from 0.1 to 0.7 (with step 0.1) were determined, d) With respect to the precipitation of each probability level and subtracting it from annual precipitation data during record period, dry periods were indicated and drought duration for each dry period was computed, and e) The two statistical characteristics of drought including return period and risk were calculated with different mathematical equations. The results based on the annual precipitation series of four aforesaid stations showed: a) For a given drought duration, as the probability of occurrence of precipitation increases the return period decreases, or with a certain probability of occurrence of precipitation, as increasing drought duration the return period also increases, b) For the probability of occurrence of drought 30 %, the longest drought in Isfahan (i.e., six-year duration) and other stations (i.e., three-year duration) occur once each 2000 and 51 years, respectively, and d) for a certain probability of drought occurrence, as increasing the basic period, the risk of drought increases. For instance, for the probability of occurrence of drought 50 %, it was identified that the drought risk in Bushehr and Tehran station is more than Isfahan and Mashhad.