Classification of changes in the length of rainfall-dependent dry periods in Iran

To study the behavior of dry period lengths, precipitation data were used on a daily scale for 45 synoptic stations in Iran (1985-2017). In order to spatially distribute the dry periods, sequences of 10, 20, 30 and more than 30 days were used and turned into zones. The results showed that the highest frequency of long-term dry periods (30 days and more), with 86 events is related to the south-eastern part of Iran and Iranshahr station. The lowest frequency with 3 cases belonged to Rasht station on the southwest coast of the Caspian Sea in northern Iran. The second-order Markov probability distribution was used to evaluate the type of dry period distribution, return continuity and their probability of occurrence. Probability matrix and return period for 10, 20 and 30 day continuities were calculated on a monthly scale and it was determined that June and April were the shortest dry period return periods (18 days) in the arid central and eastern regions of the country with the highest probability of occurrence ( 89%) and the longest return period is related to October and November in the wetlands of the north and northwest coast of the country (338 days) and the lowest probability of occurrence (28%).