Background and Objectives: Climatic characteristics (average weather) of Semnan province due to its proximity to the central desert of Iran and southern slopes of Alborz have great variety, so that with decreasing altitude from north to south, the temperature increases and the amount of precipitation decreases. Knowledge of the distribution of the probability of precipitation provides a suitable basis for planning water and soil resources, as well as flood control and erosion management.

Materials and Methods: Competence of Markov chain model for estimating changes in dry days and probabilistic zoning (classification of land based on land use) has been studied and evaluated in many provinces of Iran. In this article, the behavioral pattern of the length of dry periods was investigated using daily precipitation data (1985-2017) for all synoptic stations in the Semnan province and a number of selected climatological and rain gauge stations using the Markov chain empirical distribution model.
Results: Results showed that the highest frequency of long-term dry periods (30 days and more) in Semnan province was related to the Hosseinian station in the south of Damghan city. The lowest frequency was related to the stations on the border of Semnan province with Mazandaran and Golestan provinces, such as Rezvan and Mojen stations in the northeast of the province. Markov probability distribution was used to evaluate the continuity, sequence, and recurrence of dry periods and the probability of their recurrence. The results indicate that Markov method is a powerful and capable method in investigating and predicting dry times and has provided results very close to the time of occurrence.
Conclusion: Based on the fact that in recent decades, the climate of Semnan province, following the climate of Iran, has been reported to be increasing in temperature, decreasing in precipitation, and increasing in drought. However, the variables and results indicate that the decrease in precipitation and increase in the number of dry days are quite evident in most areas of Semnan province, and minimum and maximum daily air temperatures will also increase significantly in the northeast of this province.