Evaluation the length of dry spells dependent of Precipitation with approach of climate change in Kohgiluyeh and Boyer-Ahmad provinces

Introduction
The study variability of precipitation change in Iran is more important than other climatic variables. Precipitation changes can control the length of dry spells in arid and semi-arid such as Iran. Extraction and analysis of precipitation, is a good criterion for understanding the behavioral mechanism of Dry Spells. Recognition the change in length of Dry Spells such as the frequency of events, the length of periods, its severity and continuity is very important in environmental planning. Therefore, study of the occurrence and sequence of dry Spells is great importance in climate change studies.
Materials and methods
for identify and describe the precipitation, the firt step, provided database matrices of precipitation data in a period of 30 years from (since1990-2020) for 8 precipitation stations in the province area. In second step for evaluate the behavioral similarity of station data and assignment of all stations to a homogeneity criterion, was performed Homogeneity test (HT). by used and the help of programming and coding in MATLAB and R software, is calculated on a seasonal and monthly scale with a threshold of 5 mm to be the basis for extracting and zoning dry Spells in ArcGIS software. The behavior of dry Spells with different continuities was analyzed and zoned using the second-order Markov statistical chain. In the last step for predict the behavior of dry spells in the middle decade (2050) by referring to the scenarios of HADGEM2-ES model related to the fifth report of the intergovernmental panel of climate change (IPCC-IR5), to downscalling and reproduce data in the statistical environment of SDSM software for all stations. In the final step, the seasonal behavior and the area of dry periods in the future horizon were plotted. How to do each step will be described in the research sections.
Results and discussion
 In order to determine the degree of homogeneity and to determine the stations that are in the spatial range τ_4 and τ_3, the Husking torque homogeneity test was performed to determine the data and the outlying and heterogeneous stations. According to the definition of stations, the test component H₁ for Lcv, H₂ for the combination of Lcv and Lskew, and finally H₃ for the combination of Lkurt and Lskew, where the statistics inconsistency is less than 1, are considered homogeneous stations. Stations with more than one statistic were excluded from the study and other stages of the research. Lodab and Abdehgah stations had such conditions. Mean and output of all stations for statistics at level /.43. Adjusted (average of H₁ to H₃ statistics). In fact, this method minimizes inter-station variations in linear torque to increase the reliability of the network data usage.
 In general, the study of wet and dry monthly events showed that throughout the province, dry periods with a duration of 30 days or more have the highest probability of occurrence and the shortest return period. Based on the output of monthly emission scenarios of climate change models, the length of dry spells in the province in the cold seasons of the year with an average (29%) has a more significant increase than the warm periods of the year with an average (19%). It can be said that in tropical and arid regions the return period with short-term continuities and in mountainous and cold northern regions, the return period with long-term continuities is more dominant. Wetlands of the province are more vulnerable to drought and climate change and from west to east and from south to north increases the area of dry areas in the province. In other words, the border of arid regions shifts to the east and north.
Conclusion
The change in the length of dry periods in the arid and super-arid zones of the south and west of the province, especially cities such as Gachsaran, Imamzadeh Jafar and Dehdasht, which have polluting industries such as petrochemical and oil industry, is a serious warning for the dominant industries such as water resources, agriculture and tourism. Is. Challenges arising from the drying up of water resources such as Hur al-Azim wetland and declining discharges of rivers and dams in the neighborhood in the southern regions of the province, the challenges arising from climate change such as increasing the length of dry spells intensify. It’s necessary for planning and adaptation to climate change and management of available water resources in all areas of the province, especially in the northern and eastern area, where the possibility of long-term dry periods is high and inevitable.