Synoptic analysis of extremely heavy rains and its effect on the peak discharge of Dez river floods (floods of 1993 and 2005)

Introduction
The importance of heavy rainfall and its consequences have caused this phenomenon to be of special importance in environmental planning and water resources management. Using statistical methods and the discharge data of the Dez river, the heavy floods of this river were extracted to identify the effective factors in their creation. Among all systems leading to heavy rains, the Sudanian system has been the main factor of moisture advection in the lower layer of the troposphere with its expansion to the north and northeast. Due to its thermal characteristics, this system has been prone to receive considerable moisture from the surrounding warm seas. This system has provided the necessary conditions for the creation of surface instabilities in most transitional systems independently and in a smaller number in the integrated state with the Mediterranean system. In the investigation of the middle layer of the troposphere in all the systems that led to the floods of 1993 and 2005, it was observed that the anticyclone cell of Arabia was established over the Arabian and Oman seas and the east of the Arabian Peninsula by moving eastward. In this situation, a very deep trough has formed in the west of Iran, and the southern end of the trough has extended to southern Sudan and northern Ethiopia. The eastward movement of high pressure in Arabia and the significant southward expansion of the Mediterranean trough and the formation of a low-altitude center over Iraq is the most suitable pattern for the heavy cloud rains in the basin.
 
Materials and Methods
In terms of geographic location, the Dez basin is limited between 48°10' to 50°21' east longitude and 31°34' to 33°7' north latitude. The studied area is a part of the Dez river basin, which is located almost in the middle part to the end part. After the Karun branch of one of the largest and longest Dez rivers, Dez is formed from two main branches named Caesar and Bakhtiari, and after leaving the mountainous region north of Andimeshk and Dezful, it enters the plain of Khuzestan. For comprehensive and accurate interpretation of pressure systems and assessment of their environmental effects on the earth, the maps of the earth's surface and level of 500 hPa are very efficient. In most cases, the wind direction and temperature distribution at the 500 hPa level are completely affected by the topographic arrangement of the geopotential height at the 500 hPa level. Geopotential height maps are one of the most important and efficient atmospheric maps in synoptic analysis and interpretation. Heavy rains on representative days are analyzed, interpreted, and explained. Two statistical and synoptic methods were used for a more detailed investigation of the synoptic situation of heavy rains in the Dez river basin. In the statistical part, factor analysis, which is one of the widely used statistical methods in climatology, was used, and in the synoptic part, maps of different atmospheric levels were extracted and analyzed using Gardes software for the specified days. To determine the members of each group, the correlation of the scores of each factor with the sea level pressure map in the time period (1964-2020) was calculated, and the members of each group were determined. To determine the representative days, the correlation of the SLP maps of the days of each group was used, and the day that had a high correlation with more days was selected as the representative day. To perform factor analysis, the elements of the database were transferred from the Excel environment to the S-Plus2008 software environment, and then by performing various inferential analysis calculations, the most suitable method was selected to identify the main and effective factors.
 
Results and Discussion
As can be seen, the heavy rains of 2013 lasted for 6 days, and the system of 2011 lasted for 7 days. Of course, as mentioned, in some systems, days before or after the end of the activity period of the main system, scattered rains, even with high intensity, have been recorded in some stations. But these scattered rains are considered local rains. The highest peak flow rate of 8556 m³ belongs to the system in February 2013. This system has lasted for 4 days. It took 31 hours to reach the peak of the flood.
 
Conclusion
In the investigation of the middle layer of the troposphere in all the systems that led to the floods of 1993 and 2005, it was observed that the Arabian anticyclone has been established over the Arabian and Oman seas and the east of the Arabian Peninsula by moving eastward. In this situation, a very deep trough has formed in the west of Iran, and the southern end of the trough has extended to southern Sudan and northern Ethiopia. Therefore, the eastward displacement of the high pressure in Arabia and the significant southward expansion of the Mediterranean trough, and the formation of a low-altitude center cut over Iraq is the most suitable pattern for heavy cloud precipitation in the Dez basin. The peak discharge of floods with a delay of 12 to 24 hours from the day of peak rainfall has created a very intense discharge system. So, the floods of 2005 with a peak discharge of 8556 m³ s^-1 and the flood of 1993 with a peak discharge of 4022 m³ s^-1, while they were very heavy floods, there were two study periods.