What is the type of precipitation in Iran? Can a time series for wet and dry periods be predicted for the next 50 years of Iran's rainfall? Can the water crisis in Iran be prevented through river optimization, such as watershed management? Is dam construction good?

Iran is a country with an arid and semi-arid climate, with an average annual rainfall of about one-third of the world's average. Parts of the Idran-Nid-Jad rainfall belt are considered to be very low-rainfall areas in the southeast, and the main characteristic of these areas is high-intensity atmospheric circulations. In this study, data on daily precipitation of 45 climatic stations from the very low precipitation area of ​​southeastern Iran, which have a high statistical record of 30 years, were collected, and then a standardized matrix with the order (45×80285) was prepared in the Matlab software, whose rows indicate the number of days and its columns indicate the statistics related to the station location, with a Euclidean distance of 8. Using the clustering method, the occurrence of 35 one-day precipitation events was finally identified as days of heavy and torrential precipitation. Efforts to identify the dynamic, thermodynamic, and synoptic factors affecting the occurrence of the events were made. Six-hourly data on geopotential height, sea level pressure, air humidity, specific humidity, orbital component and meridional component were used for latitudes 0 to 20°N and longitudes 0 to 380°E, and the maps were drawn with a cell size of 4.8×4.8 arcseconds. The findings indicate that the occurrence of heavy rainfall in the very low rainfall area of ​​the southeast of the country is due to one or more different weather patterns, but in most of the places where such rainfall occurs, the encounter of the low pressure near the equator with the low pressure over the Arabian Sea provides severe storm conditions in the northwest-southeast direction.Precipitation is a climatic phenomenon that is of significant importance compared to other climatic variables due to the dependence of living organisms on it and its spatial and temporal fluctuations. Due to its geographical location, Iran is located near three oceanic high-pressure systems, and the climatic conditions resulting from these systems make it one of the rainiest regions in the world (Ashjaei Bashkand, 2003: 33). The spatial differences in rainfall in Iran are very large. The average rainfall in Iran is about 509 mm, but the rainfall received in the Kaviri region is less than 100 mm, and in some places on the western shores of the Caspian Sea, the annual rainfall is about 3099 mm (Masoudian, 2003: 355). If we consider the amount and time of the rainfall area simultaneously, Iran can be divided into four main rainfall zones (very low rainfall, low rainfall, semi-heavy rainfall, and heavy rainfall). In In his research on identifying Iran's rainfall zones, Ian Rasata Masoudian (3339) identified homogeneous rainfall areas based on the differences in the timing and amount of rainfall received by each of these regions. The very low-rainfall region of southeastern Iran, due to its dry and arid climate, has a flood climate. One of the characteristics of rainfall in this region is heavy downpours with a short duration (Arnavon, 3373:5 05). A flash flood event such as heavy rainfall causes major floods, which cause great damage to many of the natural resources needed by humans, such as soil and water, as well as economic activities such as water structures, agriculture, tourism, and transportation. Therefore, the most important goal of the Harar study is to investigate and identify the synoptic systems that cause the aforementioned phenomena in the very low rainfall area of ​​southeastern Iran. The results of this research not only clarify the mechanism of occurrence of such precipitation, but also pave the way for predicting their occurrence in the future and play an important role in managing unexpected events such as floods and optimal spring rainfall.

The time series of precipitation trends is one of the principles of identifying the climate of statistical identification in the modern era. In this regard, the present study is important in terms of its content and novelty and its relationship with blocking, which we are witnessing in the Khorasan region. Blocking is a phenomenon that can affect atmospheric flows and their formation. It creates a type of atmospheric laziness and stagnation in the atmosphere of the region and is considered a type of atmospheric blockage. And at high levels, it provides a dry or wet hole that will cause dampness or drought in the desired region. The results of research and studies have shown that the presence of blocking leads to negative anomalies of the orbital component and positive anomalies of the meridional component of the wind, strengthening relative and potential torsion and increasing vertical velocity, relative humidity and precipitation rate. In addition, blocking strengthens and increases the lifespan and changes the path of cyclones to lower latitudes. Closed systems can be either closed high-pressure systems or closed low-pressure systems. The establishment of such systems plays a role in the formation of dry or wet periods. The flow of upper winds rarely follows straight lines and is generally wavy or spiral. In addition, air in many cases rotates around a center (axis). In conditions where the movement of air is circular, it is possible that the center of rotation is located in a low-pressure or high-pressure area. Thus, as on the surface of the Earth, a cyclonic or anticyclonic flow is established in the upper atmosphere. In these conditions, under the influence of the Coriolis force and in the northern hemisphere, the movement of air around the low-pressure and high-pressure centers is in a counterclockwise and clockwise direction, respectively, which is known as the wind gradient. High-pressure and low-pressure systems can be located and studied using maps of different atmospheric levels. Atmospheric anomalies are caused by changes in pressure systems at the Earth's surface and in the upper atmosphere. Band systems are one of the most important atmospheric phenomena, causing significant meteorological and hydrological anomalies at the regional and global levels.Dry and wet periods can be very effective. (Zolfaghari, 2012) Whenever an anticyclonic center separates from the Azores overcast or independently settles at higher latitudes (N70-N50) and remains for a few days, the barrier phenomenon occurs. This is because the overcast prevents the westerly winds from blowing or diverts them from their main path, resulting in severe deviations in the pressure and temperature distribution patterns of a part of the globe. (Alijani, 1994:45). In describing barrier systems, it should be noted that they disrupt the regularity of the westerly winds and convert the orbital currents into meridional ones. The mid-latitude westerly stream and the eastward expansion of synoptic systems are constantly interrupted by long periods of atmospheric turbulence. (Zolfaghari, 2012) .

These systems sometimes appear in larger dimensions and wavelengths and have slower movement and may even appear stationary or with reverse movement in atmospheric currents. Under such conditions, it is seen that the currents in the middle and upper layers of the troposphere, which have a west-east movement, separate and branch as they approach the warm ridge that has settled. A branch on the right side of the flow deviates towards low latitudes and creates a cold trough, and another branch in the evolutionary process may be associated with the trough, with a low-altitude cell or cells, and the warm ridge with a high-altitude closed cell or cells. This system may remain around a meridian for several weeks and, due to the existence of such a system, disrupt the westerly currents in the middle latitudes. As a result, due to the existence of such a system, the western currents cannot move in their usual path and must choose a more northern or southern path and continue. The high-pressure and low-pressure systems on the earth's surface also move around this blocked system due to the flows in the middle levels of the troposphere. This phenomenon is called blocking. (Omidvar, 2014). The first studies were conducted by Rex in 1950. His studies showed that the bandals are dependent on the anticyclone on the earth's surface and in terms of geographical distribution, he mentioned the northeastern Pacific Ocean and the Atlantic as important areas in creating this phenomenon. (Zolfaghari, 2012).Research results:

Precipitation maps were prepared based on the clustering of 17 synoptic stations in northeastern Iran using the Surfer software. The answer to the thesis hypotheses regarding the effect of the Bandal pattern (atmospheric blocking) and the general circulation of the atmosphere in northeastern Iran and based on the regional slope direction map (MDE) in the northeastern region of Iran can be analyzed and explained as follows. The Markov chain model and the probability of drought and dry spells and their return period in the northeastern region of Iran can be interpreted and analyzed and its relationship with the SPI precipitation index can be understood.

The probability of annual drought occurrence and the return period of droughts is 2 to 5 days. If we consider probability as the relative frequency limit of an event, then the meaning of 0.83 (83 percent) of the probability of no rain in a place is that in the atmospheric conditions of a place, there will be about 83 days out of every 100 days without precipitation. Therefore, during the 365 days of the year, there will be about 303 days without precipitation. According to the Markov chain probabilistic model, the probability of occurrence of dry days with a precipitation threshold of 0.1 (a percent) was analyzed. According to the results of the present study, the study area experiences high dry days on average. This value has been observed from about 70 percent in the Torbat Heydariyeh synoptic station to over 91 percent in the southern area and the Nehbandan synoptic station. The above numbers are indicative of the persistence of drought and dry days in the study area. In general, it can be said that the probability of occurrence of dry days with a precipitation threshold of 0.1 (1 percent) mm increases from the northeastern area of ​​the study area to the south with a steeper slope and to the west with a gentler slope. One of the applications of the Markov chain technique is to estimate the probability of occurrence of an event with a duration of m days. For example, the persistence of a dry day is the number of days in which precipitation has occurred. For example, a two-day drought means drought for two consecutive days, but rainfall occurred before the first day and after the second day. The probability of m days is obtained based on the probability of a dry day (p) and its absence (q) from the following relationship:

pm=1/((p^m-1)*q)

In the present study, the duration of drought up to 5 days was studied and analyzed.

According to the method used, the probability of drought occurrence with a threshold of 0.1 (1 percent) in the southern region of the study area was observed with a probability of 93 percent, and the minimum probability of drought occurrence was also observed in the northeastern part of the study area with about 80 percent, which is located within the Torbat-Heydariyeh synoptic station. The maximum probability of drought occurrence was also observed in the Nehbandan region and after that in the Tabas station area. Among the studies that are of considerable importance, in addition to the probability of drought, is the study of drought persistence, which is very important in water resource management and activities related to the persistence of drought and wetness (including agricultural activities). The analysis of the three-day duration of drought indicates a return period of 7 to 15 days with the minimum return period being located in The northern part (Torbat-e Heydariyeh synoptic station) of the study area and its maximum in the southern part (Nehbandan synoptic station) were accompanied. In the four-day continuation of the drought, the return period values ​​did not show any change, and only the amount of return periods increased, with a minimum return period of 10 and a maximum return period of 16 days observed in the northern and southern parts of the study area, respectively.

The 5-day continuation of the drought is evidence of a 12-16 day return period of 5-day droughts in the study area, which continues to experience the minimum return period in the northern part of the study area and the maximum return period in the southern part.

The probability of an annual wet event and the wet return period is 2 to 3 days. If we consider the probability as the relative frequency of an event, then the meaning of 0.17 (17 percent) of the probability of rainfall in a location is that in the climatic conditions of a location, there will be about 17 days of precipitation out of every 100 days. Therefore, there will be about 62 days of precipitation during the 365 days of the year. According to the Markov chain probabilistic model, the probability of wet days with a precipitation threshold of 0.1 (a percent) was analyzed. According to the results of the present study, the study area experiences few wet days on average. This value has been observed from about 17 percent in the area of ​​the Mashhad synoptic station to over 91 percent in the northern area and the area of ​​the Bojnourd and Mashhad synoptic stations. The above numbers are indicative of the persistence of humidity and wet days in the study area. In general, it can be said that the probability of wet days with a precipitation threshold of 0.1 (1%) mm decreases from the northeastern part of the study area to the north with a steeper slope and to the south and southeast with a gentler slope.

Dams, essential hydraulic structures in water planning and management, have several benefits for a society like providing irrigation, electricity generation, flood control, water supply and recreation. However, many dams have almost reached or exceeded the estimated construction life; thus they risk structure stability. Even though it is comparatively rare, dam failures resulting from flood wave propagation can cause immense damages. Although dams are trustworthy, many disasters that occurred in the past show that a dam can fail in different ways, such as piping or overtopping. Since the collapse of a dam will directly affect most people’s lives, analyzing the dam failure and determining the flood area caused by the failure becomes significant. Two-dimensional (2D) numerical simulation proved to be an effective tool for understanding flood events caused by dam failure. HEC-RAS 2D, one of the most popular hydraulic models, is implemented to estimate the breaching parameters and flow conditions. In this study, the downstream of a consecutive dam system, consisting of Elmalı 2 concretebuttress dam, which is unique among dam failure studies located upstream and Elmalı 1 earthfill gravity dam located downstream of the watershed is selected as the study area. Since heavy traffic highway and densely residential regions are located just around the downstream of the consecutive dam system, this vulnerable study area is preferred. The study aims to evaluate the dam failure analysis of Elmalı 2 dam with more water holding capacity under different scenarios. Moreover, the inundation regions are also evaluated visually, and areal differences are analyzed comparatively. The dam failure simulation scenarios are examined for three significant criteria: breach formation time (BFT), the number of failed buttresses for Elmalı 2 and reservoir volume rate of Elmalı 1 (RVR). Therefore, peak water depth (Hp), peak flow rate (Qp), peak velocity (Vp) and time to reach the peak (tp) are discussed for the study area by comparing the scenarios. The results show that breach formation time and the number of failed buttresses effectively affect the peak flow rate, peak velocity, peak depth and time to reach the peak. On the other hand, it has been observed that there is no severe effect of the reservoir volume rate of Elmalı 1 in the consecutive dam system. Hence, the effects of these criteria on the susceptible city area located downstream of the possible failure of Elmalı 2 is presented.Dams benefit societies in many different ways (e.g., water to drink, water supply, irrigation, hydroelectric power production and so forth); on the other hand, complete or partial failure of dams for different reasons can cause critical problems for the same societies (Singh and Scarlatos, 1998; Altinbilek, 2002). As a result of any dam failure, it is vital to determine the flood in case any possible collapsing of structures especially in areas close to the settlements. More than one hundred dam failures have occured since the 1700s, and thousands of people have died including environmental damages worldwide (URL-1, 2021). For this reason, studies on dam-break modellings are of great importance as they directly affect human life and property (Fread, 1996). The main causes of dam failure are overtopping and piping (Costa, 1985; Foster et al., 2000). Even if the reason is different, almost all failures begin with a breach formation. The breach is defined as an opening formed in the dam body, and gradual expansion results in a large volume of water in the reservoir propagating to the downstream parts destructively (Wahl, 1998). Breach geometry (breach depth and width, breach side slope factor), timing (initial breach time, breach formation time, etc.), failure mode, and breach progression need to be estimated well in dam-break modelling. Also, flow conditions and dam body type can affect estimation peak hydrograph, which occurs after the breach progress (Brunner, 2014). All in all, there are two fundamental tasks in dam break analysis: the prediction of the reservoir outflow hydrograph and the routing of that hydrograph through the downstream (Wahl, 1998). The primary purpose of the present study is to reveal and evaluate the flood mitigation that will occur due to the possible dam failure in Istanbul, Turkey. Analyzes are evaluated for the study area includes the downstream regions consisting of high-populated residential areas and dense traffic highway of two consecutive dams named as Elmalı 2 (a concrete-buttress dam) and Elmalı 1 (an earth-fill gravity dam in the upstream part of Elmalı 2). It is a critical situation that the dams are consecutive, and these dams cause risks by being located in the upstream part of the residential area. In this study, Dam break simulations are performed with three criteria: Breach formation time and the number of failed buttresses of Elmalı 2 and the reservoir volume rate of Elmalı 1. The simulations are realized as 88 runs under three different simulation sets based on these criteria. The results provide information like the peak values of the water depth, the flow rate and the velocity, the time to reach the peak, and flood maps for the scenarios. Flood maps in the simulation results are compared spatially and the effect of the three criteria on the flood is analyzed. It is aimed to compare the results of the possible dam failure disaster that may occur in the study area under different criteria by examining the simulation results. Within this scope, evaluating the effects of different criteria on the results is essential in the operation methods of dams and taking measures to reduce the harmful effects of dam failure. In addition, determining the areas affected by dam failure is of great importance in taking precautions to mitigate flood hazards.