Assessment of Sudden Drought and Flood Occurrence (Case Study of the Karun River Basin)

The Middle Eastern countries, due to their location in the dry belt, are facing the problem of water scarcity and recurring droughts, as well as destructive and devastating floods. Drought-flood is one of the most serious, costly, and widespread natural disasters in the world. This complex natural phenomenon is caused by insufficient or excessive rainfall compared to long-term recorded averages, which can pose major threats to human society, agricultural production, ecosystems, and water resources. Sudden drought-flood recurrence events (DFAA) in a specific region or basin are a new type of extreme hydrological events that are caused by the coexistence and rapid alternation of drought and flood. DFAA events can create more serious disasters than a drought or flood event and lead to more casualties and economic losses. Global climate change and human activities, including abnormal atmospheric and oceanic circulation, are factors that have led to an increase in the frequency and intensity of DFAA events. Previous studies around the world have shown that the spatial distribution of DFAA events is expanding.In this study, Persian satellite images were first extracted at the level of the Karun Grand Basin and converted to satellite data. Then, using the Rstudio software, the degree of rainfall concentration (PCD) and the rainfall concentration period (PCP) were calculated for the Karkheh and Karun basins. Next, the MANN-KENDAL trend test was evaluated for PCD and PCP. The results showed that the increasing trend of rainfall concentration degree and rainfall concentration period for the period 1980-2017 was in line with the increasing trend of rainfall data. The Sudden Drought-Flood Recurrence Assessment Index (LDFAI) was also evaluated. The results showed that the trend of the sudden drought-flood recurrence index is directly related to the degree of rainfall concentration and the rainfall concentration period.This study is important because it provides new insights into the causes and trends of DFAA events in the Middle East. The results of this study suggest that DFAA events are becoming more frequent and intense in this region, and that this trend is likely to continue in the future. This is a serious concern, as DFAA events can have devastating impacts on human society and the environment.The study's findings have several implications for policy and decision-making. First, they highlight the need for increased investment in adaptation measures to climate change. Second, they suggest that policies to reduce human-induced climate change and other factors that contribute to DFAA events are needed. Third, they call for improved monitoring and early warning systems to mitigate the impacts of DFAA events.

The Karun Grand Basin is one of the most important water basins in the country in terms of water resources, and the abundance of rainfall, especially in the headwaters of these two basins, has created a significant potential for surface and groundwater resources.From the point of view of geographical location, the Karun basin is located in the range of 48°39′77˝ to 48°17′39˝ east longitude and 30°39′11˝ to 37°39′01˝ north latitude, and Karkheh is located between the geographical coordinates of 34 to 56 degrees north latitude and 30 to 58 degrees east longitude.The Karun Grand Basin includes the provinces of Chaharmahal and Bakhtiari, large parts of the provinces of Lorestan and Khuzestan, and a small part of the provinces of Kohgiluyeh and Boyer-Ahmad and Isfahan. The Karkheh Grand Basin includes more than half of the provinces of Lorestan and Kermanshah, and parts of the provinces of Kurdistan, Hamadan, Ilam, and Khuzestan. The main waterway of the basin is the Karkheh River, which is formed by the confluence of the Simorgh and Zale rivers in the south of Pol-e Dokhtar. This basin is part of the second-degree basins of the country in terms of national divisions. The area of Karkheh is equal to 51527 square kilometers. In the Karun Grand Basin and Karkheh, the highest monthly rainfall of 111.4 millimeters and a share of 19.3% in the year is related to December. After December, November with 114.8 millimeters and 19.7% has the most rainfall. In this water basin, the rainfall of the months of June to September accounts for less than 1% of the total annual rainfall. In the Karkheh and Karun Grand Basin, annual rainfall varies from 113 to 7000 millimeters, with an average of 837 millimeters.Here is a more concise translation:The Karun Grand Basin is a major water source in Iran, with abundant rainfall, especially in the winter months. The basin covers parts of seven provinces, and its area is about 51,500 square kilometers. The average annual rainfall is 837 millimeters. I hope this helps! Let me know if you have any other questions. The method for calculating the degree of rainfall concentration index (PCD) and rainfall concentration period index (PCP) was first proposed by Zhang et al. (2003) in a study of compound drought-flood events in the Yangtze River basins. This method can quantitatively describe the spatial and temporal characteristics of regional rainfall. The concept of the PCD and PCP calculation methods is to describe the total monthly rainfall as a vector quantity with magnitude and direction (Li et al., 2010). The method used to calculate PCD and PCP is as follows:To calculate the monthly direction angle, the angle corresponding to the middle of the day of each month is considered. According to Equation (1), the mean daily rainfall depth is equal to 0.986. The method used in this study to calculate PCD and PCP is as follows. (Zhang et al., 2003). MANN-KENDAL trend estimation In time series, especially climate data time series, the need for trend analysis is felt. One of the practical and non-parametric methods is the Mann-Kendal method, which is widely used. The Mann-Kendall method was first presented by Mann (1945) and then expanded and developed by Kendall (1970). The zero hypothesis of the Mann-Kendall test indicates the randomness and absence of trend in the data series, and the acceptance of the one hypothesis (rejection of the null hypothesis) indicates the existence of the trend in the data series. The Mann-Kendall test is used to determine whether a time series has a uniform upward or downward trend. The data need not be normally distributed or linear. It requires that there is no autocorrelation.In the relation (6 n) of the total number of data, p indicates the cumulative number of years when the data value is greater than the value of the previous year, and in other words, if xi is the data value per year, p is the number of observations that Xji. Finally, positive values of T indicate an increasing trend in the time series, while negative values indicate a decreasing trend over time (Yu et al., 2002). Also, if the absolute value of T is greater than 1.96 in The probability level of 5% has a significant trend and for the absolute value of T greater than 2.575, the probability level of 1% has a significant trend (Hobbins et al. 2001).

In spatial scale analysis, PCD rainfall concentration and PCP rainfall concentration period show a scattered increasing trend from the southern and southeastern points to the northwestern points. The spatial distribution of these parameters is relatively consistent. In the PCD scale, the southeastern station has the highest value and the western station has the lowest value, and the range of these values varies from 0.36 to 0.54. On the other hand, in the PCP scale, the range of values varies from 0.254 to 1.165. The spatial distribution of PCD and PCP in the Karkheh and Karun region is relatively consistent, and it seems that various fluctuations in weather conditions may affect the spatial and temporal changes of these two parameters. In spatial scale analysis of the LDFAI index (sudden drought-flood recurrence index in the short term), it is seen that there is a decreasing trend from the northern and northwestern regions to the south. The spatial distribution of this index has a significant match with the degree and period of rainfall concentration.In the Karkheh and Karun Grand region, it is observed that there are significant differences in the frequency of LDFAA events (drought to flood and flood to drought). DTF and FTD events from the northern to southern region of Karkheh and Karun Grand show different characteristics. These two events are seen in some areas with significant differences in frequency. Although the spatial pattern of FTD events agrees with DTF, in general, the frequency of DTF events has been higher than FTD. These results show that in this region, drought to flood (DTF) events are seen more in some areas than flood to drought (FTD) even.