Introduction
Flooding is a significant natural hazard that affects millions of people around the world every year and causes extensive damage to property, infrastructure, and ecosystems. Investigating changes in floodplains is very important to understand and reduce these effects. Scientific research reveals that floodplains are affected by various factors, including climate change, urbanization, and river management practices. Notably, recent climate shifts have intensified torrential rains, expanding flood zones and elevating flood risk. Concurrently, urban development and residential expansion contribute to increased flood volume and flow by altering surface runoff patterns and reducing natural water infiltration areas. To optimize flood management strategies, the initial step involves identifying flood-prone regions, creating flood area maps, and monitoring changes over time. While post-event flood zone mapping and continuous monitoring pose challenges, satellite imagery remains the most practical solution. Advances in earth surface monitoring using satellite technology have significantly enhanced global flood monitoring capabilities. Leveraging satellite imagery and remote sensing technologies, platforms like Google Earth Engine provide valuable data for analyzing temporal and spatial variations in flood-prone areas.
Materials and methods
In the current research, we discuss the continuous monitoring of flood events in relation to the instantaneous maximum flow at the hydrometric station of the Kohak Dam. The Hamuns of Sistan, especially in recent years, have relied solely on water inflow from Afghan rivers during floods. Consequently, we conducted monitoring of the water area of the Hamuns of Sistan during flood events and tracked the water area of the Sistan plain over time using a time series of satellite images spanning from 1984 to 2021. For this purpose, we utilized Landsat satellite images (employing the NDVI index, transition algorithms, and seasonality blue zone identification algorithms) within the Google Earth Engine environment. The initial step involved extracting flood dates from the hydrometric station at Kohak Dam, located on the Sistan River at the entrance of the Hirmand River to Iran. Subsequently, we downloaded Landsat satellite images for various dates, including pre-flood, the first image after the flood, and post-water withdrawal (based on the daily hydrograph of the Kohak Dam station). To process and extract information from the images, we performed pre-processing and identified catchment areas of Sistan’s Hamuns during flood events using the NDVI index and infrared and red bands. Additionally, we employed GSWE (Global Surface Water Explorer) algorithms to monitor temporal and spatial water extent distributions globally over the past three decades.
Results and Conclusion
Using the dates of flood occurrence at the hydrometric station of Kohak Dam located on the Sistan River (at the entrance of the Hirmand River to Iran), we analyzed three distinct time periods: before the flood, the first date of the satellite image after the occurrence, and after the completion of water withdrawal (based on the daily hydrograph of the Kohak Dam station). During these periods, we downloaded Landsat satellite images. The obtained results reveal that in the water year 1988-1989, the maximum instantaneous discharge was equal to 901.83 cubic meters per second, occurring on 1989/04/06. Monitoring Landsat satellite images during the three time periods before, during, and after the flood shows that with the arrival of flood flow from the Hirmand River to Iran, the water area of the Hamun wetlands increased. Specifically, after the flooding event, the water area of the Hamun wetlands in Sistan reached 2614.4 square kilometers. In the water years 1990-1991 and 2015-2016, the instantaneous maximum discharge was equal to 1178 and 624.6 cubic meters per second, respectively, occurring on 1991/06/01 and 2016/04/16. The extraction of water areas of Hamun wetlands using the NDVI index from Landsat satellite images shows that in these two flood events, the maximum water area of the Hamun wetlands resulted from flood flow and subsequent inundation. Specifically, the water area was 4477.6 and 1319.8 square kilometers, respectively.
Conclusion
The study of three maximum annual flood events in the Sistan plain showed that the water intake of Hamun wetlands is completely affected by the occurrence of floods. A step-by-step comparison of the beginning to the end of the flood hydrograph with the flood zones extracted from satellite images in the same time steps, shows the complete matching of the surface of the water zone with the size of the flood discharge. Also, the study of the changes in the water areas of the Hamuns of Sistan using the GSWE layer shows the changes that have occurred in these areas between 1984 and 2020. Therefore, the results obtained from the study of the water zones of Hamuns using GSW maps showed that the water area of Hamuns in the long term shows a sharp decrease, so that the changes occurred in such a way that in the year In 2020, compared to 1984, more than 74% of the area of Sistan's Hamuns is drained seasonally. From the point of view of studying the situation of monthly water intake based on the monitoring of long-term average water areas, the results showed that only a small part of the Sistan plain (about 11 thousand hectares) has water on average throughout the year, and this area also belongs to wells. It is half and covers about 0.5% of the studied area. Also, more than 2 million hectares of the surface of the studied area, on average, have water only in 1 month of the year, which is equivalent to 88.4% of the total area of the studied area (includes Hamuns, the rivers adjacent and the wells). |
