Introduction
Drought is one of the environmental hazards that refers to long - term water scarcity in an area, and it can occur in all weather conditions , whether humid or dry . This water scarcity can be caused by reduced natural precipitation, excessive evaporation, changes in rainfall patterns, or overuse of water resources. Drought can have serious and widespread impacts on the environment, agriculture, economy , and society . This phenomenon can lead to a decrease in agricultural production , water shortages , price increases , reduced quality of life, and disruption of the natural balance in the region. Drought gradually evolves into a natural disaster and dominates the environment . Drought is considered a complex risk that depends on the climatic and weather conditions of each region . Additionally, drought has a significant impact on reservoirs . During drought periods, precipitation decreases, resulting in reduced inflow of water into reservoirs . These droughts can cause a decrease in water levels in reservoirs and subsequently reduce the water storage capacity . This can directly affect drinking water supply , irrigation, and hydroelectric power generation from hydropower plants . In addition to the decrease in water volume in reservoirs, drought can also lead to a decrease in water quality . When water levels in reservoirs decrease, the water can become relatively more polluted, with increased salinity and contamination parameters . This can result in a decrease in water quality and create difficulties in using it for various purposes . In general, drought can lead to a decrease in water reserves in dams , a decrease in electricity production , and problems in providing drinking water and watering plants . This shows that the management of water resources and planning according to drought conditions is necessary to maintain and optimally exploit dam reservoirs .

Methodology

In recent years, multiple droughts have occurred in various regions of the country , and the city of Marvdasht and the Doroodzan Dam have not been exempt from this issue . The Doroodzan reservoir dam is located 100 kilometers north of Shiraz on the Kar River . The construction of the dam began in 1966 and was completed in 1972 . The purpose of building this dam was to provide water for agriculture , drinking , and electricity supply to the region . The annual adjustable water capacity of this dam is 526 million cubic meters . The dam is of an earth-fill type with a clay core , with a crest length of 710 meters , a height of 60 meters from the foundation, and 55 meters from the riverbed . The body volume of the dam is 5.70 million cubic meters , the normal storage volume is 960 million cubic meters, and the useful volume is 860 million cubic meters .
In this study , the status of water level changes in the Doroodzan reservoir lake was examined using the Normalized Difference Water Index (NDWI). In this research, to investigate the changes in water resources in the Doroodzan reservoir lake as the main and important surface water source in the region, 80 images from Landsat 5, 7, and 8 satellite images were used, and then the water zone maps of the Doroodzan reservoir lake and its area were extracted during a 20-year statistical period. The satellite images were processed using Landsat 8, 7, and 5 . Three different methods are used to separate surface waters from other phenomena on the ground surface, including : 1) use of spectral band information, 2) supervised and unsupervised classification methods and 3) use of remote sensing indices for weather detection. The accuracy of the results obtained from the water indices , as well as the ease of implementation, has led to widespread use of this type of method . The Normalized Difference Water Index (NDWI) , proposed and presented by McFeeters in 1996, is the first index that reveals water through remote sensing images . Since water has a high reflectance in the green band and low reflectance in the near-infrared band in satellite images .

Conclusion
According to the results, the largest area of water zones in the Doroodzan reservoir lake during the study period was in 2004 with an area of 39.28 square kilometers, followed by 2019 with an area of 39.11 square kilometers . The smallest area of water zones was in 2016 , with an area of 24.83 square kilometers , followed by 2010 with an area of 25.31 square kilometers . Additionally , to classify the water zones , the TM , ETM , and OLI sensors of Landsat 5 , 7 , and 8 satellite images were used during the 20-year statistical period . It was shown that the largest area of water zones in the Doroodzan reservoir lake during the study period was in 2004 , with an area of 39.28 square kilometers, followed by 2019 with an area of 39.11 square kilometers . The smallest area of water zones was in 2016, with an area of 24.83 square kilometers, followed by 2010 with an area of 25.31 square kilometers .According to the results, the largest area of water zones in the Doroodzan reservoir lake during the study period was in 2004 with an area of 39.28 square kilometers, followed by 2019 with an area of 39.11 square kilometers . The smallest area of water zones was in 2016 , with an area of 24.83 square kilometers , followed by 2010 with an area of 25.31 square kilometers . Additionally , to classify the water zones , the TM , ETM , and OLI sensors of Landsat 5 , 7 , and 8 satellite images were used during the 20-year statistical period . It was shown that the largest area of water zones in the Doroodzan reservoir lake during the study period was in 2004 , with an area of 39.28 square kilometers, followed by 2019 with an area of 39.11 square kilometers . The smallest area of water zones was in 2016, with an area of 24.83 square kilometers, followed by 2010 with an area of 25.31 square kilometers .