بررسی تأثیرات خشک‌سالی بر کیفیت منابع آب سطحی پُل دختر

Introduction and Goal
Understanding water in terms of quality, quantity and its supply is an essential step towards optimizing consumption. In recent years, Iran has suffered from prolonged droughts, causing significant damage to various parts of the country each year. Severe droughts in arid and semi-arid regions caused by inappropriate human interventions have reduced the quantity and quality of surface water resources in these regions. Overall, the amount of precipitation, the duration and intensity of droughts, climatic conditions, and industrial and agricultural activities are among the factors that significantly affect the quality of surface waters. To this end, researchers have used various indices to monitor drought and its impact on water quality. Therefore, it is essential to use appropriate methods to study the effects of phenomena such as drought on the quality of surface waters. This is a tool for water resources in critical conditions. The main objective of this research was to identify wet and dry periods and the droughts that occurred using the SPI index. Then, the sensitivity of surface water chemical quality indicators in the Keshkan watershed during drought periods and their changes over a 35-year period were examined. Additionally, in this context, the frequency and intensity of hydrological drought events, as well as the magnitude and range of flow changes affected by droughts in the studied watershed, were determined.
Materials and Methods
The studied area is part of the Keshkan watershed, covering an area of 9,560 km², which accounts for approximately 22% of the larger Karkheh watershed. It is located 950 km southwest of Tehran in the central Zagros region. In this research, the chemical indicators of surface waters were measured. Then, the periods of meteorological drought occurrences, as well as the relationships between these events and the quality of surface waters in Lorestan Province, were determined. To this end, the chemical quality of rivers over a 35-year period was analyzed using existing quality data from hydrometric stations, including anions (Cl⁻, SO₄²⁻, HCO₃⁻) and cations (Na⁺, Mg²⁺, Ca²⁺, K⁺), as well as other variables such as SAR, pH, TH, TDS, and EC during the common statistical period (1981-2016). Although there are many challenges in obtaining water quality data today, fortunately, in this long-term study, the number of samples, data homogeneity, and the common period were adequate. Considering that the studied period included normal, dry, and wet periods, the quality of the research was also appropriate. Additionally, it should be noted that the Water Resources Development Company, for various reasons related to validation, always publishes data several years later. Therefore, it was not possible to examine a longer statistical period. On the other hand, the most extensive droughts in the region occurred during this time frame.
Results and Discussion
Although dolomitic, calcareous, and marly formations play a role in reducing the quality of surface runoff passing through them, based on the results of this research, the occurrence of extensive and numerous droughts has also led to an increase in the concentration of salts per unit volume,  specifically, the pH of the water increased from 7.61 to 7.68, and the EC of the water rose from 300 µm/cm in 1981 to 570 µm/cm in 2016, yet it still remained in an acceptable quality class for irrigation. Using correlation and bivariate regression methods, the relationships and effects between the independent variables of drought and flooding with the dependent variables related to the chemical quality indicators of surface waters in the watershed were examined using SPSS and Excel software. The results of the analysis of anions and cations indicated that their levels during drought and flooding periods were significant at the 1 and 5% levels. The levels of the SAR, SSP, and TH indices were more significant under drought conditions. Additionally, the changes in EC levels during drought periods were significant at all levels.
Conclusion and suggestions
In general, droughts have a close relationship with water crises and the reduction of surface water resources. Therefore, examining the impact of drought on the quantity and quality of surface waters is important. Using the results of this research, a better understanding of the factors affecting the quality of surface waters can be achieved. The results of this research indicated that despite the decrease in quality, surface waters remain suitable for irrigation in the agricultural sector. It is recommended that the role of industrial uses and pollutants in surface waters be examined as well. Especially in the case of stations located downstream of residential and industrial areas, monitoring changes and the type of pollution is of great importance.