Introduction

Snow cover plays a crucial role at both global and regional scales, as it is essential for local water supply, river runoff, and groundwater recharge. Therefore, accurately understanding the extent and dynamics of snow cover is imperative. Snow is a significant type of precipitation in the hydrological cycle, and a substantial portion of precipitation in the mountainous regions of watersheds occurs as snow. Consequently, snowmelt replenishes groundwater, lake, and river water resources and plays a vital role in generating surface runoff. The Karun River, as one of Iran's most voluminous and largest rivers, is instrumental in providing drinking water, agricultural and industrial uses, and electricity distribution in Khuzestan Province and the entire country. On the other hand, the snow cover in the Zagros Mountains watershed, which is the headwaters of this river, plays a vital role in supplying water and flow to the river. Snowmelt during the spring and summer gradually affects the river flow, causing changes in its discharge.

Methodology

In this research, by utilizing MODIS satellite data over a 22-year period (2001-2022), snow cover changes in the Zagros watershed were obtained with a spatial resolution of 500 meters and were studied after undergoing corrections. To achieve this, MODIS satellite data were initially processed on the Google Earth Engine (GEE) cloud computing platform. GEE, as a powerful tool, provides easy access to a vast amount of satellite data, including the retrieval and adjustment of data and the performance of complex calculations. After data loading, satellite images were clipped using the geometric shape of the Karun watershed, and snow cover data for this region were adjusted and analyzed on a monthly and seasonal basis. Subsequently, the obtained snow cover images were converted into binary images to separate snow-covered areas from snow-free areas. Then, the Normalized Difference Snow Index (NDSI) was calculated to determine the snow cover extent in each pixel. In the following step, the pixel values of snow cover images were converted to binary numbers (0 and 1), and the NDSI snow cover was divided into six classes. In this classification, the closer the number is to 1, the deeper the snow, and the closer it is to 0, the shallower the snow. Furthermore, to investigate the quantitative relationship between snow cover and Karun River discharge, Pearson correlation was employed.

Findings

The results of this study indicated that there is a simultaneous and time-lagged correlation between the Zagros snow cover in different classes (low, medium, high, dense, and total snow cover) and the Karun River discharge. Particularly in the winter months, with the increase in the extent of snow cover in the high and dense classes, the discharge of the Karun River also increases in January, February, and March. In general, the correlation is stronger in winter and spring compared to autumn and summer, indicating that the impact of snow cover on river discharge varies in different seasons. In winter, there is a significant correlation at the 0.05 level between high snow cover and total snow cover, and a significant correlation at the 0.01 level with a correlation coefficient of 0.57 between dense snow cover and Karun River discharge. In other words, with an increase in snow cover in the high, dense, and total snow cover classes, the discharge of the Karun River in winter also increases significantly. Furthermore, by comparing the snow cover (dense, high, and total classes) of the Karun watershed and the discharge of the Karun River in winter, it is evident that all variables exhibit significant fluctuations over time. These fluctuations can be influenced by temperature, precipitation, and atmospheric conditions, and represent winter variations. This strong correlation indicates that snow cover in the Zagros watershed is a significant factor in feeding the Karun River in winter. Additionally, in spring, there is a considerable correlation with a correlation coefficient higher than 0.30, without a significant relationship, between all snow cover classes and the Karun River discharge. In other seasons, the correlation between snow cover and Karun River discharge is weak.

Discussion and Conclusion

The results obtained demonstrate a significant correlation between snow cover and Karun River discharge, particularly during the winter months. It was confirmed that as the extent of snow cover in the high and dense classes increases, the river discharge also increases simultaneously and with a time lag. Furthermore, the results revealed that changes in snow cover in the preceding months of spring have a substantial impact on river discharge in subsequent months. However, due to the decrease in snow cover, no significant correlation was observed between snow cover and Karun River discharge in the summer. Nevertheless, in autumn, especially in November and December, changes in snow cover