Water quality monitoring and assessment in drinking water resources are of great importance. The entry of pollutants and nutrients into water resources may cause problems in public health and increase the bioaccumulation and biodiversity risks. This issue is more critical in dam lakes with more retention time, a primary source for drinking, agriculture, fish farming, and industry. This study aims to apply the universal water quality index (U-WQI) to evaluate the dam lake quality in an arid area and identify the sources of upstream pollutants over time (2016-2020). U-WQI was calculated for 16 physical, chemical, and microbial parameters in three sampling points of the Ilam Dam lake to quarterly evaluate the water quality in drinking water and cold water fish farming. During the monitoring period, the average U-WQI for drinking and fish farming was 142.79 and 124.84, respectively. Seasonal changes in the index suggest that fish farming in spring is more suitable than in other seasons since the U-WQI decreases in summer and winter.
