In semi-arid regions, climate fluctuations and recurrent drought episodes strongly affect both the quantity and quality of surface water stored in reservoirs. In this context, the Beni Bahdel Dam, located in western Algeria and constituting an important source of irrigation water, is increasingly experiencing the effects of current climate change. This research investigates how wet and dry hydrological periods influence the hydrochemical properties of the dam water and its appropriateness for agricultural use. The study relies on the assessment of major hydrochemical variables together with several irrigation water quality indices, namely sodium adsorption ratio (SAR), Kelly’s ratio (KR), sodium percentage (Na%), magnesium adsorption ratio (MAR), cation ratio of structural stability (CROSS), and irrigation water quality index (IWQI). The results highlight a significant impact of hydrological variability on water quality. In particular, they reveal a doubling of the average electrical conductivity (EC), increasing from 402.46 (μS/cm) during the wet period to 810.25 (μS/cm) during the dry period. Furthermore, the Irrigation Water Quality Index (IWQI) showed a decrease in its median value, from 71.19 to 65.73, indicating a shift toward a “moderate restriction” class. The CROSS index also proved to be a particularly sensitive indicator of aridification conditions, with an increase of 32.93% (from 1.64 to 2.18). Finally, the Magnesium Adsorption Ratio (MAR) exceeded the critical threshold of 50 during the dry period, reaching an average value of 53.60. Descriptive statistical methods and Pearson correlation analysis were applied to explore the relationships between ionic composition and irrigation suitability indicators. The findings contribute to a better understanding of the effects of climate variability on water quality and provide useful evidence for improving sustainable reservoir management in semi-arid environments.