This document presents a probabilistic decision support system for water quality management that accounts for risk and uncertainty. It proposes a framework using cluster analysis to reduce monitoring points, factor analysis to construct water quality variants, and time series analysis of historical variant data. A decision tree is developed to classify water quality conditions and uncertainty levels, aiding management decisions. The framework was applied to 3 study areas in Egypt comprising 32 monitoring sites. Results demonstrated the approach for quantifying uncertainty and risk in variant time series to support probabilistic decision making under uncertain conditions.