Aim of the study:
This study aimed to evaluate the water quality of Sempor Reservoir, Indonesia, over five years (2019–2023) based on physicochemical parameters and the Cumulative Pollution Index

Material and methods:
Water quality data were obtained from routine monitoring conducted during dry and wet seasons at three sampling locations: Sampang, central area, and Kedung Waringin. The observed parameters included total dissolved solids, total suspended solids, biochemical oxygen demand, chemical oxygen demand, total phosphorus, nitrate, sulphate, and detergent. Water quality was assessed using the cumulative pollution index. The normality of spatial datasets was evaluated using the Shapiro–Wilk test. Because most datasets were not normally distributed, non-parametric tests were applied: the Friedman test to assess spatial differences and the Wilcoxon test to assess seasonal differences. A significance level of p < 0.05 was used for all statistical tests.

Results and conclusions:
The results showed that most physicochemical parameters were within the Class II water quality standard, although occasional exceedances were observed for biochemical oxygen demand, total phosphorus, and detergent. Cumulative Pollution Index values indicated that water quality ranged from clean to moderately polluted. The absence of significant spatial and temporal differences in Cumulative Pollution Index indicates relatively homogeneous overall pollution conditions throughout the reservoir during the monitoring period.