Phnom Penh, the capital city of Cambodia, has had a rapid increase in urbanization in recent decades. As impervious area increases, urban runoff becomes an increasingly more important component of urban non-point pollution. Boeng Trabek catchment is highly susceptible to flash flooding during heavy storm events. This study evaluates the relationship between the peak discharge and water quality, and assess the impact of land use scenarios on urban water quality by applying PCSWMM model. The sample were collected and used to analyze for the particle of the Total Suspended Solid (TSS), Chemical Oxygen Demand (COD), Phosphate (PO4) and Nitrate (NO3). The land use scenarios were applied by increasing the percentage of 10% and 20% from the baseline imperviousness. Flow and water quality data were monitored during three storm events for calibration and validation. Land use Editor was assigned to achieve the water quality calibration, with the manual determination of buildup and washoff coefficients. The result showed that Chemical Oxygen Demand, Total Suspended Solid and Phosphate corresponded to the peak values during both storm events on September 22nd, 2018 and September 28th, 2018. However, Nitrate contrasted with the peak flow. Furthermore, the overall simulation accuracies of the pollutants concentration for the calibration were R²0.7, while NSE0.5 for both storm events. Land use scenarios indicated that the discharge peak and pollutants concentration were increased as the imperviousness increased. However, Q-mean increased slightly about 3% from the baseline, which from
8.098 m³/s to 8.975 m³/s when the increasing imperviousness 20%. The study found that the perviousness in Boeng Trabek has less impact on the average flow in the channel. Regarding the pollutants load at the outfall of the catchment, the all targeted pollutant loads has increased between 6% to 9% from the baseline scenario when increasing the percentage of 10% and 20% from the baseline imperviousness. Consequently, understanding the impact of the imperviousness on urban area could be the useful practice for management and decision making in planning and developing on the wastewater treatment plant planning regarding the impervious scenarios.