Efficient operation of urban water distribution systems requires not only accurate design but also innovative calibration frameworks that address data limitations and reflect real demand dynamics to ensure reliable service, optimize resource use, and prevent failures. This study develops and validates an integrated hydraulic model of the water supply network of Lamia, Greece, which comprises both an outer aqueduct conveying water from the Gorgopotamos River and Taratsa springs to the main reservoirs and an inner aqueduct distributing water throughout the city to multiple consumption zones. The network has a total length of 253 km, including primary and secondary pipelines, and incorporates major reservoirs, pumping stations, and pressure-reducing valves as key structural elements. To overcome incomplete records and spatial gaps, WaterGEMS, AutoCAD, ArcGIS, Google Earth, and official cadastral maps were combined to establish a reliable network baseline. Simulation and data extraction were conducted in the WaterGEMS environment, using a 72 EPS cycle to capture hourly variations over a three-day pattern, enabling targeted interventions to minimize losses and optimize distribution. The calibration and validation compared simulated reservoir levels against SCADA observed data, resulting in high model performance with averaged metrics of NSE = 0.78, R² = 0.82, and VE = 0.97, demonstrating strong agreement. Our results showed average nodal pressures of 546 kPa and peak velocities below 1.7 m/s, ensuring service reliability across low- and high-demand conditions. Beyond a design tool, the model provides a foundation for a Digital Twin of Lamia’s water supply system. This study provides a transferable framework for urban hydraulic modeling, demonstrating how digital tools, real-time monitoring, and performance-based calibration can facilitate the resilient, efficient, and adaptive management of urban water networks.