A simulation model and a digital twin are often sold as the same thing. They are not. A simulation model represents a system so you can ask what would happen under conditions you choose. A digital twin is a simulation model that is continuously updated from the real system, so that it stays a faithful copy of the asset as it is today.
What is the difference between a digital twin and a simulation model?
The difference is the live connection. A simulation model is built, calibrated and run for a study, then put away until the next one. A digital twin is fed by telemetry and asset records, recalibrated as the system changes, and run continuously so that its state matches the real network at any moment.
- Simulation model: built for a question, such as sizing a main or testing a pressure zone. Snapshot inputs. Updated when someone commissions an update.
- Digital twin: built for operations. Live inputs from SCADA and meters, current asset data, automatic comparison of modelled against measured values.
Is a digital twin just a simulation?
No, but every useful digital twin contains one. On a water network the simulation inside the twin is usually EPANET or a commercial equivalent; in a drainage system, SWMM or InfoWorks ICM. What the twin adds is everything around the model that keeps it true: data pipelines, calibration that runs as conditions change, and an interface that operators use rather than a report that engineers read.
When do you need a twin, and when is a model enough?
A model is enough when the question is a design or planning decision taken once: a new main, a new zone, a master plan. A twin earns its cost when the questions are operational and recur every day: which valve to close for a burst, how to schedule pumps against tomorrow's demand, whether a pressure drop is a leak or a meter fault.
The test is how often the model would be run. If it is a few times a year, keep a well calibrated model and update it for each study. If it is every shift, the cost of keeping it live is worth paying.
What a digital twin costs that a model does not
The model is the cheap part. The cost is in the live data link: telemetry that is reliable enough to drive a model, asset records kept current as mains are replaced, and a calibration process that notices when the model and the network have drifted apart. A twin whose data feeds are broken is a simulation model with an expensive dashboard on top.
Be most careful with a twin offered without a calibration plan and a field measurement campaign. An uncalibrated twin reproduces the network as drawn, not as built.
Where AI fits
Machine learning helps a twin at the edges: forecasting tomorrow's demand to drive it, flagging a sensor that has drifted before it corrupts the calibration, and emulating the model when a scenario has to run in seconds. It does not replace the hydraulic model at the centre, for the reasons set out in AI in water management. Where an agent sits on top of a twin is covered in AI agents vs digital twins.
Smart Bhujal builds the hydraulic models at the centre of water network twins, in EPANET and InfoWorks WS, through our water distribution modelling work, and the operational layer around them as a water decision support system.
