In the book “Advanced Water Distribution Modeling and Management” I found the following:
If the differences in pressures and flows between actual conditions and predicted conditions are so great that unrealistic and unexplainable pipe roughness values (less than 30 or more than 150) or major adjustments in demands must be used to achieve calibration, then chances are good that the discrepancy is the result of a closed or partially closed valve or errors in system mapping.
The numbers less than 30 or more than 150 are Hazen-Williams c-factor. But when talking in terms of Darcy-Weisbach roughness height (e) what would be the extremes that one could draw the same conclusions from?
The key here is that you should not focus your calibration solely on pipe roughness but determine WHY the model and the field data differ and correct the parameter that is different. Here is a list of a few of the things that can make the model and field data differ. You need to be a bit of a detective to determine which applies in your situation.
Physical
Pipe size/location
Pipe connectivity
Pipe roughness
Pressure zone boundary
Pump curves
Pipe material/age in GIS
System changes since model built
Elevation data
Operational
Valve open/closed/throttled status
Control valve operation/settings
Transient events
Actual operations not matching control rules
Unusual operations when data were collected
Tank water levels
Pump status/speed
Lack of sufficient sensors/gages
Water quality reaction rates
Demands
Spatial allocation
Model does not reflect conditions when data collected
Large customers with atypical demand patterns
Not accounting for seasonal changes in demand
Data
Inaccurate/uncalibrated gages/meters
“Latched” data from SCADA
Understanding SCADA data – average vs. instantaneous
Note that I have incorporated Dr. Walski's advice into the following related article: Water Model Calibration Tips
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.