The purpose of this article is to provide guidelines and best practices for hydraulic model calibration of water systems.
When a water distribution system model is first constructed, the model results need to be compared with field data to ensure that the model accurately represents the real world. Initially, most models do not agree well with the field data. The model inputs must be adjusted (or in some cases incorrect field data must be discarded or corrected) to bring the model into acceptable calibration. Calibration is defined in attachment #1 at the bottom of this article.
The difficulty with calibration is that there are many reasons that a model and field data will not be in agreement. The primary problem is not making the adjustments, but understanding what is causing the discrepancy in the first place. Some of the reasons a model may appear uncalibrated would include:
*Note: typically roughness does not have a dramatic effect on carrying capacity with few exceptions such as cast iron. As an example, the City of Charleston, SC, which has the oldest cement mortar lined pipe (dating back to 1922), conducted a C-factor test (see YouTube video at the bottom of this article). The test showed that the roughness had not changed over 97 years. So, if Darwin Calibrator recommends a C-factor of 80 for a cement mortar lined pipe, it is probably compensating for something else such as a closed pipe or mistake in data entry somewhere.
#Note: when performing calibration it is best to have a model that is as close to the real system as possible. For this reason having an over skeletonized or simplified model would lead to unaccountability of the entire carrying capacity of your system. Using a model that accurately represents the system with all its pipes would help in getting better calibration results.
It takes considerable judgment and experience to identify the source of the discrepancies. Users are often overwhelmed by the possible choices. It is best to take a logical set of steps through the calibration process. Attachment #2 at the bottom of this article provides a procedure to approach calibration.
Darwin Calibrator (available in WaterGEMS) provides a tool to adjust the first three parameters from the above list once the user has identified that one of them is the source of the discrepancy. If a user decides to adjust the wrong parameter in order to make the model look calibrated, it is referred to as “Calibration by compensating error” which uses one error to correct for another. It may be acceptable to assume that a certain parameter is the sole source of discrepancy and allow Darwin Calibrator to make adjustments, but these adjustments must be used with caution.
Darwin Calibrator uses the head loss between the water source as the primary driving force for its solution. It is therefore essential to ensure that the head loss between the source and the pressure measuring point is significantly greater than the error in measurement. For example, if the head loss from a tank to a pressure gage is 1 m, and the error in measurement is +/- 2 m, this data should not be used. For more on data accuracy, see attachment #3 at the bottom of this article.
Before beginning to work on calibration, users are encouraged to read the attachments to this article and take the Bentley training classes on Calibration and Darwin Calibrator:
(Bentley LEARN Server) Water Distribution Design and Modeling Advanced using WaterGEMS CONNECT Edition (see "Automated Calibration" on-demand lecture and workshop)
(OpenFlows YouTube channel) Water Advanced Training (WaterGEMS) (See part 1 and 2 on Darwin Calibrator)
Attachment #1 - PDF(JAWWA, 2013)
Attachment #2 - PDF(JAWWA, 2017)
Attachment #3 - PDF(JAWWA, 2000)
Using Darwin Calibrator
Calibrating a model based on hydrant flow tests
Darwin Calibrator Performance Improvement Tips for Large Models
Tips on Sewer Calibration
OpenFlows | Hydraulics and Hydrology Forum (search for many conversations on the subject of calibration)
Automated Calibration with Darwin Calibrator in WaterGEMS:
Water Model Calibration Tips and Tricks