Modeling

Uncertainty Analysis 161
area, New Jersey. (Water Science and Technology Vol 36 No 5 pp 141-
148.(Anonymous, 1997).
The American Water Works Association (AWWA) Engineering Computer
Applications Committee indicate that true model calibration is achieved by
adjusting whatever parameter values need adjusting until a reasonable
agreement is achieved between model-predicted behavior and actual field
behavior (AWWA Engineering Computer Applications Committee 1999). Once
a model is considered to be calibrated, it can then be used, among other
purposes, to estimate hydraulic characteristics of the real-world system at
locations where measured data are unavailable or unknown, spatially and
temporally.
In the US, definitive standards to assess the accuracy of water distribution
model calibration have yet to be agreed upon or established. However, the
following calibration criteria have been suggested:
1. An average pressure difference of ±2.2 psi with a maximum difference
of ±7.3 psi for a good data set, and an average pressure difference of
±4.3 psi with a maximum difference of ±14.2 psi for a poor data set
(Walski 1983); and
2. The difference between measured and simulated values should be ±5 psi
to ±10 psi (Cesario and Davis 1984).
According to the AWWA Engineering Computer Applications Committee
(1999), ten sources of possible error could cause poor agreement between water
distribution model values and measured field values. These sources of error,
which provide a potential list of factors that can be adjusted during the modelcalibration
process, are:
1. errors in input data (measured and typographic),
2. unknown pipe roughness values (i.e., Hazen-Williams C-factors),
3. effects of system demands (distributing consumption along a pipe to a
single node),
4. errors in data derived from network maps,
5. node elevation errors,
6. errors introduced by time variance of parameter values such as storage
tank water levels and pressures,
7. errors introduced by a skeletal representation of the network as opposed
to modeling all small-diameter pipes,
8. errors introduced by geometric anomalies or partially closed valves,
9. outdated or unknown pump-characteristic curves, and
10. poorly calibrated measuring equipment including data loggers, tank
water-level monitors, and SCADA systems.
Model calibration entails adjusting model parameter values until an acceptable
match is achieved between measured data and model-simulated values (e.g.
pressures at the test hydrants, water levels in the storage tanks, flows from
booster pumps, and pumpage from groundwater wells). The ten sources of
possible error that could lead to model simulated values not agreeing with