I fully agree with Mr Ben Harding's view on model calibration. This illustrates the need for developing methods for water quality measurements. Especially source tracing would be very useful.

In most cases however, a model which is calibrated for flows under normal conditions will be acceptable for water quality analyses. Unfortunately flow measurements in a distribution system are indeed a "luxury" and simpler methods need to be developed. In Swedish modelling practice, demand values are normally based on annual billing measurements and scaled according to instantaneous values at supply points. Diurnal variation is deduced from large groups and imposed on every single user, possibly divided into two or three different types. This is a sufficient level of ambition for a normal municipal application, but it is not sufficient for correct modelling of flows (let alone quality parameters) in the detailed distribution network.

Lacking other parameters, pressure measurements have been used widely for model calibration in Sweden (as - I suppose - in other countries). However this gives very little information on the model performance under normal conditions. Pressures may be significant under peak flow conditions, for which the networks are normally designed. But under normal conditions energy losses are simpy too small to be a good flow indicator.

To summarize:

- The purpose of the model is crucial to the calibration requirement
- Simple methods for source tracing and flow measurements would mean a breakthrough for the calibration of network models
- Pressure is probably the least significant parameter for model calibration

Yours,
Krister Törneke
Municipality of Botkyrka, Sweden