Definition: The annual volume input to the water supply system.
In case the entire system input is metered, the calculation of the annual system input should be a straightforward task. The regular meter records have to be collected and the annual quantities of the individual system inputs calculated. This includes own sources as well as imported water from bulk suppliers.
The accuracy of the input meters should be verified on an annual basis, using portable flow-measuring devices, or if possible by conducting volumetric comparisons via reservoir drop test, for example. If any inaccuracies of the system input meters are revealed it is necessary to further investigate the problem, and if necessary, the recorded volume of water has to be adjusted to account for the inaccuracy of the system input meter. It is recommended that as well as verifying the accuracy of the meters, the entire data recording chain from the raw 4 to 20 mA signal produced by the meter to the Supervisory Control and Data Acquisition (SCADA) archive is checked when testing the input meters.
If there are unmetered sources then the annual flow has to be estimated by using any (or a combination) of the following:
• Temporary flow measurements using portable devices
• Reservoir drop tests
• Analysis of pump curves, pressures, and average pumping hours
It is important to realize that the accuracy and reliability of the water balance results are directly linked to the accuracy of the figures used for the system input volumes. It is recommended that system input meters are tested for their accuracy at least once a year so they can be recalibrated if necessary.
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