Top Down Water Balance

The first step in this analytical process of assessing and calculating the volume of real and apparent losses is to undertake a IWA/AWWA recommended standardized top-down water balance (see Chap. 7 for a detailed guidance on how to undertake a water audit). Good management of any resource requires that the supplier maintain accurate records of transactions and deliveries of the commodity provided to its customers. A water balance has exactly that goal, tracking and accounting for every component of water in the cycle of delivery. The water balance tracks the flow of water from the site of withdrawal or treatment, through the water distribution system up to the point of customer consumption. The water balance usually exists in the form of a worksheet or spreadsheet that details the variety of consumption and losses that exist in a water system. The water balance itself is a summary of all the components of consumption and losses in a standardized format. Every unit of water supplied into the system needs to be assessed and assigned to the appropriate component. It is certainly not best practice to have units that are unaccounted for.

It is quite common that the calculated volumes of real and apparent losses have a relatively low level of confidence the first time a water balance was established. There are many reasons for a low level of confidence in the calculated water loss volumes the first time a standardized water balance is established however the most common ones are that some of the water balance components are not metered and/or the data used has not been validated. Therefore, it might be necessary to first increase the confidence in the calculated water loss figures by validating all the volumes entered in the water balance through meter accuracy testing, improvement of record keeping, and estimation practices and if necessary installing new system input and/or export meters. The water utility will realize that the auditing process is a revealing undertaking that provides great insight to the auditor on the type and volumes of water loss (real and apparent losses) occurring in the utility.

The real loss volume calculated through the water balance includes real losses from leaks that have been repaired (through an active or reactive leakage management policy), the volume of background losses, and real losses that are due to leaks still running in the system. The losses caused by leaks which still need to be detected and repaired by the utility are called hidden losses. However, just by establishing the water balance it is not possible to estimate the volume of hidden losses. It is recommended as best practice by the International Water Association (IWA) and American Water Works Association Water Loss Control Committee (AWWA WLCC) that the assessment of real losses using a "top-down" water balance should be complemented by the following two methodologies:

• Component analysis of real losses: A technique which models leakage volumes based upon the nature of leak occurrences and durations (see Chap. 10 for more details)

• "Bottom-up" analysis of real losses: Using district metered area (DMA) and minimum night-time flow (MNF) analysis (see Chap. 16 for more details)

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