Unaccounted for Water (UAW) is a commonly used statistic to gauge the health of water distribution systems. It is the percentage of the annual withdrawal that is lost with no indication of where it went. There are times when UAW is a great measurement like when high percentages of water are lost. A recent news story highlighted several communities that lost over half of the water they pumped. The problems with UAW arise as conservation efforts become more successful, systems use less water and the systems age.
The main issue is that UAW is an indirect measure of a key piece of information that is readily available; in fact it is used in the calculation of UAW - the volume of lost water. An example is a system that looses the same volume for two consecutive years but withdraws more water the second year than it did the first year. In this case the same volume is lost but the UAW declines which indicate an improvement that did not occur. Similarly if a system looses the same volume each year but withdrawals go down in the second year then UAW goes up even though less water was used.
UAW is often used to compare multiple water distribution systems since it normalizes the amounts withdrawn and lost for easy comparison. What it does not convey are the factors that make water systems different such as their size, age, number of connections, population, water quality and other characteristics. A simple example is two systems with the same UAW percentage. One covers a large amount of land area, serves a small population and uses a small amount of water. The second system covers a much smaller area but serves a large population and uses a great deal of water. Which system is healthier? It's hard to gauge with the limited amount of information given but is impossible with just the UAW.
The best way to gauge a system's health is to compare the actual volume lost over several years. Reducing that volume over successive years is what matters the most and should be the goal. One of the confounding issues is that UAW was not regulated by the Department of Environmental Protection until recently. Now that it must be within certain bounds, system managers will likely allocate more time for better record keeping and develop better estimation methods. The result is a more accurate gauge today but one that is not really comparable to previous decades when UAW was not as much of a concern and less time and care was spent calculating the value. DEP will also spend more time reviewing the water supplier's calculations to make sure they are correct. A similar issue exists with residential per capital use, especially for systems that do not serve entire towns or serve parts of several towns.
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