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Calculating the water balance

The UK uses a top down, bottom-up hybrid methodology for calculating the water balance, which takes an estimate of each water balance element and compares it to the sum of the distribution input of the system. This is different to the normal international methodology, due to the way that leakage is measured as well as the limited customer meter penetration that is seen.聽

Due to the nature of these estimates the values are unlikely to match exactly, and so a residual is distributed across each of the components based on the uncertainty that exists within the element method. This is known as the confidence interval and is used to reduce the water balance gap. Ofwat have published suggested ranges for these confidence intervals that companies should meet. The difference in water volume between distribution input and the sum of all components must then meet the ultimate threshold of 卤5%, and anything that is outside this 5% is added to leakage.聽

Unmeasured non-household consumption contributes least to the water balance, as there are only a very small number of non-household properties that are not metered. Distribution system operational use, and water taken legally and illegally make up the minor components of the water balance, contributing 5.7%, and understanding within this area is in need of improvement.聽

Components that have particularly high levels of uncertainty include water taken legally unbilled, water taken illegally unbilled, and unmeasured consumption. This is due to the high number of assumptions that are used to calculate these values. For example, for unmeasured households a small number of properties are monitored for a period of time then this is applied to all households, thus assuming that they all act the same. This is an inherently inaccurate representation of the population.聽

Making a difference

Within my role as an Analyst, I have worked on several projects for UK water companies to calculate water balance confidence intervals. We have worked to help them improve the methods used for calculation of the intervals along with recommendations on data sources that could enable better reporting in the future. I found this work particularly interesting as were able to use our industry knowledge to tailor methods to the water company鈥檚 data and work on ways that reporting can be improved. I identified the areas where confidence intervals were coming out too high, indicating a low confidence in the data, and this has allowed water companies to focus their efforts on components that are most in need of improvement.

I have also worked on projects to generate a minor components review, focusing on this area of the water balance only. As minor components is such a small element of the water balance, it is generally less well understood, and less investment is given to improving the knowledge within this area. However, as there are a large number of elements that contribute to these components, it is important that data sources are improved so that they may be better understood in the future.