Cleaning Reservoir Data to Understand Water Age

Cleaned historical reservoir data tells a story about water age, which impacts on water quality. Excessive water ageing is likely to lead to a reduction in residual chlorine concentration, and potentially poor disinfection for communities.

Reservoirs can experience considerable differences in winter and summer water age especially where there is high summer tourism.

Cleaning and organising historical reservoir data can give practitioners a useful overview of water age at different times of the year. This can be used for general guidelines for seasonal reservoir operation and inform decisions about chlorine dosing.

Let’s look at some data…

The figure below captures about 20 years of water level data from a reservoir located in a residential area. You will notice that the data is messy! Which can lead to it being undervalued or overlooked during decision-making.

You will notice spikes and gaps in the data and on closer inspection there are shifts in the data linked to changes in cut in-levels. And the large data gap on the right-hand side suggests the reservoir was taken offline for maintenance.

Figure 1 Water level data from a reservoir located in a residential area

Despite Figure 1 appearing to be messy, with data cleaning, valuable metrics around water age can be learnt.

For example, if outflow data is available a simple ‘plug flow’ model can be used. Where, at any time (T) the duration (D) for cumulative outflow to exceed the volume at time T can be estimated. This duration is a lower bound on the age of the water in the reservoir at time T+D. So, if the duration D exceeds the desirable water age there is definitely a problem. Plug Flow analysis is conservative because it does not include any mixing effects.

Plug flow durations provide an approximate but useful indication of relative differences in water age between low demand winter months and high demand summer months. It can guide seasonal operation rules, such as reduced volume in winter months.

Because chlorine reactivity is dependent on concentration, mixing in the reservoir is important. Scheduling of chlorine dosing requires residual measurement – most importantly at the outflow. Although chlorine residual measurement is desirable, water age analysis can give a broad indication of the likely ratio of winter to summer dosing.

In conclusion…

Cleaning and organising historical reservoir data can lead to an improved understanding of seasonal water age in reservoirs. This can lead to better operational rules of thumb to limit the impacts of water age.

FSA Data can easily clean, organise and present visualisation of your water data. This can be used to inform decisions about water infrastructure. Please contact us for more information.