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Water quality methodology

This note summarises proposed methodology for including water quality impacts in the 2050 Calculator. Comments on this methodology would be very welcome.

Background

The objective of this work is to identify the impact on the UK water environment of different pathways towards 2050 emissions targets. The method used must be compatible with the existing approach, which identifies a number of different measures for changing emissions. These include demand-side measures (reductions in energy use), supply-side measures (changes in the way that power is generated) and changes to sectors that contribute directly to greenhouse gas emissions (for example, transport and agriculture). Some of these changes will affect the aquatic environment, sometimes for the better. The objective is to identify the net effect of a complete pathway on the aquatic environment.

Approach

For each measure the change has to be measured against a unit output, so that change can be scaled according to how much of that measure is introduced. This is conceptually easy with electricity generation, where it will be easy to establish a relationship between water use and power generated. For agriculture, the units will be per head of cattle or per hectare of land, depending on the output. For industrial output, it may be easiest to relate to production.

The schematic demonstrates the approach. For each component, calculate:

Change in volume of water used (up or down from baseline)
Change in chemical water quality (again, up or down). Note the need to define a single scale of water quality changes that can be added sensibly – this is conceptually challenging. However, the matrix approach means that the scale may only need to be relative.
Direct change in other factors (eg biodiversity). This may only apply to one or two components – for example, hydropower, where fish migration is of concern. Again, a scale for this will have to be defined.

Each will need to be on a scale that can be accumulated. All of the changes of each impact are accumulated – this means, for example, that positive and negative changes can cancel out. The approach here does not provide a combined score for each of the three impacts but keeps them separate. This allows a more flexible approach to a scoring matrix that calculates total impact and places this in a class on the matrix. The scoring matrix requires expert judgement, which is also subsequently needed to link it to a change in WFD classification.

One possible approach to water quality scoring is to work out the worst possible combination of options and assign this a score of 100. All others can then be scaled relative to this score, reducing impacts proportionally. This again requires expert judgement but this can be recorded and therefore tested.

Figure 1: Diagram illustrating water quality methodology