Author:Melissa M Foley
Source:Auckland Council Research and Evaluation Unit, RIMU
Auckland Council (and Auckland Regional Council prior to 2010) has been reporting overall water quality scores since 2007 using the water quality index (WQI) developed by the Canadian Council of Ministers of the Environment. Prior to the use of this index, water quality was reported using individual parameters. As individual parameters can show different trends (some increasing, some decreasing), it can be difficult to interpret what the overall state of water quality is.
The WQI index helps to simplify communications around water quality by providing a single score for overall water quality that takes into account fluctuations in individual parameters. It also provides one simple metric for communication of water quality state and changes to a wide audience. Methods for calculating the WQI have evolved over time. Standardising how the WQI score is calculated is important for assessing water quality against static guidelines, as well as for analysing water quality trends in the Auckland region.
This report documents how methods for calculating the WQI have changed over time, proposes new methods for future calculations, and compares the outcomes of both methods. Using the new method does produce some different state results to those reported previously, particularly for open coast sites. However, the overall patterns in water quality generally remain the same. Scores at sites that were in poor condition using the previous methods stayed in the poor category with the new method although scores declined slightly due to more stringent objectives. Sites that were in good to fair condition had broadly similar scores for the two methods between 2014 and 2017.
Using the new method to calculate water quality back to 2007, water quality scores increased across most sites, consistent with the state and trends analysis on individual parameters for this same period of time (Foley et al. 2018).
Auckland Council technical report TR2018/027