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Factsheet: Calculating water quality state for lakes

Why are we recording the current state of our freshwaters?

We measure the current state of our rivers, streams and lakes to get an idea about whether our water is suitable for use for various purposes, and about the effect of different pressures (e.g. urbanisation, farming, etc.) on the condition of our freshwater resources.

Some influence from people on our waterways is necessary to support our communities and agriculture, and it is not possible to return most water bodies in New Zealand to their pre-human state. Even where this is almost achievable, such as within national parks, introduced aquatic species such as trout, as well as native and exotic birds, insects and pest mammals, and natural processes such as erosion and atmospheric pollution, impact our waterways to some extent.

How is State calculated for our lake sites?

State in the LAWA Lakes topic is described using NOF band scores.The NOF band scores are an absolute measure, evaluating each site against expectations of water quality values in the National Policy Statement for Freshwater Management 2020.

NOF Band Scores

The National Policy Statement for Freshwater Management 2020 (Freshwater NPS 2020) requires the management of freshwater in a way that ‘gives effect’ to Te Mana o te Wai.  The National Objectives Framework (NOF) in the Freshwater NPS 2020 sets up requirements for regional councils and unitary authoritaries in setting objectives, policies and rules to manage freshwater in their regions.  One component in the NOF is defining of 'attribute states' or 'bands', which range from A to D (or E).  ‘Attributes’ in the Freshwater NPS 2020 are characteristics of the water that need to be managed by regional councils, the equivalent of LAWA's 'indicators'.

Monitoring data from lake water quality indicators are evaluated against NOF attribute states, and LAWA displays this as the A, B, C D or E NOF bands (along with descriptions for context) so you can see which attribute state the water is currently in.

 

 

Minimum data requirements for determining NOF bands:

For all physical-chemical water quality indicators, LAWA requires:

  • at least 50% of monthly data available over the last five years (2015-2019), (i.e. at least 2.5 years' worth of data over this time period).

For E. coli, LAWA requires:

  • a minimum of five years of data
  • at least 54 of 60 samples available over the last five-year period.

 

Attribute states for indicators are defined by the lowest band from the numeric attribute state metric (e.g. the annual median, the 95th percentile or the annual maximum result).  For example if the ammoniacal nitrogen annual median result was an A NOF band and the annual maximum result was a B NOF band, then LAWA shows this as a B NOF band.

An example of the descriptions for the attribute states (e.g. total phosphorus for Lakes) is given below:  

 

Councils, along with their communities, need to set ‘target attribute states’ (NOF bands) for sites in their region. For many attributes there is a national bottom line. That means councils need to aim for the bottom line or better, unless it is considered appropriate to set the NOF band below the national bottom line (e.g. the existing freshwater quality is caused by naturally occurring processes). As part of setting target attribute states, councils will also set timeframes as to when these will be achieved.