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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 an inevitable result of supporting our communities and agriculture, and it may not be 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?

LAWA evaluates conditions (state) at sites nationwide against attribute bands described in the National Policy Statement for Freshwater Management 2020 (NPS-FM 2020).

The NPS-FM 2020 requires the management of freshwater in a way that ‘gives effect’ to Te Mana o te Wai, and sets up requirements for regional councils and unitary authorities in setting objectives, policies and rules to manage freshwater in their regions.  ‘Attributes’ in the NPS-FM 2020 are characteristics of the water that need to be managed, and are the equivalent of LAWA's 'indicators'. 

The 'current state' for 2020 at each site is based on data over the last five years (2016-2020).  Each indicator's state is defined by attribute bands, from A (good) to D or E (poor).

 

Councils, along with their communities, need to set ‘target attribute states’ (attribute bands) for sites in their region. For many indicators there is a national bottom line value and councils and their communities need to aim above that bottom line, unless there is a specific justification why that would not be achievable (e.g. the existing freshwater quality is caused by naturally occurring processes, or a specific time-limited exemption applies). As part of setting target attribute states, councils will also set timeframes as to when these will be achieved.

 

Minimum data requirements for determining attribute bands

For all lake water quality indicators, LAWA requires:

  • at least 50% of monthly data available over the last five years (2016-2020), (i.e. at least 30 monthly measurements over this time period).

 

For E. coli, LAWA requires:

  • at least 54 of 60 samples available over the last five years (2016-2020).

 

For cyanobacteria, LAWA requires:

  • at least 12 samples from the last three years (2018-2020).

 

Several attributes (indicators) have multiple band definitions, referring to different statistics calculated from the set of monthly data, such as the median or maximum.  The attribute band assigned to such indicators is the worst of those derived for different statistics of each attribute.  For example, if the chlorophyll median result was qualified as an A band and the maximum result as a B band, the site is assigned the B attribute band for chlorophyll a, and this B is displayed on LAWA.

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