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Factsheet: Calculating water state

Why are we recording the current state of our freshwaters?

We measure the current state of our rivers, lakes and streams to get an idea about whether our water is suitable for use for various purposes, and the effect of different pressures (i.e. 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, have an impact.

How is State calculated for our freshwater sites?

River water quality monitoring sites

The state presented for the river water quality sites on LAWA compares the median of monitoring results for river sites using the last five years data (2013-2017).   A state is calculated when there is at least 50% of the data available over this time-period, (i.e. at least 2.5 years' worth of data over a five-year period). 

For river water quality indicators, the median for a site can be compared to all other sites around the country, as well as those sites with similar land use and altitude. The land use classification for a reach is determined by the predominant upstream land use.  For example, if a reach is surrounded by 50% of more of forest, the reach’s land use is classified as forest. These land use classifications include forest, urban and rural.

The results are then presented in four groups from those that are within the best 25% (Quartile 1), to those in the worst 25% (Quartile 4):

QuartileIconQ1

Q1:  Best 0-25% 

of  sites

QuartileIconQ2

Q2: Best 25-50% 

of sites

QuartileIconQ3

Q3: Worst 25-50% 

of sites

QuartileIconQ4

Q4: Worst 0-25%

 of sites

 

National Policy Statement for Freshwater Management (NPS-FM): Lake and river water quality sites

Lake and some river water quality indicators on LAWA show how the current state of the water compares to the ‘attribute states’ in the National Policy Statement (NPS) for Freshwater Management 2014 (amended 2017), which is the Government’s direction to regional councils about how to manage fresh water.

‘Attributes’ in the NPS are characteristics of the water that need to be managed by regional councils. The attributes are grouped into those that are important for managing the health of ecosystems (e.g.  phytoplankton, total nitrogen (TN), periphyton, ammonia) and human health for recreation (E.coli, cyanobacteria).  The NPS includes a table of ‘attribute states’ which ranges from A to D, and in the case of E. coli  from A to E.    

LAWA shows the current state for freshwater quality indicators 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.  Attribute states for most indicators are defined by the annual median result for the site (which would indicate the overall environmental significance of the quality of the site), the 95th percentile result or the annual maximum result (which gives an indication of the environmental significance of what the worst quality at the site may be).  LAWA displays the lowest NOF-band from these calculations.  For example, if the ammonia annual mean 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 (total phosphorus for Lakes) is given below:  NOF band legend for Phosphorus

Councils need to set ‘freshwater objectives’ for these attributes, describing the band their communities want for each attribute. For each attribute (except E. coli) there is a national bottom line (between the C and D band). That means councils need to aim for a C or better, unless it is considered appropriate to set the freshwater objective below the national bottom line (e.g. the existing freshwater quality is caused by naturally occurring processes).  If they are not already achieving these freshwater objectives they need to work towards achieving them over time.

 

A note on State for water quality indicators:

 Viewing state by quartiles is a useful method for comparative purposes.  It shows us, in relative terms, how much better or worse a site is compared to other sites.  This comparison measure does not tell us how good or poor the water quality for a given indicator is at a site.  The NOF-band score  does give us an indication of water quality for some indicators at sites.  For some sites the water quality may be good (e.g. score an A NOF-band), yet the sites fall in the worst 25% of sites.  This is because while the water quality result for this indicator at a site is considered ‘good’, when compared to other sites, the water quality medians are in the lowest 25% of the ‘good’ water quality range.   An example is shown here:

 State quartile and NOF view

 

River ecological health monitoring sites

LAWA shows three indicators (macroinvertebrate community index (MCI), taxonomic richness and %EPT) which are used to measure the ecological health of river sites.

The state presented for these sites uses the median calculated from data over the last five years (2013-2017).  A minimum of three data points over the last five years are required for a median value to be calculated for sites that are sampled once a year.  Sites that are sampled twice per year need a minimum of six samples over the last five years.

 

Find out more

Calculating water quality trend factsheet