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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 resource.

Some influence from people 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.

Regional State

How is regional state calculated?

The state of freshwater quality presented on the LAWA website compares the median of monitoring results for river sites using the last five years data, and lake sites using results from data collected over the last calendar year.

Lake and some river water quality indicators show how the current state of the water compares to the ‘attribute states’ in the National Policy Statement (NPS) for Freshwater Management 2014, 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. For some of the main attributes that are important for managing the health of ecosystems and people (e.g. types of contaminants like E.coli, Phytoplankton, Total Nitrogen, Ammonia etc.), the NPS includes a table of ‘attribute states’ which ranges from A to D. 

LAWA shows the current state for freshwater quality indicators as the A, B, C or D band (along with narrative for context) so you can see which attribute state the water is currently in.  This is 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 annual maximum result (which gives an indication of the environmental significance of what the worst quality at the site may be).  An example for 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 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.

 *Please note that the Government agreed to amend the NPS for Freshwater Management 2014 in August 2017.  The amendments came into force on 7 September 2017.  As part of the changes, there are new attribute tables for primary contact recreation in lakes and rivers, and the attributes for E. coli and planktonic cyanobacteria (toxic algae) have  been replaced.  The data on LAWA has not yet been updated to reflect the new requirements.  

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. Note that this is different to the way the land use is categorised for the national picture.

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 25% of sites

QuartileIconQ2

Q2: Best 50% of sites

QuartileIconQ3

Q3: Worst 50% of sites

QuartileIconQ4

Q4: Worst 25% of sites

National State

How do we estimate current national state?

We estimate water quality across the whole country by using models, which are based on data collected from hundreds of regional councils and National Institute for Water and Atmospheric Research (NIWA) monitoring sites. In the majority of cases this reflects a five year period. 

How is national state calculated?

Due to limitations in the available data at the national scale, we aren’t yet able to assess many of the human pressures that impact on the state of our rivers. However, one important attribute for which we do have good national scale information on is land cover. Land cover is associated with many pressures such as run-off and point-source (piped) pollution, altered hydrology, channel straightening, and riparian vegetation modification. Landuse serves as a good proxy for the majority of these pressures and is the only component of human impact incorporated into the modelling used in this indicator. For this reason we break down the results by land-cover classes in our analysis.

Approximately 48.4% of New Zealand’s river length is fed by catchments which are ‘predominantly indigenous’ (i.e., native Forest (22.5%), Tussock (15.4%), Bare Ground (5.5%) and Scrub (4.9%)), 45.7% mainly pasture, 5.1% exotic forest, and 0.8% urban land-cover classes. Pasture refers to all intensive agricultural use and includes pasture, cropping and horticulture.

Exotic forest and indigenous land cover categories are considered dominant where they make up the greatest proportion of the catchment. However, a reach is considered urban if its upstream catchment area is more than 15% urbanised and pastoral if its upstream catchment area is more than 25% agriculturally used. If both pastoral and urban exceed their respective thresholds, then urban is assumed the most dominant. These land cover categories are derived from the New Zealand River Environment Classification (REC) which is based on information derived from the New Zealand Land Cover Database (LCDB). You can find out more details on how land use is classified for the national picture here http://www.mfe.govt.nz/sites/default/files/environmental-reporting/about-environmental-reporting/classification-systems/rec-user-guide-2010.pdf. Note that this is different to the way regional land use is classified.

Modelling and reporting by land cover also helps deal with the limited number and therefore limited representativeness of monitoring sites. Many monitoring sites tend to be situated in areas with more pressures such as lowland plains, cities and intensive land uses, while pristine areas go unmonitored. Without modelling, these sources of bias would mean the site data would not represent wider patterns across the whole of New Zealand.