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Factsheet: Calculating water quality trends

LAWA displays trends for the last five and ten years for the river and lake water quality data. Data are evaluated to determine whether water quality indicators are showing improving or degrading trends.  An indeterminate result occurs when the available data do not show a significant trend. Sites that are 'not assessed' are monitored, but the data do not meet the criteria to be eligible for a trend analysis.

Why do we calculate trends?

LAWA calculates trends to show how the quality of water in rivers and lakes has changed at each site over time. Changes in water quality often take several years to be seen and longer trends are generally more reliable.  This is because some physical and chemical measures of water quality can be influenced by flow, seasonal and climatic cycles (weather patterns). 

What river water quality parameters does LAWA show trends for?

LAWA shows river water quality trends for eight water quality indicators: water clarity (black disc), turbidityE. coli, total nitrogen, total oxidised nitrogen, ammoniacal nitrogen, dissolved reactive phosphorus and total phosphorus.   In 2018, LAWA started determining 10-year trends for the macroinvertebrate community index (MCI), which is an indicator of the ecological health of a river.

LAWA shows lake water quality trends for six water quality indicators: total phosphorus, total nitrogen, ammoniacal nitrogen, chlorophyll a, Secchi disc and E. coli.

LAWA trend categories for rivers and lakes

LAWA categorises trends into more classes than in the previous year.  These categories are based on the probability of trend occurrence and provide more detail about trends that would have previously been identified as 'indeterminate'.  There are five categories: Very likely degrading, likely degrading, indeterminate, likely improving, very likely improving.

 

Improving Trends 

Very likely improving trend Likely improving trend
  Very likely improving trend Likely improving trend

 

 

 

 

 

 

 

 

For sites that are showing improving trends, these are either classified as 'very likely improving' or 'likely improving'.

A 'very likely improving' trend is given when there is 90 - 100% certainty of an improving trend.  A 'likely improving' trend is given when there the certainty of the trend ranges from 67 - 90%.  The lower certainty tells us that there is an indication of an improving trend, however there is less statistical certainty that this trend exists.

The increasing trend symbols are used for sites that show an improving trend in water quality.  An improvement is generally a reduction in concentration of a water quality parameter, for example, phosphorus and nitrogen concentrations.  However, for visual clarity indicators (e.g. black disc and Secchi disc) and the Macroinvertebrate Community Index (MCI), an improvement is represented by an increase in these values.  

With enough data, the trend evaluation method can detect even a very small trend, in these cases the trend can be classified as improving (or degrading), even if there is only a small change in water quality.

 

Indeterminate Trend

Indeterminate trend_Sept2018
Indeterminate trend

This classification is given to sites where there is insufficient evidence to clearly say if water quality trend is improving or degrading.  An indeterminate trend may mean that the water quality at a site has stayed the same or that the data available do not show an upward or declining trend direction with statistical certainty. 

 

Degrading Trends

Very likely degrading trend Likely degrading trend
Very likely degrading trend Likely degrading trend

 

Sites that are showing a decline in water quality are either classified as 'very likely degrading' or 'likely degrading '.

A 'very likely degrading' trend is given when there is 90 - 100% certainty of a degrading trend.  A 'likely degrading' trend is given when there the certainty of the trend ranges from 67 - 90%.  The lower certainty tells us that there is an indication of a degrading trend, however there is less statistical certainty that this trend exists.

The degrading trend symbols are used for sites that show a degrading trend in water quality.  A degrading trend is generally an increase in a water quality parameter, for example, phosphorus and nitrogen concentrations.  However, for visual clarity indicators (e.g. black disc and Secchi disc) and the Macroinvertebrate Community Index (MCI), degradation is reflected by a decrease in these values.  

 

Trend Not Assessed 

Not assessed for a trend
Not Assessed

Sites are not assessed for trends when they do not meet the criteria to be included in the trend analysis (e.g. there were not enough samples over the period).  

 

How do we calculate water quality trends? 

Data requirements

Trends are calculated for the last five and ten years. Data are evaluated to determine whether water quality is showing improving, degrading, or indeterminate trends.

The data used to calculate water quality trends for rivers is collected monthly, every two months or quarterly.  Macroinvertebrate data is collected annually or twice per year.  Sites that are sampled monthly will have more data than sites that are sampled quarterly.  Generally, the more data points we have available for a site, the more robust the trend analysis will be. LAWA calculates and displays trends for the last five and ten years for sites with monthly data, but only ten-year trends are calculated for sites which are sampled less frequently. 

For ten-year trends, river and lake water quality sites were excluded from the analysis if they had less than 90% of the data expected, and data from less than nine out of the last ten years.  For macroinvertebrate community index trends, sites with less than eight years of data points over the last ten years were excluded from the analysis.

For five-year trends, the river and lake water quality sites were excluded from the analysis if they had less than 90% of measures over five years.  LAWA does not calculate five-year trends for macroinvertebrates.

 

A bit more detail on river trend analyses 

To determine whether water quality at a river site is showing improving, degrading or indeterminate trends, LAWA follows the methodology of McBride (2018), as implemented in R functions (R Core Team) provided by LandWaterPeople. 

Trends are calculated on data for one site/measurement combination at a time. 

 The data for each combination were tested for seasonal effects, and then analysed with either a seasonal or non-seasonal version of the Mann Kendall Slope Test.  This seasonal test compares the water quality data of each season separately (January with January, February with February, etc) which means if any changes are detected they are not hidden by seasonal patterns.  

Probability values from this test are interpreted in a Bayesian context as the probability of a mis-classification.  The confidence is then derived as one minus half the probability of mis-calculation.  The Mann Kendall probability value classifies each site/parameter combination into one of five categories (very likely improving, likely improving, indeterminate, likely degrading, very likely degrading).

Censored values (data that are less than or greater than laboratory detection limits) are included in the trend analysis.  Trends are not calculated where there are fewer than five total and three unique, non-censored observations.  Sites which did not meet the criteria for analyses are reported as 'not assessed'.

Ideally, data should be flow adjusted before trend analysis to remove any effects of variation in water quality measurements caused by variable stream flow.  However, many councils do not measure river flow at their water quality sampling sites which is why all data used for trend analysis on LAWA are not flow-adjusted. 

 

Where do I find more information? 

LandWaterPeople (LWP).  https://landwaterpeople.co.nz/

Larned S, Snelder T, Unwin M, McBride G, Verberg P & McMillan H. 2015. Analysis of water quality in New Zealand lakes and rivers. Prepared for Ministry for the Environment. NIWA Client Report No. CHC2015-033. 

Larned S, Snelder T, Unwin M & McBride G. 2016.  Water quality in New Zealand rivers: current state and trends.  New Zealand Journal of Marine and Freshwater Research 50:3, 389-417.

McBride, G. 2018.  Has water quality improved or been maintained?  A quantitative assessment procedure.  Paper submitted to Journal of Environmental Quality.

Our fresh water 2017.  A joint report by the Ministry for the Environment and Stats New Zealand.

R Core Team.  2018. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. hyyps://www.R-project.org/

Snelder T & Fraser C. 2018. Aggregating trend data for environmental reporting.  LWP Client Report 2018-01.

Stocker T, Qin D & Plattner G (Editors). 2014.  Climate Change 2013: The physical science basis: working group | Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.  Cambridge University Press.