Development of the scientific rationale and formulae for altering
RIVPACS predicted indices for WFD Reference Condition
WFD72b
September 2006
EXECUTIVE SUMMARY
Background to research
With the advent of the EU Water Framework Directive (WFD), the concept
of the ‘reference condition’ (RC) against which
assessments of biological degradation must be compared has become
explicit within the legislative framework of the European Union
(Council of the European Communities, 2000). It is therefore essential
that member states can demonstrate that the biological datasets and
models used to define RC meet the WFD criteria and set the same
standards for all types of river site.
The selection of the RIVPACS (River InVertebrate Prediction And
Classification System) reference sites and development of the RIVPACS
methodology and software system for assessing the ecological status of
UK rivers preceded the WFD. For several years it has been a concern
that several, or even many, of these reference sites may not have been
in WFD RC at the time of macroinvertebrate sampling, but merely
represented the “best available” sites for each
type of river site. In particular the predictions of expected fauna for
some types of river site will be based on inadequate quality reference
sites, leading to under-estimation of RC values for biotic indices and
over-estimates of the RIVPACS observed to expected (O/E) ratios (termed
Ecological Quality Indices (EQI)) and site quality for
macroinvertebrates.
To help address this problem, the Centre for Ecology and Hydrology
(CEH) collated and assessed the available pressure data on each site
(SNIFFER project WFD46). Subsequently UK agency aquatic ecologists
provided an assessment score (1-6) of the perceived WFD ecological
status class of each reference site (1 = top of high, 2 =
middle of high, 3 = high/good boundary, 4 = middle of good, 5 =
good/moderate boundary, 6 = worse).
In the current UK classification scheme, an EQI value of 1.0 is used to
set the “high/good” boundary; a river site has to
achieve biological index values equal to or in excess of those
predicted by RIVPACS in order to be classified as the highest status.
This means that, in effect, that roughly half of the RIVPACS references
sites are assumed to be of “high” ecological status
and roughly half of “good” status. The problem is
that this assumption has been applied across the board for all types of
river sites in the UK.
UK regulatory agency ecologists have been devising methods of adjusting
the RIVPACS expected (E) values or EQI values of indices (ASPT and
number of BMWP taxa (TAXA)) by determining the weighted average
assessment score of the reference sites involved in the prediction for
any particular test site. However, these approaches are, in effect,
determined solely by the EQI values used to set the good/moderate
boundary.
Objectives of research
To develop a robust defensible mechanism for adjusting the RIVPACS
expected values of biotic indices for any specific test site according
to the perceived ecological condition (at the time of sampling for
RIVPACS) of the RIVPACS reference sites actively involved in the
prediction for that test site. The resulting adjusted O/E values should
then more evenly reflect the WFD concept of ecological status across
all UK river types
To provide the necessary formulae for implementing this adjustment
mechanism for the environmental regulatory agencies’
classification sites.
Key findings and recommendations
The research approach used was based on statistical modeling to assess
and quantify the actual relationship between assessment score (1-6) and
the observed values of TAXA and ASPT amongst reference sites within
RIVPACS site types (TWINSPAN groups).
Negligible differences in average index values were found between
reference sites with assessment scores of 1,2 or 3. Observed index
values are lower relatively for reference sites with assessment scores
of 4 and especially 5. (There were insufficient reference sites with
scores of 6 (worse than good/moderate boundary) to estimate its
adjustment factor and it is recommended that any such sites are treated
as having assessments scores of 5 in the adjusted of test site expected
values.)
Models were fitted separately to data for each RIVPACS module (GB
Northern Ireland (NI), Scottish Islands (SI) and Scottish Highlands
(SH)) and for samples based on each the seven possible combinations of
one, two or three RIVPACS seasons. There was insufficient range of
scores in the high quality SI and SH sites to determine any significant
relationships. Therefore a recommended single UK-wide adjustment model
was fitted using all UK reference sites and samples. Although actual
effects of changes in site quality on index values may vary with site
type, to achieve adequate precision and for simplicity, the best single
overall estimates for the adjustment factors were derived.
Models were also fitted based on grouping sites by WFD System A
typology; some estimates of adjustment factors were greater, but
overall model fits were poorer.
The recommended statistical model for estimating the adjustment factors
was model M4 which estimates the average proportional increase or
decrease in index values due to each level of site assessment score
relative to sites with scores of 3 (“target”
high/good boundary) within the same TWINSPAN group. Recommended
adjustment factors are given in Table 30.
Procedures and formulae are derived to combine these score-specific
adjustment factors with information on the proportion of reference
sites in each site group with each assessment score to calculate the
recommended adjustment factors for expected values for any test site
based on its RIVPACS probability of belonging to each TWINSPAN group.
An EXCEL spreadsheet adjustment “calculator” with
encoded formulae to automate this procedure for adjusting RIVPACS
expected values of any UK RIVPACS test sites has been produced and is
available as an project deliverable and output.
One advantage of the M4 type of model is that the approach could also
be used in other European ecoregions where reference condition expected
values of metrics for a test site are based on some average or
percentile value of the reference sites in the same WFD
System A or B stream typology (Council of the European Union, 2000)
(i.e. stream type replaces TWINSPAN group in the models)).
Keywords: Water Framework Directive (WFD), Reference Condition,
RIVPACS, Adjusted expected
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