DEVELOPMENT OF ENVIRONMENTAL
STANDARDS (WATER RESOURCES)
STAGE 3 REPORT:
ENVIRONMENTAL STANDARDS
WFD48 Stage 3
March 2006
EXECUTIVE SUMMARY
The WFD 48 project carried-out the work necessary to revise water
resource regulatory standards covering abstraction and impoundments for
rivers and lakes, throughout the UK based upon ecological status. This
report details the results of Stage 3 of WFD 48, which determines the
appropriate environmental standards (i.e. the required thresholds) for
water resources parameters for UK river and lake water
bodies. The set of standards are appropriate to deliver the
WFD and relate to the boundaries for all five WFD classification bands:
high (HES), good (GES), moderate (MES), poor (PES) ecological status.
The standards do not relate to other impacts on ecological status of
rivers, such as physical obstructions to fish migration or temperature
changes to riparian tree clearance. Stage 1 of WFD 48
reviewed the appropriateness of different parameters, such as river
flow and lake level, as environmental standards. Stage 2 reviewed the
potential typologies of rivers and lakes for defining water bodies in
which different standards would be appropriate. Stage 3
produces the procedures for classifying any water body into the various
types and defines the thresholds for the parameters within each type.
The UK has relatively few species and few people reliant directly on
river resources for their livelihoods. Consequently, environmental
standards for water resources can be quite simple. However, the
resulting standards should not be used outside of the UK.
Developing a typology for
river water bodies
The four elements of fish, macrophytes, macro-invertebrates and
physical structure provide good indicators of river ecosystem status.
The initial intention of the project was to adopt a typology for each
element (e.g. RIVPACS for invertebrates). However, the only appropriate
typology of UK rivers based on ecological data that can be used for
water resources standards is that defined using macrophyte communities
from 1500 sites (Holmes et al. 1998). This classification was
simplified to give 8 generic river water body types (A1, A2, B1, B2,
C1, C2, D1, D2). The types differentiate, for example,
lowland, low gradient, clay substrate river water bodies (A1), lowland
chalk streams (A2) and steep, upland, coarse-grained substrate river
water bodies (D2). Chalk streams were further sub-divided
into headwaters and downstream areas. Expert consensus was that this
typology was suitable for setting standards for macro-invertebrates and
macrophytes. The experts recommended the use of the 8 fish
community types defined by Cowx et al. (2004) for setting standards for
fish. In only one type (salmonid spawning and nursery areas), standards
for fish exceeded those standards set for the 8 generic types (defined
for macro-invertebrates and macrophytes). Consequently, only this fish
type was used.
Physical river water body
types
Since the environmental standards needed to be applicable to all river
water bodies without any site visit, classification was based on
variables that could be quantified using existing datasets.
To understand the variables that differentiate UK river water body
types, catchment characteristics were derived for a set of 781 river
flow gauging stations, giving good geographical coverage over the
UK. The catchment characteristics included topographical,
climatological, soil, flow variables. Principal Components Analysis
(PCA) was used to define uncorrelated linear combinations of these
characteristics, which permitted the selection of a small number of
dominant characteristics, that most strongly differentiated different
river water body types. Rainfall, slope and altitude broadly
differentiate north west UK (wet, steep, high) from south east UK (dry,
low gradient, low altitude). Drainage area differentiated
water bodies with catchments of different scale. Base flow index (BFI)
differentiate flashy from base flow dominated water bodies. Together
these three components account for 61% of the variance in UK water
bodies and can be interpreted intuitively. From the PCA, rainfall
(SAAR), altitude (ALTBAR), slope (DPSBAR), drainage area (AREA) and
baseflow index (BFI) were selected to characterise variation along the
first three components and hence to provided a basis for discriminating
between types.
Linking biology to
physical river variability
The WFD System A catchment typology that uses altitude, catchment area
and geology was examined, but the broad classes meant that it did not
discriminate between the generic river water body types in the UK.
Recursive partitioning analysis (Rpart) was employed to classify the
generic river water body types according to the physical
characteristics selected from the PCA. Rpart constructs hierarchical
binary classification trees, giving a series of splits based on
cut-points in the explanatory variables. SAAR, AREA and BFI
were able to predict membership of all classes except C1 and D1. Class
C1 contains relatively few sites which have a wide geographical
distribution and are not individually distinctive sites. Consequently
it was of crucial importance for the model to differentiate this type.
Class D1 sites have specific locations in English Lowland Heaths (e.g.
New Forest), Scottish Flow Country and Western Isles and can thus be
located geographically. Separate analysis was conducted for
different hydro-eco-regions, but this did not improve predictive power
in the model. The use of substrate data was also explored.
Some general patterns emerged justifying future research, but no
significant improvement in predictive power was gained. No
method was available to define salmonid spawning and nursery areas,
other than use of local knowledge.
Maps were produced to demonstrate how river water body generic types
vary along major UK rivers from, for example, D2 in the headwaters to
B2 near the mouth for the Rivers Tweed and Exe.
Defining environmental
standards for river water bodies
Regulatory standards for each river water body type were defined
through an expert consensus workshop approach. The experts were invited
to define thresholds of flow alteration that would ensure good
ecological status (GES) in water bodies, based on two scenarios:
abstraction and impoundments. Experts in macrophytes and
macro-invertebrates adopted the generic typology and defined thresholds
for abstraction for each river water body type to achieve GES. The fish
experts defined standards for fish community types. All standards were
very precautionary based on indicating points at which experts could no
longer be certain that GES would be achieved. The thresholds were
broadly in the range of 10-20% permissible abstraction above flows of Q95
with hands-off below Q95; these stringent levels
reflected uncertainty in precise threshold levels. All experts felt
that standards for impoundments should be the same as those for
abstraction and that long periods of constant compensation flow
releases from impoundments could achieve Good Ecological Potential
(GEP) but would not achieve GES, which requires maintenance of flow
variability.
No precise method was available to identify fish community types for UK
river water bodies. A fish atlas is available for Great Britain at 10
km grid scale, but this does not include spawning and nursery
areas. In general terms, fish community 1 (chalk river fish)
relates to generic type A2 (chalk rivers); fish community 2
(eurytopic/limnophylic fish) relates to A1 (lowland clay-substrate
rivers). The other fish communities cut across generic types;
rheophilic cyprinids could occur in types B1, B2, C1, C2 and adult
salmonids and salmonid spawning could occur in types B1, B2, C1, C2, D1
and D2. Future research is required to be able to predict
these latter three fish community types in river water bodies.
Recursive partitioning analysis (as used for the generic types)
provides one possible approach.
Results of analysis of LIFE score data were not able confirm any
variations in standards between river water body types so long as the
flow regime was standardised by both mean and flow variation. Analysis
of changes in physical character of river water bodies (based on wetted
river width) reinforced the significance of Q95
as a threshold at which sensitivity to flow changes.
In all cases, except salmonid spawning and nursery areas, standards set
for the generic types (based macro-invertebrate and macrophyte flow
requirements) were more strict than those set for fish community
types. Consequently, only the salmonid spawning and nursery
area type was retained as an explicit river water body type for WFD 48,
in addition to the 8 generic types.
Practical standards, less stringent than those precautionary standards
defined by experts (which are likely to “guarantee”
a particular status will be met), were derived by the project team by
taking a risk-based approach. This approach accepts that with more
relaxed standards, some river water bodies may fail to achieve the
desired ecological status, but these would be identified by appropriate
monitoring. In this way the team defined standards for the lower limits
to achieve different levels of ecological status:
% of flow
Lower limit for flow > Q95
Lower limit for flow < Q95
High Ecological Status
10
5
Good Ecological Status
15-35
7.5-20
Moderate Ecological Status
25-45
15-30
In general the strictest standards are those for steep upland rivers
(D2) and chalk streams (A2) whilst the least stringent tend to be for
lowland clay-substrate rivers (A1). All standards
were defined in terms of % of flow on the day abstraction. The experts
felt that it was not possible to specify a constant volume that could
be abstracted at any flow and still achieve GES, except by defining the
volume as a percentage of the very lowest flow. As a fail-safe, it was
suggested that any abstraction should not reduce flow at Qn99 by more
than 25%.
Although not strictly part of the project specification, the team
analysed the experts’ views on releases from impoundments
that would achieve GEP. These included release of flood events at key
times of the year and variations and fluctuations in compensations
flows.
Defining a typology for
lake water bodies
The fundamental approach to defining a typology for lake water bodies
was to adopt the lake reporting typology adopted for Great
Britain. This is based essentially on chemistry, in turn
reflecting geology and salinity, and giving basic classes for peat,
low, medium and high alkalinity, marl and brackish waters. It
was recognised that the typology could allow for classes based on these
types to be split or combined, in order to increase sensitivity to
factors specifically relevant to the fundamental hydrological variable
of water level, or to avoid duplication, respectively as
appropriate. Such an approach offers the advantage that
supporting biological data are now being collected and analysed.
Physical lake types
Further to the chemical basis of the typology reflecting geology and
salinity, a second tier of the typology reflects lake depth (two
classes for mean depth less than or greater than 3 m), and further
lower tiers are based on altitude (three classes divided at 200 m and
800 m) and lake size (three classes divided at water area 10 ha and 50
ha). Strong control on the geographical distribution of these
classes is exercised by geology: deep lakes are much more concentrated
in the north while shallow lakes are much more abundant in the
south. A further physical characteristic is basin form,
whereby a further two-fold division of classes has been proposed
following the work of Håkanson.
Linking biology to
physical lake variability
The typology has been developed on the basis that water level
alteration is the principal hydrological parameter to which aquatic
communities are sensitive, while the degree of sensitivity is a
function of many other lake characteristics, as identified
above. The expert workshop identified that there were
substantial deficiencies in the knowledge necessary to confidently
predict the threshold hydrological alterations which would lead to
changes in ecological status for lakes of various physical and chemical
characters. The incorporation of effects was achieved by
using the chemical types to define basic levels of sensitivity, located
by reference to the limited opinions expressed at the expert workshop
and as refined according to the literature, and then defining
sensitivity modifiers according to each of the additional factors
outlined above. Basic levels of sensitivity ranged from
10-20% deviation in naturally occurring lake levels. A
sensitivity calendar was used to identify and collate the seasonal
nature of sensitivity effects for different species and groups of
organisms. A risk-based approach was developed, whereby the
number of sensitivity-increasing factors applying to an individual lake
in an individual season was used to identify the degree by which the
basic sensitivity threshold should be reduced. The principal
threshold of interest was that representing the boundary between Good
and Moderate Ecological Status, and was initially expressed as a
proportion of naturally-occurring lake level on any day, relative to
the sill or control structure over which the outflow drains.
Defining environmental
standards for lake water bodies
To provide environmental standards in terms of water flows, allowing
regulators to work towards licences in volumetric or flow terms, it was
necessary to relate water level deviations to flows. This was
possible using the assumption that flow over a sill or other outflow is
related to level by a rating relationship with a stage exponent greater
than unity. This is as indicated theoretically by the Chezy
equation and has been confirmed empirically in this study by reference
to data from a necessarily small number of sites at which levels and
flows are available. Assuming the Chezy exponent of 1.5,
permitted abstraction fluxes were found to be more lenient than their
corresponding (and more ecologically relevant) permitted water level
deviations. Level restrictions ranged from a mere 5% for some
peat lakes to 20% for some brackish lakes, corresponding to abstraction
restrictions of 7% to 28% respectively.
This system of defining environmental thresholds provided for
individual differences attributable to specific physical controls to be
reflected through the concept of risk, but led to results with an
unjustifiable level of apparent accuracy. As a means of
addressing this concern, the final table of environmental standards,
expressed in flow deviation terms, therefore introduces a rounding to
the nearest 5%: the loss of unjustifiable minor differences in
threshold is argued to compensate for possible exaggeration of
threshold values in some cases where similar lakes fall just either
side of a 5% boundary. The set of thresholds for the
Good/Moderate Ecological Status boundary was then taken as a starting
point to define threshold values for the other ecological status
classes, while maintaining the assessments of relative difference in
sensitivity.
By ultimately defining standards in flow terms, it becomes possible to
assess the possible effects of a water use proposal not only in
relation to the adjacent rivers but also any lakes on the same river
system, and so therefore be able to identify whether the river or the
lake provides the more stringent environmental requirement.
Given the assumption of the Chezy equation applying to outflow ratings,
it is likely that lakes will often require less stringent provisions
than rivers.
Proposals for future work
Recommendations for future research are provided including: a method to
predict which river fish community types occur in which water bodies;
use of site variables such as channel geometry and substrate using RHS;
applying environmental standards to licensing; involving researchers
more closely with development of standards; and defining standards for
flows to estuaries. For the proper setting of environmental
standards for lake abstractions, further recommendations are made,
especially in relation to increasing the amount of monitoring of lake
levels and their associated outflows.
Copies of this report are available from the Foundation, in electronic
format on CDRom at £20.00 + VAT or hard copy at £35
.00, less 20% to FWR members.
N.B.
The report is available for download from the SNIFFER Website