Geoscience Reference
In-Depth Information
Monitoring data may also generate bias in
subsequent research, because of factors such as the
selection of sampling locations. Site access is an
important issue for selecting frequently sampled
survey sites; for example, those visited for monthly
water chemistry sampling are usually located close
to bridges and roads. While this may have little
effect on water chemistry, it means that certain
types of site will be over-represented in the dataset,
while others will be under-sampled, so this has to
be taken into account in subsequent data analysis
(Vaughan and Ormerod, 2010). Similarly, because
most river management and associated monitoring
in the UK occurs in medium-sized or larger
channels, the majority of headwater streams are
under-represented (Environment Agency, 2010b).
Therefore, careful specification of subsequent
study objectives and appropriate data analysis are
required.
The opportunities and problems of using
monitoring data for other purposes, and the
limitations and benefits of combining datasets to
extend the range of enquiry, are discussed by
Vaughan and Ormerod (2010). Inevitably, studies
that use data for purposes beyond their original
remit have to deal with marked limitations.
Fortunately, within even those limits there is
a wide range of possibilities that could make a
valuable contribution to river science, needed
for improved understanding and management.
This reflects the benefits of the unique array of
variables in space, time and both biological and
physical domains provided by river monitoring in
the UK. The following section provides two simple
illustrations.
monitoring information from several sources to
identify regions affected by multiple pressures. The
catchment (area 4136 km
2
) is sparsely populated
in the upper and middle reaches, where improved
pasture is the major land-use, along with rough
grazing and conifer forestry (Edwards and Brooker,
1982; Jarvie
et al.
, 2003). The lower reaches
are characterized by arable agriculture and dairy
farming, and these also flow through the major
population centres (Figure 7.2b; Jarvie
et al
.,
2003). The catchment is of significant conservation
importance, with parts designated as Special Areas
of Conservation under the EC Habitats Directive
(Council of the European Communities, 1992),
while the river itself
is an important salmonid
fishery.
We chose to focus on two potential pressures:
organic pollution, measured by Biochemical
Oxygen Demand (BOD) and channel resectioning.
Median BOD concentrations were calculated for
1994-1996 from 234 locations across the Wye
catchment, using the Environment Agency's
Water Information Management System database.
Information on channel structure for 495 RHS sites
was extracted from the national RHS database,
comprising data from the two national baseline
surveys, as well as other sites sampled within the
catchment. Channel resectioning was quantified
using a standard RHS index ranging from zero
(no modification) to 100 (channel and banks
resectioned over the complete 500 m length;
Vaughan, 2010).
Basic mapping of BOD and channel resectioning
immediately highlighted those river reaches
exposed to both pressures, as well as illustrating
the excellent data coverage across the catchment
(Figure 7.3). More detailed results for a wider
range of determinands can be found in Jarvie
et al
. (2003). Interpolation methods can be used
to clarify congested dot maps and predict values
for unsampled sites, thereby giving complete
catchment or river network coverage (Haining,
2003). From this, areas of concern can then
be delineated using simple rules such as those
exceeding a threshold (e.g. BOD concentration
or extent of channel resectioning) or the top
n
-percentile of locations. Trans-Gaussian kriging
Characterizing joint pressures
in the River Wye catchment
Characterizing the correlations among potential
human pressures (stressors), and mapping their
spatial distributions and trends over time, are
obvious applications for river monitoring data. The
wealth of data from the River Wye catchment
that spans the border between England and Wales
(Figure 7.2a), demonstrates the potential of using
