Environmental Engineering Reference
In-Depth Information
Fig. 3.1
Pontbren and Rhos aflo catchment
showing themain streams and gauging sites.
a relationship does not exist, but rather that noise
in catchment-scale measurements and the mul-
tifaceted nature of catchment change, combined
with climate variability, do not allow such ef-
fects, even if potentially substantial, to be
discriminated.
There is therefore a need for an alternative
approach. There are two basic issues that arise.
The first is the availability of appropriate data
and models to characterize local-scale effects; and
the second is the development of an appropriate
modelling strategy to address the generic problem
of upscaling from local to catchment-scale. In this
chapter we report the development and applica-
tion of such a methodology, based on multiscale
experimental data from Pontbren, a tributary of
the Severn inmid-Wales, and discuss the potential
of this approach for extension to national applica-
tion, linking back to the use of regional datasets,
such as the HOST classification, and the simpli-
fied methods of O'Connell et al. (2004).
improved pasture (drained, ploughed, reseeded and
fertilized) andwoodland (Fig. 3.1). Elevations range
from170 to 438mAOD (Above Ordnance Datum),
and the soils are clay-rich, mainly from the Cegin
and Wilcocks series, which are common in Wales.
The soils have low-permeability subsoil, overlying
glacial drift deposits, and are seasonally wet or
waterlogged. Field drainage is ubiquitous where
pasture has been improved. The predominant land
use is grazing, mainly for sheep.
The farmers' perception is that changes to land
management, and in particular changes to grazing
densities and animal weights, have changed runoff
response. Although land use has changed relative-
ly little since the 19th century, between the 1970s
and 1990s dramatic changes in farming intensity
took place; sheep numbers increased by a factor of
six and animal weights doubled (R. Jukes, personal
communication).
The farmers have recently reinstated woodland
areas and hedgerows, and preliminary research by
the Centre for Ecology and Hydrology, Bangor
(Carroll et al. 2004) on the infiltration rates of the
grazed hillslopes and woodland buffer strips dem-
onstrated a dramatic change in soil response to
rainfall. Infiltration rates on the grazed pastures
were extremely low, but within a few years of tree
planting, soil structure and permeability in buffer
strips showed significant improvement. However,
these results needed to be extended, to evaluate
The Pontbren Multiscale Experiment
Background and experimental design
Pontbren, situated in the headwaters of the River
Severn in Wales, is a farmers' cooperative con-
cernedwithsustainableupland agriculture, involv-
ing 10 hill farms and over 1000ha of agriculturally
