Geology Reference
In-Depth Information
are only used to distinguish between the hydrau-
lic properties of the different areas. Shallow flows
and low slopes on the areas feeding the rills with
water and sediment tend to produce less sedi-
ment than the concentrated flow paths where
water is deeper and moving more quickly.
5.3 Methodology
5.3.1 Field site
The field site was a fodder field (Fig. 5.3) with
an estimated area of 5.44-6.81 ha, depending on
whether a potential contributing area upslope is
included or not, within the Den Brook catchment,
Devon, UK (UK grid reference: SX 67712 99685).
Note that, as is often the case in agricultural
catchments, delineating the catchment bounda-
ries and area is itself an uncertain process - this is
explored more fully in Section 5.5. The catch-
ment is described in more detail by Page
et al
.
(2005). Den Brook has an estimated area of
46.43 ha, soils are poorly draining, often-water-
logged clays (clayey non-calcareous pelostagnog-
ley (Avery, 1980), a Typic Haplaquept (USDA,
1975) of the Hallsworth Series. Land use at Den
Brook is intensively managed grassland, with the
grass used for grazing animals, silage and fodder
production. Increasingly, farmers in the UK are
incorporating fodder fields into their grassland
farms to provide food for the stock over the win-
ter months. Such fields are vulnerable to soil ero-
sion because of poor canopy cover during the
winter and spring, combined with row cultiva-
tion. At Den Brook, kale (
Brassica oleracea
) was
grown with grass undersown. Sheep and lambs
were allowed to graze the field during the winter
months. Den Brook receives substantial amounts
of rainfall, with a 40-year mean annual total of
1050 mm (Page
et al
., 2005).
Rainfall was measured using a tipping-bucket
rain gauge (Rainwise, Bar Harbor, ME) which
recorded the total number of tips per minute
(each tip equivalent to 0.254 mm rainfall).
Discharge from the fodder field was measured
as it flowed over a V-notch weir at the lowest cor-
ner of the field. Depth of water flowing through
Fig. 5.3
Outline of the simulated field with 1 m
contours. The measurement flume is on the northeast
corner of the field. The contributing area consider-
ing the field boundaries is 5.44 ha, but it is likely that
the area further upslope overflows into the field
during large storm events, making the contribut-
ing area 6.81 ha. The NEXTMap Britain
orthorectified radar image Intermap Technologies
(2007) was provided courtesy of NERC via the
NERC Earth Observation Data Centre
(NEODC).
the weir was recorded using a bubble meter
logged by an ISCO 24 bottle pumped sampler,
which also took water samples from which sus-
pended sediment concentrations were deter-
mined gravimetrically. The weir was calibrated
by the bucket and stopwatch method and the
uncertainties surrounding measurements at the
same stage quantified using the fuzzy rating
curve approach of Krueger
et al
. (2010) (an exten-
sion of a concept presented by Pappenberger
et al
., 2006). Suspended sediment concentration
uncertainty was not quantified in this study due
to the absence of field measurements to do so,
but the potential for this type of measurement
uncertainty was acknowledged in the course of
model evaluation.
