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Using this method of classification allowed a much more realistic assessment of the
level of risk that each field plot contributed to water quality in the catchment. These
results, Figure 1 and summarised in Table 4 below, show there were no fields in the
very low risk category. This is because the proximity to water course weighting
forces a minimum possible score of 5 points.
Table 4:
Summary results of risk assessment of 2002 land use
Number of field plots
in category
Approximate area Approximate
(ha)
N loss (mg/l)
Very low risk 0 0 0
Low risk 922 3120 184
Medium risk 414 2219 101
High risk 569 3717 363
Very high risk 191 3256 345
Modelling Land Use Change Scenarios
Although it is not possible to predict a precise moment when nutrient loss will exceed
the EU limit, the export coefficient model can be applied to predict the impact of
changing land use on nutrient losses at the field scale. This modelling will be useful
to the farming community as it can provide information to be used in their decision
making processes. Currently the catchment includes 74% arable land use which is
responsible for 87% of total fertiliser input and 92% of the total predicted nutrient
loss. The four scenarios described below involve changing the way in which arable
farming is practised.
The predicted impacts of the scenario modelling are shown in Table 5 below. The
first land use change scenario involves the installation of fixed width grassland
buffers at 5 or 10 m, to all water courses in the catchment. These buffers would
remove approximately 150–320 ha of land from production, of which approximately
90–190 ha are currently used for arable production. As part of the management of
these buffers, it is assumed that fencing is installed to prevent livestock accessing the
stream; application of chemicals including fertilisers ceases; and most importantly, all
field drains discharging to streams are blocked to prevent nutrient losses by-passing
the buffer. In the short-term, vegetation would return to rough grassland, although
further management of the buffer could include planting native species woodland
which would increase nitrate removal in these zones. In these two scenarios nitrogen
input is limited to that from atmospheric deposition (3.15 kg/ha/year) therefore the
total N loss is recalculated using the export coefficient for land in set-aside/woodland.
Installing fixed width buffers results in N losses being reduced by 0.16–0.25% for the
whole catchment. In terms of benefits for the farming community, fertiliser usage
is reduced by 1.17% to 2.47% of the existing use and this would reduce the cost.
However, this would have to be balanced by the loss in income from grain sales on
these buffers.
The buffer scenario was further investigated by changing the buffer to 50 m. In
an intensive arable regime, the land within 50 m of the water course can make a
significant contribution to nutrient loss. Under current farming practices, there are a
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