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• numbers and distribution of livestock in the catchment;
• input of nutrients to the catchments through nitrogen fixation and atmospheric
deposition; and
• export coefficients derived from literature sources to determine the rate of loss of
nutrients from each source to the surface drainage network.
This research had produced a land cover map for 2002 classified from multispectral
remote sensed data and aerial photography (Widdison, 2005). This was used as the
base map to calculate the extent of risk of N loss using the equation:
n
L = ∑Ei[Ai(Ii)]+ p
(Johnes, 1996)
i=1
where: L is loss of nutrients; E is export coefficient for nutrient source I; A is the area
occupied by land use type, number of livestock type i; I is the input of nutrients to
source; p is the input of nutrients from precipitation.
ArcGIS software was used to calculate total N loss kg/ha/year on a per-field basis.
Using the 2002 land cover as a base map provided a spatial dimension of predicted
output relevant to the main stakeholder group (i.e. the farming community). Results
of the preliminary modelling found the agricultural area within the Leet catchment to
be 11,213 hectares. Calculated total nutrient input (organic and inorganic fertilisers)
was a little over 2 million kg resulting in a total loss of nitrogen from the catchment
of 285,540 kg, an average of 25.5 kg/ha/year. Table 1 summarises the results for
land cover associated with the main farming activities. Cereal crops are the most
significant land cover in terms of area, input and losses of total nitrogen, accounting
for approximately 74% of the total land use and nitrogen input in the catchment but
approximately 92% of the nutrient losses.
Table 1:
Nutrient export from agricultural sources in the Leet catchment
(2002)
Land cover source
~Area
ha
% land
cover
Fertiliser
inputs (kg)
Total export of
nitrogen (kg)
% of the total
loss of nutrients
Pasture 1403 12.52 264017 17622 6.17
Rough grazing 121 1.08 381 0.13
Cereal crops* 8302 74 1743427 263168 92.2
Wood & Hedge 530 4.73 1670 0.6
Fallow & Set-aside 333 2.97 1050 0.4
*Includes three fields of potatoes (9.37 ha) which accounted for nutrient loss of 377kg.
Furthermore, when the impacts of winter sown wheat and oilseed rape are examined,
these account for approximately 47% of the total area and ~59% of nutrient losses
(Table 2).
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