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Table 2: Nutrient export from winter wheat and winter oilseed rape (2002)
Land cover source
~Area
Ha
% land
cover
Fertiliser
Inputs (kg)
Total export of
nutrients (kg)
W. wheat 4763 42.5 1000261 135736 47.5
W. oilseed rape 549 4.9 115296 35091 12.3
% of total loss of
nutrients
These results illustrate the significance of cereal cropping, and in particular the
significance of winter sown crops to the problems of nutrient export in the catchment
as a whole.
In the monitored sub-catchments, the average annual loss of N ranged from 18.25
to 164 mg/L. The highest N losses (> 146 mg/L p.a.) occur in areas contributing to
gauging stations in sub-catchments where arable land use is more than 80% so this
can be seen to be one of the contributing factors to such high losses. Potential risk
of N loss was attributed to each field plot by establishing which plots lie within 50 m
of the watercourses and therefore pose the greatest threat to water quality. Using a
combination of factors, the spatial distribution of N loss was presented in ArcGIS as
risk maps to be used by stakeholders as part of a suite of land use decision-making
tools.
The Nutrient Export Risk Maps for the Leet Water Catchment
Relative risk was assigned using weighted values for a range of criteria; when
these are combined with a weighting for proximity to a water-course, a combined
risk score can be calculated. Weighting values ranging from 1–5 were given to the
parameters shown in Table 3 below. For land use, nutrient input and export the full
range of weighted values were available for each field. However, for distance from
watercourse, the weight range was restricted to reflect the significant contribution of
field drains to potential nutrient loss.
Table 3:
Weighted values for parameters in risk assessment
Land use
Nutrient input
kg/ha/year
Daily nutrient
export N
(mg/L)
Distance from
water course (m)
Woodland 0 0–0.09 1
Fallow/set-aside 1–99 0.1–0.19 > 50 2
Grazing 100–199 0.2–0.29 3
Spring cereals 200–249 0.3–0.39 11–50 4
Winter cereals > 250 > 0.4 0–10 5
Weighted
value for each
variable
This enabled a total risk assessment score of up to a maximum of 20 points to be
assigned to each field plot. Risk was then classified into five categories based on
the following intervals:
Very low risk
Low risk
Medium risk
High risk
Very high risk
1 – 4 points
5 – 8 points
9 – 12 points
13 – 16 points
17 – 20 points.
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