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could be misleading. The results of the first scenario can be attributed to the following
factors:
• The proportion of winter crops in the Lunan catchment is very low, and specifically
winter crops in the catchment are limited to winter cereals, which compose just
6.7% of the overall cereal crops, based on 1999 agricultural census data;
• The amount of potatoes could also be considered as winter crop, given that
potatoes increase the risk of run-off;
• Finally, the third assumption that was made could be questioned.
The range of P export coefficient used by PLUS for arable land is 0.40–2.00 kg/ha/
year, while the range of P export coefficient for arable land used for the purpose of the
second scenario is 0.24–1.20 kg/ha/year. In order for this assumption to be justified,
results of P export studies that were carried out on two main subcatchments/ farms
in the Lunan Lochs chain are used:
• Craigie Burn/ Hillocks of Gourdie Farm
• Cattymill Burn/ Meadows of Ballied Farm
Based on the samples (Table 2), the exports are higher than 1 kg/ha in specific
areas in the catchment. This figure weakens the argument that the PLUS exports
(0.40–2.00 kg/ha) are too high. It has to be kept in mind though that the ditch that
drains the field where repeated applications of poultry manure takes place drains
afterwards into Black Loch. So, the high value of P loss could be attributed to the
poultry manure application. However, the Cattymill catchment is mainly arable. More
accurate conclusions regarding the range of P exports could be drawn if a series of
samples well distributed across the whole Lunan catchment was available.
Table 2:
Craigie
Burn
Cattymill
Burn
Summary of P export from three catchment areas
Catchment
area (ha)
Sampling period
Mean [TP]
(mg/L)
P loss
(kg/ha)
Ditch draining a field 18 13/10/04 to 13/3/05 0.27 0.86
with poultry manure
Black Loch outlet 60 8/4/05 to 30/7/05 0.49 1.17
Meadows of Ballied 13/10/04 to 1/2/05 0.34 0.99
Chapelton 2453 13/10/04 to 19/1/05 0.22 0.60
CONCLUSIONS – FUTURE WORK
Taking under consideration the change into winter/spring cereals distribution has
some positive effect on estimating TP in loch concentration, but the difference is not
statistically significant at 95% confidence. However, this is attributed to a number of
factors related to Lunan catchment and to the assumptions of the scenario.
Changing P export coefficients for arable land, based on literature search, resulted
in a decrease of the calculated TP in the lochs by up to 20%. This demonstrates
that model calibration by inspecting and adjusting the export coefficients is required
and has the potential to result in more accurate estimation of the total in-loch
concentrations.
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