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A third scenario was applied to Clunie subcatchment with the aim of investigating
any potential reduction in phosphorus in-loch concentrations, resulting from installing
buffer strips on both sides of Lunan Burn in the Clunie subcatchment.
First Scenario’s Description and Assumptions
The aim of this scenario was to check how sensitive the model is to the proportions
of winter and spring cereals. Three sets of data were used: 1989, 1994 and 2004. The
assumptions made for this scenario are listed below:
• First assumption: P loss coefficients for arable land, in the P export rates table
provided by MLURI for the purpose of this project, are representative of the
proportion of winter and spring cereals in Lunan catchment for 1999 (control
year).
• Second assumption: The model works for all five subcatchments, so the final
outputs are not to be questioned. The numbers used in order to draw conclusions
are not the actual output numbers for P loads in the lochs but the difference in the
values between 1999 and 1989, 1994, 2004.
• Third assumption: Relative P loss coefficient for winter cereals is equal to 1
and for spring cereals is equal to 0.39 based on literature. Winter cereals were
considered to be the same as having no best management practice applied and
spring cereals were considered the same as conservation tillage.
Applying this scenario involved modifying the export coefficients to better reflect
land use (spring/winter cereals). In order for this to be accomplished the following
steps were followed:
a. A formula that calculated correction factors for P export rates based on relative
areas of winter/ spring crops was created. The relative areas were calculated
using agricultural census data per parish.
b. P export table was transformed according to the correction factors and a different
table for each application (1989, 1994, 2004) was produced.
S
TA . S + e
W
TA . W e
Formula used to calculate P export coefficients: A e
=
(1)
Where:
A e is the P export coefficient for arable land.
S and W are the areas of spring and winter cereals, respectively.
TA is the area of total arable land.
S e and W e are the P loss exports for spring and winter cereals,
respectively.
Based on the third assumption: S e
- 0.39 . W e (2)
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