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CAN WE IMPROVE PREDICTION OF P CONCENTRATION
IN LUNAN LOCHS BY CHANGING THE PLUS MODEL?
I Papadopoulou 1 , A Vinten 2 and J DeGroote 2
1
Miaouli 24, Kordelio 56334, Thessaloniki, Greece, E-mail: ioanna.papadopoulou@
gmail.com; 2 The Macaulay Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK
SUMMARY
This project involves changing different factors of the PLUS model in order to test
how sensitive the model is to these parameters. The first factor examined was
whether modifying export coefficients to better reflect land use (spring–winter cereals)
improved prediction of P loading. After checking the impact of crop choice, another
scenario was set aimed at investigating how sensitive the model is to changes in
the P export coefficient table used in previous PLUS application. In addition, an
attempt was made to represent structural Best Management Practices (BMPs) in the
GIS framework and predict potential reduction in phosphorus in-loch concentrations
resulting from installing buffer strips.
INTRODUCTION
PLUS is a manageable model, constructed to predict the loss of phosphorus from
a complex catchment to its drainage system. The model uses an export coefficient
approach, calculating the total phosphorus (TP) load delivered annually to a water
body as the sum of the individual loads exported from each nutrient source in its
catchment.
The catchment of the Lunan Burn, in Scotland, was selected as the study site for the
present project. The Lunan chain of Lochs in Perthshire is of high conservation status
and requires informed and targeted environmental protection.
A study comparing predictions of PLUS with monthly observations of P status
of the Lunan chain of Lochs (SEPA, 2004) found a marked discrepancy between
observations and predictions. This discrepancy could be attributed to a number of
factors, but the most likely issues identified were the inadequacy of the land cover
data, the use of inappropriate loss coefficients and imperfect knowledge of septic
tank outputs.
Comparison of the resulting predictions for in-loch concentrations of TP with
observed values for the lochs showed reasonably good agreement for the upper
three lochs in the Lunan chain. However, for Lochs Clunie and Marlee the predictions
of the model were significantly higher than the observed values (SEPA, 2004).
SCENARIOS APPLIED
Two scenarios were applied to Lunan catchment, with the aim of determining which
of the model parameters exert greatest influence over model output. For the purpose
of these scenarios, the year of 1999 was chosen as the control year. This decision
was taken because among the sets of years for which data are available, 1999 was
closer to the initial PLUS application during which water chemistry data from January
2000 was used.
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