Predicting surface water critical loads at the catchment scale
Current applications of the critical loads concept are geared primarily towards targeting
emission control strategies at a national and international level. In the UK maps of critical
loads for freshwaters are available at 10km2 resolution based on a single representative site
in each grid square. These maps do not take variations of water chemistry within mapping
units into account and are therefore of limited use for application to non-mapped sites. This
thesis describes the development of an empirical statistical model, which uses nationally
available secondary data, to predict freshwater critical loads for catchments lacking the
appropriate water chemistry information.
A calibration exercise using data from 78 catchments throughout Scotland is described.
Water chemistry for each catchment has been determined and each catchment is
characterised according to a number of attributes. Multivariate statistical analysis of these
data shows clear relationships between catchment attributes and water chemistry and
between water chemistry and diatom critical load. The key variables which explain most of
the variation in critical load relate to soil, geology and land use within the catchment. Using
these variables (as predictors) in a regression analysis diatom critical load could be
predicted across a broad gradient of sensitivity (R2adj = c. 0.8). The predictive power of the
model was maintained when different combinations of explanatory variables were used. This
accords the model a degree of flexibility in that model paramaterisation can be geared
towards availability of secondary data.
There are limitations with the model. These relate to the nature of the predictor variables
and the ability of the model to predict critical loads for more sensitive sites. Nevertheless the
ability of the model to differentiate between sensitive and non-sensitive sites offers
considerable scope for environmental managers to undertake national inventories of
catchment sensitivity and specific assessments of individual catchments.