Common exceedance metrics are described below. The UK trends in critical loads exceedance show the changes in these metrics over time, from 1995 to the present day.
Area of habitat exceeded
This is derived from the habitat distribution maps. For terrestrial habitats the area values are based on the CEH Land Cover Map 2000 data. If the critical load for any individual habitat is exceeded, the exceeded area is set to the habitat area within the 1x1 km square for that particular habitat. For the freshwater habitats, if the critical load is exceeded, the whole catchment is assumed to be exceeded, and the exceeded area set to the catchment area. Exceeded habitat areas are summarised by country.
Percentage area of habitat exceeded
This is calculated from the exceeded areas and the total area of each habitat mapped in each country, according to the habitat distribution maps. While this is a useful metric, it does have its limitations, for example, when comparing exceedance results from one year to another (or one deposition scenario to another), there may very small (or no) changes in the percentage area of habitat exceeded. This is because the magnitude of the exceedance may have reduced, but the area exceeding the critical load remains the same; the area exceeded will only reduce when the critical load is no longer exceeded.
Accumulated Exceedance (AE)
This metric takes into account both the area exceeded and the magnitude of exceedance:
AE (keq year-1) = exceedance (keq ha-1 year-1) * exceeded area (ha)
AE is calculated for each 1x1 km square for each habitat and then summarised by habitat and country. AE is set to zero where critical loads are not exceeded. This metric can be useful for comparing results for different years or scenarios, but because the results are expressed in keq year-1 they tend to be very large numbers and not intuitive to understand. It should also be noted that the same AE can arise from a large exceedance and small exceeded area, or a small exceedance and a large area.
Average Accumulated Exceedance (AAE)
This metric provides an exceedance value averaged across the entire sensitive habitat area, and is based on the AE for the habitat divided by the total habitat area:
AAE (keq ha-1 year-1) = AE (keq year-1) / total habitat area (ha)
AAE is calculated by habitat at the country level and is set to zero where critical loads are not exceeded. This metric provides a more intuitive value for comparing the exceedance results for different years or scenarios, and gives an indication of the reduction in the magnitude of exceedance even if there is no change in the percentage area of habitat exceeded.