The amount of excess deposition above the critical load is called the exceedance. The critical load values are compared with deposition values mapped at 5x5 km resolution for the UK; for this exercise the deposition is assumed to be constant within each 5x5 km square. For nutrient nitrogen, the exceedance is calculated for each habitat as the amount of excess total nitrogen (ie, wet and dry, oxidised and reduced) deposition above the critical load.
Deposition of both sulphur and nitrogen compounds can contribute to acidification and therefore to the exceedance of acidity critical loads. A Critical Loads Function (CLF) has been developed (Posch et al, 1995; UBA 2004) that defines combinations of sulphur and nitrogen deposition that will not cause harmful effects, ie, separate acidity critical loads in terms of sulphur and nitrogen. These critical loads incorporate the acidity critical loads values, together with data on base cation and nitrogen uptake, non-marine base cation deposition, nitrogen immobilisation and leaching and denitrification. The CLF is a three-node line graph representing the acidity critical load, and the intercepts of the CLF on the sulphur and nitrogen axes define the sulphur and nitrogen critical load values (CLmaxS, CLminN and CLmaxN on the graph below). Combinations of sulphur and nitrogen deposition above the CLF exceed the critical load, while all areas on or below the CLF line represent an “envelope of protection” where critical loads are not exceeded. Using the CLF, acidity exceedances are calculated for the habitat critical load values in each 1x1 km square in which they occur across the country. Details on the methods used to derive these critical load values for the UK, and on the calculations of exceedances can be found in Hall et al. (2015).
It should be noted that the critical loads data on which these exceedance calculations are based, are derived from empirical or steady-state mass balance methods, which are used to define long-term critical loads for systems at steady-state. Therefore, exceedance is an indication of the potential for harmful effects to systems at steady-state. This means that current exceedance does not necessarily equate with damage. In addition, achievement of non-exceedance of critical loads does not mean the ecosystems have recovered. Chemical recovery will not necessarily be accompanied by biological recovery; and the timescales for both chemical and biological recovery could be very long, particularly for the most sensitive ecosystems.