Calculating Critical Loads


Critical loads are generally defined as: “a quantitative estimate of exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge” (Nilsson & Grennfelt, 1988).

The methods for calculating critical loads are based on internationally agreed approaches and have been adapted to make use of the national data sets that are available for producing our maps. A major update to the methods used in the UK to calculate and map acidity critical loads was carried out in 2003 with further minor updates in 2004, 2008 and 2009.  Critical loads for nutrient nitrogen for the UK were reviewed and updated in 2011. Critical loads data and maps are available nationally on a 1x1 km grid for habitats that are sensitive to acidification and eutrophication and for which sufficient data exist to map their distribution nationally. The methods for calculating critical loads and exceedances are outlined below; further details on the methods, including equations and maps, can be found in the latest UK Status Report (Hall et al, 2016) found under the Publications page of the web site.

Critical Loads of Acidity

Two methods are used for calculating acidity critical loads for terrestrial habitats in the UK: an empirical approach is applied to non-woodland habitats and the simple mass balance (SMB) equation is applied to both managed and unmanaged woodland habitats. For freshwater ecosystems, national critical load maps are currently based on the First-order Acidity Balance (FAB) model. All of these methods provide critical loads for systems at steady-state; each is described briefly below.

Empirical critical loads of acidity for soils

Mineral weathering in soils provides the main long-term sink for deposited acidity. Using this principle, critical loads of acidity can be based on the amount of acid deposition which could be buffered by the annual production of base cations from mineral weathering (Nilsson & Grennfelt, 1988).

In the UK, empirical critical loads of acidity for soils have been assigned to each 1x1 km grid square of the country based upon the mineralogy and chemistry of the dominant soil series present in the grid square (Hornung et al., 1995). The data are mapped in five classes representing ranges of critical load values, with low critical loads for soils dominated by minerals such as quartz and high critical loads for soils containing free carbonates. Where a single critical load value is required, for example, when calculating the excess deposition above the critical load (ie, the exceedance), the mid-range values are applied, with the exception of those with the highest critical load, where the value at the top of the range is used (Hall et al., 2003a). However, this classification, based on weathering rates and mineralogy, is inappropriate for peat soils, which contain little mineral material. Instead, for peat soils, acidity critical loads are based on the concept of effective rain pH ie, total acidifying pollutant load divided by runoff. This method sets the critical load to the amount of acid deposition that would give rise to an effective rain pH value of 4.4, which reflects the buffering effects of organic acids upon peat drainage water pH (Calver, 2003; Calver et al, 2004). This method is analogous to applying the Simple Mass Balance equation (see below) using critical pH as the chemical criterion, but with the leaching of aluminium and base cation weathering set to zero. This method is suitable for upland and lowland acid peat soils, but not for lowland/arable fen peats, which are less sensitive to acidification and hence require a higher critical load value to be set (currently set to 4.0 keq ha-1 year-1).

Together, these methods provide a 1x1 km map of acidity critical loads for soils across the UK. These critical loads are assigned to, and mapped, for the following non-woodland terrestrial broad habitats: acid grassland, calcareous grassland, dwarf shrub heath, bog and montane (Hall et al, 2015).

The application of these methods in the UK represent a precautionary approach, setting the critical loads for soils to prevent any further change in soil chemistry as a result of deposited acidity (Hornung et al., 1997).

The simple mass balance (SMB) equation for calculating acidity critical loads for woodland ecosystems

The SMB equation is the most commonly used model in Europe for the calculation of acidity critical loads for woodland ecosystems. This model is based on balancing the acidic inputs to and outputs from a system, to derive a critical load that ensures a critical chemical limit (related to effects on the ecosystem) is not exceeded (Sverdrup et al., 1990, Sverdrup & De Vries, 1994). The equation has been derived from a charge balance of ions in leaching fluxes from the soil compartment, combined with mass balance equations for the inputs, sinks, sources and outputs of sulphur and nitrogen (Posch et al., 1995).

In the UK we apply SMB equations to managed (productive) and unmanaged coniferous and broadleaved woodland habitats. The application of the SMB equation to non-woodland systems needs further development and testing because of uncertainties in the applicability of the critical chemical criteria to other ecosystems.

The SMB equation is parameterised according to the soil and woodland types. Critical chemical criteria and critical limits are selected to protect the receptor from the adverse effects of acidification. A critical molar ratio of calcium to aluminium of one in soil solution is a common criterion applied in the SMB to protect the fine roots of trees. This criterion is applied in the UK to both coniferous and broadleaved woodland on mineral or organo-mineral soils (ie, mineral soils with a peaty top). For woodlands on peat soils the methods described above for application to peat soils are used, with a critical pH of 4.4 for upland and lowland acid peat soils. The methods used also take into account the base cation inputs from the addition of phosphate and potassium fertilisers to managed woodlands. The parameterisation for the different woodland/soil types and the equations currently being used are given in Hall et al. (2015).

Acidity critical loads for freshwater ecosystems

The acidity critical loads for UK freshwaters are based on data from a national survey of lakes or headwater streams, where a single site, judged to be the most sensitive (in terms of acidification) was sampled in each 10x10 km grid square of the country. In less sensitive regions (eg, south-east England) the sampling generally consisted of one site in each 20x20 km grid square. In 2004 this “mapping dataset” was updated to include sites from other surveys and networks, where appropriate data were available. To date the models have been applied to 1625 sites in Great Britain and 127 in Northern Ireland. Hence the freshwater critical load maps do not represent all waters in the UK and the results are mapped by site location.

The First-order Acidity Balance (FAB) model

FAB is a catchment-based model used to derive linked critical loads of sulphur and nitrogen. The model was re-formulated in 2001 (Henriksen & Posch, 2001; UBA 2004) to take account of direct deposition to the lake surface, whereas the previous version (Posch et al., 1997) assumed that all deposited nitrogen had first to pass through the terrestrial catchment before reaching surface waters.

The FAB model employs a simple charge balance for nitrogen and sulphur, along with the base cation leaching rate derived from the Steady-State Water Chemistry model (Henriksen et al, 1992; Henriksen et al, 1997). The charge balance equates the deposition inputs of acid anions with the sum of processes which control their long term storage (eg, in-lake retention of sulphur and nitrogen), removal (eg, net growth uptake of nitrogen by forest vegetation, long-term immobilisation of nitrogen and nitrogen lost through denitrification in catchment soils) and leaching exports (eg, catchment runoff). In the 2004 update the value of the critical chemical threshold of acid neutralising capacity was changed from zero to 20 meq l-1 for all sites, except for naturally acidic sites where a value of ANC 0 meq l-1 has been retained. The equations currently being used in the model are given in Hall et al. (2015).

Critical Loads for Nutrient Nitrogen

Enhanced nitrogen deposition to terrestrial and freshwater ecosystems can lead to acidification or eutrophication. The latter can have major impacts on plant communities leading to changes in species composition and the sensitivity of vegetation to environmental stresses, such as drought, frost or insect predation (Hornung et al., 1997). Therefore methods have been developed to set critical loads to protect against these adverse effects. Two approaches are currently in use: empirical and mass balance, and these are described briefly below.

Empirical critical load for nutrient nitrogen

Empirical nutrient nitrogen critical loads have been set for different ecosystem types. They are based on observed changes in the structure or function of ecosystems as reported in the refereed literature from the results of experimental or field studies, or in a few cases dynamic ecosystem modelling.

Ranges of critical load values are given for each ecosystem type to take account of: (i) intra-ecosystem variation between different regions where an ecosystem has been investigated; (ii) the finite intervals between additions of nitrogen in experiments; (iii) uncertainties in estimated total atmospheric deposition values. Each range of values is accompanied by one of the following “reliability” scores: “reliable” where a number of published papers of various studies showed comparable results; “quite reliable” when the results of some studies were comparable; “expert judgement” when no empirical data were available for the ecosystem and the nitrogen critical load was based on expert judgement and knowledge of comparable ecosystems.

The empirical critical loads for all habitat types were most recently reviewed at an international workshop in 2010 (Bobbink & Hettelingh, 2011). Critical loads were assigned to habitat classes within seven categories of the European Nature and Information System (EUNIS) habitat classification:

Woodland and forest habitats
Heathland, scrub and tundra habitats
Grassland and tall forb habitats
Mire, bog and fen habitats
Inland surface water habitats
Coastal habitats
Marine habitats

In the UK empirical nitrogen critical loads have been applied to unmanaged coniferous and broadleaved woodland categories, grassland (acid and calcareous), dwarf shrub heath, bog, montane, dune grassland and saltmarsh (Hall et al., 2015, 2011). Within each range of values for each habitat a “UK mapping value” has been set to provide a single value for the calculation of critical load exceedances; these mapping values are given in Hall et al. (2015, 2011).

Mass balance critical loads for nutrient nitrogen

This method is based on an equation, which balances all significant long-term inputs and outputs of nitrogen for terrestrial ecosystems. In this context, long-term is defined as at least one forest rotation or 100 years (UBA, 2004). Critical loads calculated using this method are set to: (i) prevent an increase in leaching of nitrogen compounds, particularly nitrate, which may result in damage to the terrestrial, or linked aquatic systems; (ii) ensure sustainable production by limiting nitrogen uptake and removal to a level which will not result in deficiencies of other nutrient elements (Hornung et al., 1997).  In principle, this approach could be used for any terrestrial ecosystem, but to date its use has been largely restricted to forest ecosystems. In the UK, the mass balance equation is currently used to calculate nutrient nitrogen critical loads for managed (productive) coniferous and broadleaved woodland habitats only (Hall et al., 2015)


The following organisations are acknowledged for the provision of data sets that have been used in the derivation of UK critical loads:

Soil Survey and Land Research Centre (now National Soil Resources Institute)

Macaulay Land Use Research Institute (now James Hutton Institute)

Department of Agriculture and Rural Development

Natural England

Natural Resources Wales

Scottish Natural Heritage

Northern Ireland Environment Agency

Joint Nature Conservation Committee

Forest Research