Water, Air, & Soil Pollution: Focus

, Volume 7, Issue 1, pp 413–419

The Applicability of National Critical Loads Data in Assessing Designated Sites


    • Centre for Ecology and Hydrology
  • Jackie Ullyett
    • Centre for Ecology and Hydrology
  • Richard Wadsworth
    • Centre for Ecology and Hydrology
  • Brian Reynolds
    • Centre for Ecology and Hydrology

DOI: 10.1007/s11267-006-9091-9

Cite this article as:
Hall, J., Ullyett, J., Wadsworth, R. et al. Water Air Soil Pollut: Focus (2007) 7: 413. doi:10.1007/s11267-006-9091-9


Critical loads have been successfully used within Europe in the development of effects-based policies for pollution abatement, including the Second Sulphur Protocol and the Protocol to abate acidification, eutrophication and ground-level ozone (CLRTAP, 1979). This success has encouraged the UK Environment Agency and Conservation Agencies to use the national critical load maps as a screening tool in assessing the threats from acidification and eutrophication to designated (Natura 2000) sites. The UK maps of critical loads are based on national-scale data sets appropriate for national-scale assessments, and were never intended for use at the site-specific level. Site-based assessments are often targeted at Special Areas of Conservation, a sub-set of the UK Natura 2000 sites. The spatial data available includes the boundaries of the sites but not the location of the designated features. Ancillary data is variable from one site to another; habitat types may be described in detail with cross-reference to classes of the National Vegetation Classification (NVC: Rodwell, 1991 et seq), but information available on soils and geology is generalised and has not been related to the habitats or species being protected. Hence it can be difficult to relate the individual sites to the national maps, even where appropriate to do so. This paper examines the underlying uncertainties in the national critical load maps showing how the maps could give misleading results if used for site-specific assessments. It also includes advice on how to determine when the national data may be appropriate as a policy-tool at the site-level.


critical loadsdesignated sitesendorsement theorynational vegetation classificationpolicyacidificationeutrophication

1 Introduction

Critical loads (CL) have proved to be a useful tool in the development and review of national (eg, UK Air Quality Strategy (DETR, 2000)) and international (eg, Protocols (CLRTAP, 1979)) policies to abate the pollutants responsible for acidification and eutrophication. The UK maps of acidity and nutrient nitrogen CL have been developed using national databases of soils and habitat information (Hall et al., 2003, 2004a). These maps provide national-scale pictures of the areas and habitats at risk from the potential harmful effects of excess acid or nitrogen deposition.

The EU Habitats Directive (EU, 1992) requires measures to be taken to maintain or restore to favourable conservation status, habitats and species of wild flora and fauna, listed in the Annexes to the Directive. In England and Wales the Environment Agency and Conservation Agencies are charged with the task of assessing the potential threats from acidification and eutrophication to the UK’s designated Natura 2000 sites. The data collated for these sites include the lists of designated habitats and species and a general description of the site characteristics. Generally there is insufficient data to calculate a site-specific CL, and unfortunately, information on soils, geology and other descriptive information is generalized and not related to the designated features. Hence the national maps have been used as a “screening tool” even though they were never intended for assessments at the site-specific scale and may give misleading results. As a consequence the UK Environment Agency has funded research to provide guidance on the use of the national maps or alternative approaches for carrying out site-level assessments.

2 Background to the National Critical Load Maps

The national acidity and nutrient nitrogen CL maps are based on internationally agreed methods (UBA, 2004) and on the best available data to provide national coverage. Critical loads are calculated and mapped for UK Biodiversity Action Plan Broad Habitats sensitive to acidification and eutrophication. The maps of acidity CL for terrestrial non-woodland habitats are based on the UK acidity CL for soils. Values have been derived separately for peat and non-peat soils; CL for non-peat soils are based on the mineralogy and weathering rate (Hornung et al., 1995). This method is inappropriate for peat soils, for which the CL are based on a critical soil solution pH (Calver, Cresser, & Smart, 2004; Hall et al., 2004a). The maps for woodland habitats are derived using a UK formulation of the simple mass balance model (Langan et al., 2004; Hall et al., 2004a) and incorporate the soil acidity CL values in setting the base cation weathering component. There are a number of limitations and uncertainties in the national CL data, for example:
  1. (1)

    The soils data are based on the dominant soil association within each 1 × 1 km grid square; other soil associations (or sub-dominant series within them) may be more or less sensitive to acidification or eutrophication and have lower or higher critical loads.

  2. (2)

    The 1 × 1 km soils databases are derived from 1:250,000 scale soil maps, therefore not all soil types within each 1 × 1 km grid square will necessarily be represented.

  3. (3)

    The habitat distributions are based on a combination of 1 × 1 km summary land cover information, species distribution data mapped at 10 × 10 km resolution and ancillary data sets at a range of scales (Hall et al., 2003). Hence the national habitat-specific CL maps will not necessarily include all small areas of sensitive habitats.

  4. (4)

    Features may be designated because they are rare or infrequent; they are therefore difficult to map.

  5. (5)

    Within the CL models some input parameters, such as the uptake and removal of base cations and nitrogen resulting from the harvesting of productive forestry are based on data for a limited number of sites.

  6. (6)

    The critical chemical criteria on which the national CL maps are based (eg, the critical molar ratio of calcium to aluminium in soil solution) may be inappropriate for the protection of the designated feature(s) on the site, but suitable criteria may not be available.


A formal assessment of the uncertainties in the national CL data has been carried out (Hall, Ullyett, Heywood, Broughton, & Fawehinmi, 2004b) and a comparison of the uncertainties at the national and local scales is currently being undertaken (Heywood, Skeffington, Whitehead, & Reynolds, (this issue); Skeffington et al., 2005). Despite their limitations the UK CL maps provide an adequate picture of the sensitivity of habitats to acidification and eutrophication both nationally and regionally. Critical load maps have been widely accepted by policy-makers, partly because they are simple to understand as well as providing an effects-based approach to pollution abatement. They are routinely used by the UK Department for Environment, Food and Rural Affairs to assess the potential impacts of future emission and deposition scenarios and they form the official UK data set used for activities under the CLRTAP. However, at the local or site-specific scale the maps may give misleading results if they are not used with care and the associated uncertainties taken into account. The following sections present and discuss a practical framework aimed at guiding the policy maker in the use of national CL maps or viable alternatives.

3 A Framework for Assessing Designated Sites

In an attempt to bridge the gap between the uncertainties in the use of the national-scale data and the lack of site-specific data, we propose a hierarchy (Fig. 1) to consider where, when and how the national data may be used, or an alternative site-specific approach. The approach is illustrated in terms of acidity below.
Fig. 1

Proposed framework for assessing designated sites

3.1 Stage 1: Assessing Whether the Designated Feature is Sensitive to Acidification

In the UK, Special Areas of Conservation (SACs) and Specially Protected Areas (SPAs) represent the “Natura 2000” sites, designated for protection under the EU Habitats Directive. Each site is designated to protect one or more features (habitats or species). The “Natura 2000 standard data form” lists these features in terms of the Habitats Directive Annex 1 Habitats or Annex II Species. At this stage the user needs to determine if the designated feature is sensitive to acidification; this may be done by consulting relevant experts or the literature (eg, UK Biodiversity Action Group, http://www.ukbap.org.uk or the UK Air Pollution Information System, http://www.apis.ac.uk). If the designated feature is not sensitive to acidification then there is no requirement to determine an appropriate CL or progress to the next stage.

3.2 Stage 2: Using the National Critical Loads Map

In this stage the “variance” in CL values within each 1 × 1 km grid square is examined (the national maps have been described in Section 2). This is demonstrated by the maps in Fig. 2 which are based on the soil acidity CL values only, rather than national habitat-specific CLs. In this way the CL is set to protect the soil upon which a habitat may occur or depend. The link to habitat in terms of vegetation community comes into Stage 3 of the framework. The maps in Fig. 2 compare the CL values for all soil associations in each 1 × 1 km grid square. The maps identify areas where (a) CL values are all the same; (b) CL for a sub-dominant soil is the lowest. It should be noted that this map does not include those 1 × 1 km2 dominated by peat soils. Therefore, if a designated site is in an area (or a 1 × 1 km2) where all the critical load values are the same for all soil types (Fig. 2a), there can be said to be low variance in the critical load.
Fig. 2

Variance in soil acidity critical loads a critical loads for dominant and sub-dominant soil the same, b Sub-dominant soil most sensitive (lowest critical load)

If the site is small (<1 km2), and there is no variance in the CL, then the national value can be used, although the uncertainties in the national map (Section 2) should still be taken into consideration. If the site is larger there are several additional issues to consider. Firstly, the grid reference provided for each site is for the centre point, and the site may consist of one or many adjacent or non-adjacent land parcels. Therefore this centre point may not be representative of the site as a whole, in terms of the soil CL values, or of the habitat or species for which the site is designated. For example, the CL may vary not only within each 1 × 1 km2 but also from square to square within a site. If the within-square variance in critical loads is low for each square the site covers, the values for all squares within the site could be extracted, and if done within a Geographical Information System, an area-weighted mean CL could be determined. But as large sites may contain a wide variety of soil and habitat types (as well as designated features) such an area-weighted mean value could also be inappropriate. If appropriate data are to be extracted from the national map then it is important that the locations of the designated features are known; currently this information is not readily available outside the Conservation Agencies and in some cases may not even exist.

If the variance in the critical load values for each 1 × 1 km2 is high then it is important that the relationships between the soils and the habitats are known or can be inferred. In such instances we recommend moving onto Stage 3.

3.3 Stage 3: Using the National Vegetation Classification

The National Vegetation Classification (NVC, Rodwell, 1991 et seq) is frequently used in the UK to describe natural and semi-natural vegetation communities. This stage of the hierarchy is aimed at defining the designated features in terms of NVC classes, required for Stage 4. The SAC site information held by the Joint Nature Conservation Committee (http://www.jncc.gov.uk) typically includes descriptions of the designated Annex I Habitats in terms of NVC classes. Therefore for many sites the designated features are already related to an NVC class or classes; where this is not the case the user can consult a relevant expert or the National Biodiversity Network Habitats Dictionary (http://www.nbn.org.uk/habitats) which provides information on the relationships between Annex 1 Habitats and NVC classes.

3.4 Stage 4: Applying Endorsement Theory

An Endorsement Theory approach has been developed by Wadsworth and Hall (this issue) to determine CL for individual NVC classes. Endorsement Theory (Cohen, 1985) is ideally suited to problems where data are incomplete and the evidence uncertain, as is the case with designated sites which tend to lack detailed information relating the location of habitats or species to their associated soil or geology types. The Endorsement Theory method uses all available published information on the soils and geology for each NVC class (contained in the five volumes of Rodwell, 1991 et seq) and relates this to the soils and geology information used in setting critical loads (Loveland, 1991). It uses a “symbolic” (rather than numeric) approach to derive an evidence based endorsement for each “CL class” (identical to the class ranges used by Hornung et al., 1995). An endorsement can be: “definitive,” “confident,” “likely,” “weak” or “very weak” or none. The amount of information available for each NVC community is very variable so for some communities no CL class receives more than a “very weak” endorsement, other communities are better described and a stronger endorsement may be given to one or more CL classes. The authors have designed a database that generates the CL endorsements for any terrestrial NVC class (that is, excluding the aquatic communities).

3.5 Stage 5: Combining Local (Endorsement Theory) and National Critical Load Estimates

There are uncertainties in the estimate of CL whether they are taken from the national map or from the “local” estimate based on Endorsement Theory. Two approaches for combining the estimates are possible; symbolic and numeric.

A symbolic approach can be considered as a form of quality assurance. The endorsement for each CL class (for the site relevant NVC communities) can be compared to the national estimate. The degree to which they agree (or conflict) can be used to decide whether further investigation or screening is required. For example if the NVC community had a “definitive” or “confident” endorsement for the same CL class as the national data, that might be considered good confirmation. If there was only a “weak” or “very weak” endorsement for the CL class, but no other class had a strong endorsement then the national estimate could be considered plausible, where there is a strong endorsement for a different CL class that would indicate further investigation was required.

In a numeric approach the different levels of endorsements need to be converted to “probabilities”. This is the same process as deciding on the number of endorsement categories and their labels, ie, it is an expert opinion. In this case our expert considers that a “definitive” endorsement is equivalent to a 0.9 probability. In an analogous manner the CL variance map can be used to provide an estimate of confidence in the national CL value. The numeric values may then be combined using a Dempster-Shafer formalism (Dempster, 1967; Shafer, 1976) as this enables an explicit representation of the uncertainty. Dempster–Shafer is mathematically equivalent to Bayesian statistics if the uncertainty is zero, and like Bayes it is essentially concerned with the revision of belief following additional information. A worked example is given in Wadsworth and Hall (this issue).

4 Conclusions

There are uncertainties in using national CL maps at the national, regional and local scales. Uncertainties in the national CL calculations have been quantified (Hall et al., 2004b). However, due to the nature and scale of the underlying data, and the incomplete data for designated sites, there are additional uncertainties in using the national maps for site-specific assessments. With appropriate guidance the national soil CL map can be used as a first-level screening tool for assessing designated sites as demonstrated by the framework presented above. The CL “variance” map provides additional information at the national scale to help determine if soil acidity CL values extracted from the national map are appropriate, especially for small sites. For the larger or more complex sites it is even more important to gather information on the spatial location of the designated features and their relationships to habitat and soil types. The Endorsement Theory approach helps provide a further quality assurance measure of the appropriateness of CL values extracted from the national map and applied to designated species or habitats.

The practical hierarchy presented deals only with acidity; though a similar approach could be adopted for nitrogen. In a regulatory framework it is important that the uncertainties in the use of the national maps are fully understood and taken in to account. In an ideal world site-specific data would be collected for all sites of interest, and habitat- or species-specific dose–response relationships for acidity and nitrogen derived for all designated features. However, given the number of different designated habitats and species this would be an enormous, time-consuming and very costly task; and one no policymaker is likely to fund unless they could be certain at the outset that the results would be so very different from what can be obtained by using the national data to screen sites. The proposed hierarchy provides a practical series of steps to guide the policymaker in screening and assessing designated sites, using national maps where possible and appropriate, and providing additional tools to determine the robustness of the CL values used. It will also highlight where the national data are inappropriate and therefore where research and funding need to be focused.


The authors gratefully acknowledge the UK Environment Agency for their contribution to the funding of this research. However, the views expressed are those of the authors.

Copyright information

© Springer Science+Business Media B.V. 2007