Uncertainties in Critical Loads and their Exceedances

The national critical loads and exceedance data sets and maps were developed to provide a national-scale picture of the areas at risk from the adverse effects of acidification and eutrophication. The data and maps are largely derived from national-scale input data sets, appropriate for national-scale assessments; they were not intended for local-scale or site-specific assessments. The national critical loads data are based on data sets of different resolutions and from numerous sources, and include default parameter values by habitat or soil type where more detailed information is lacking.

Two main studies have been undertaken to assess the uncertainties in the calculation of critical loads and their exceedances. A study funded by Defra carried out a literature review and uncertainty analyses on the national data. This was followed up by a study funded by the Environment Agency that examined uncertainties at the site-specific, local, regional and national scales and suggested frameworks to allow the Agency to incorporate critical load uncertainty in its regulatory role.

Sensitivity analysis, which aimed to identify the key parameters contributing to uncertainty, showed variations from one site to another, so that almost any input parameter could potentially be important. Comparison of critical loads calculated using national and site-specific parameters showed a number of differences, highlighting the problems of using national scale data for site-specific assessments. Uncertainty analysis for both aquatic and terrestrial ecosystems demonstrated that uncertainties in the critical load values were generally considerably less than the uncertainties in the individual input parameters, possibly due to a “compensation of errors” mechanism.

The main sources of uncertainties in the national data were identified as:

  • Uncertainties inherent in the underlying data sets:
  • Land cover (based on classified satellite imagery) and ancillary data used to map broad habitats.
  • Soil data and allocation of base cation weathering rates to soil types.
  • Default values (eg, base cation uptake) for a limited number of soil or vegetation types, often based on a limited number of sites across the UK.
  • Deposition data (base cations, sulphur, nitrogen).
  • Uncertainties due to the resolution of the data used:
    • Habitat maps based on 1km summary land cover data (derived from 25x25 m data) combined with ancillary data of different resolutions, for example, species distribution data, at 10x10 km resolution.
    • Acidity critical loads (and nutrient nitrogen critical loads for managed woodlands) based on 1x1 km resolution soils data.
    • Deposition data at 5x5 km resolution; actual deposition may vary within the grid resolution of the data.
  • Uncertainties in the methods used:
    • Mapping of broad habitats and estimates of UK habitat areas.
    • Basing acidity (and for managed woodlands, nutrient nitrogen) critical loads on the characteristics of the dominant soil type in each 1x1 km grid square.
    • Suitability of criteria (eg, Acid Neutralising Capacity (ANC), critical soil solution pH, nitrate leaching) used in models.
    • Empirical nutrient nitrogen critical loads based on ranges set for European habitat classes (European Nature Information System: EUNIS), and the selection of appropriate “mapping” values within each range for the UK.

These topics are discussed in more detail in the literature.


Skeffington, R.; Hall, J.; Heywood, L.; Wadsworth, R.; Whitehead, P.; Reynolds, B.; Abbott, J.; Vincent, K. 2007 Uncertainty in critical load assessment models
Heywood, E.; Whyatt, J.D.; Hall, J.; Wadsworth, R.; Page, T. 2006 Presentation of the influence of deposition uncertainties on acidity critical load exceedance across Wales
Heywood, L.; Hall, J.; Smith, R. 2006 Uncertainty in mass balance critical loads and exceedance: Application to a UK national data set
Skeffington, R. 2006 Quantifying uncertainty in critical loads: (A) literature review
Skeffington, R.; Whitehead, P.; Abbott, J. 2006 Quantifying uncertainty in critical loads: (B) acidity mass balance critical loads on a sensitive site
Heywood, E.; Hall, J.; Reynolds, B. 2006 A review of uncertainties in the inputs to critical loads of acidity and nutrient nitrogen for woodland habitats.
Hall, J.; Ullyett, J.; Heywood, E.; Broughton, R.; Fawehinmi, J. 2004 The National Critical Loads Mapping Programme Phase IV. Final Report (July 2001 – June 2004)
Aherne, J.; Farrell, E.P.; Hall, J.; Reynolds, B.; Hornung, M. 2001 Selecting chemical criteria for critical loads of acidity in maritime regions.
Hall, J.; Hornung, M.; Kennedy, F.; Langan, S.; Reynolds, B.; Aherne, J. 2001 Investigating the uncertainties in the Simple Mass Balance equation for acidity critical loads for terrestrial ecosystems.
Hall, J.; Reynolds, B.; Aherne, J.; Hornung, M. 2001 The importance of selecting appropriate criteria for calculating acidity critical loads for terrestrial ecosystems using the Simple Mass Balance equation.
Langan, S.; Hodson, M.; Kennedy, F.; Hornung, M.; Reynolds, B.; Hall, J.; Donald, L. 2001 The role of weathering rate determinations in generating uncertainties in the calculation of critical loads of acidity and their exceedance.
Smith, R.I.; Hall, J.R.; Howard, D.C. 1995 Estimating uncertainty in the current critical loads exceedance models