WP 1: Critical loads for acidity


The methodology for calculating freshwater critical loads in the UK has evolved over the past decade in response to the need to incorporate N deposition and greater use for scenario testing. The first maps were produced using the SSWC model for sulphur, which was then adapted to include N effects in the early-1990s. The major shortcoming of the SSWC model was its inability to predict future responses to N deposition scenarios except under the assumption that N retention processes were already at steady-state and leaching would only change in proportion to deposition changes. The weakness of this assumption in the face of effects work in terrestrial ecosystems and the general acceptance of the N saturation hypothesis led to the development of a more sophisticated mass-balance model for both S and N, called the FAB model. This model has been the standard for freshwater critical loads work over the last decade.

Much work has already been done in the UK to test the applicability of the FAB model and regular adaptations have resulted. In particular, a recommended method for the calculation of steady-state denitrification fluxes was rejected under a previous DEFRA contract as inappropriate for UK conditions and new empirical values are now used instead.

The latest changes in FAB model applications in the UK entailed the adoption of a new critical chemical threshold, ANCcrit (ANCcrit = 20 µeq L-1) in March 2004 and a reformulation of the model to take account of direct deposition to the surface of lakes, which was previously neglected in the mass-balances used. Hence it is an ongoing requirement to monitor new modelling developments and the publication of new data that may underpin changes in either model structure or the critical chemical threshold employed.

Work programme

The work programme for Work package 1 consists of a single task. Click on the link below for further information on an individual task

  • Task 1.1 - Watching brief on critical loads methodology and new datasets