A very simple dynamic soil acidification model for scenario analyses and target load calculations

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Abstract

A very simple dynamic soil acidification model, VSD, is described, which has been developed as the simplest extension of steady-state models for critical load calculations and with an eye on regional applications. The model requires only a minimum set of inputs (compared to more detailed models) and execution time is minimised by reducing the set of model equations to a single non-linear equation. To facilitate the exploration of model behaviour at individual sites, the model is linked to a graphical user interface (GUI). This GUI allows easy (Bayesian) calibration, forward simulation (scenario analyses) and can also be used to compute target loads and delay times between deposition reductions and ecosystem recovery. VSD compares well to other widely used more complex models and is currently used in several European countries in the support of effects-based emission reduction policies.

Introduction

Numerous models for simulating the acidification of soils and surface waters have been developed during the past decades (see, e.g., Tiktak and Van Grinsven, 1995). These models cover a wide spectrum of applications and objectives. Many of them are ‘research models’ that have been developed for a certain project and/or sites for which detailed measurements are available. Examples of such models are ForSVA (Arp and Oja, 1997) and NUCSAM (Groenenberg et al., 1995). In general, they are not intended or suitable for regional applications, as they require detailed input data not readily available on a regional scale. At the other end of the spectrum are models designed to be easily applicable at many sites: their developers try to minimise input requirements and pay attention to ease (and speed) of use. This is especially important if the model is to be used on a regional scale, where input data is sparse and fast model execution is important. Examples of such models currently in use are the soil models SMART (De Vries et al., 1989, Posch et al., 1993) and SAFE (Warfvinge et al., 1993) and the catchment model MAGIC (Cosby et al., 1985, Cosby et al., 2001).

Here we describe the ‘very simple dynamic’ (VSD) model, which is designed for sites with few data available and applications on a large regional or continental scale. The VSD model has been developed as a minimal extension of steady-state mass balance models for calculating critical loads, which have been widely used during the past 15 years in European sulphur (S) and nitrogen (N) emission reduction negotiations under the UNECE Convention on Long-range Transboundary Air Pollution (LRTAP) (Hettelingh et al., 1995, Hettelingh et al., 2001). Critical loads are steady-state quantities, which define upper limits for S and N deposition that do not cause ‘harmful effects’ on specified ecosystems. They are mostly calculated with a ‘simple mass balance’ (SMB) model, which assumes that soil processes are in equilibrium with depositions (UBA, 2004). Critical loads are based on a soil chemical criterion, such as an Al/Bc ratio in the soil solution. A deposition equal to a critical load will, in the long run, lead to the soil chemical state not ‘harmful’ to the ecosystem. Critical loads, however, do not give any information on the time when a certain soil chemical state is obtained for a given future deposition pathway. To this end dynamic models are required; and the VSD model is the simplest extension of the steady-state SMB model into a dynamic model by including cation exchange and time-dependent N immobilisation (accumulation). The simplicity of the model results in a short execution time that allows rapid scenario analyses and the calculation of target loads, i.e. deposition targets which result in a desired chemical condition in the soil (solution) in a specified year. Assessment of target loads requires the solution of an inverse problem, i.e. to find depositions which lead to a given chemical state at a given time in the future. They are determined by running the VSD model iteratively, and thus may require many model runs. Furthermore, delay times can be determined with VSD, i.e. the year when a certain soil chemical condition is met for a given deposition scenario. Despite its simplicity, the VSD model incorporates the main processes also present in more complicated models, and model comparisons have shown that results obtained are very similar for most soils (Kurz and Posch, 2002). This gives confidence that the VSD model properly describes the average chemical development of the major ions in the soil and soil solution over long time periods.

The paper presents a description of the VSD model (listing all processes and equations) followed by a description of the model's functionality, i.e. how the software developed around the core VSD model – called VSDStudio – can be used to calibrate the model and to use it for different tasks, such as scenario analyses, critical load and target load calculations as well as delay time estimates. Finally, we discuss model evaluation and list some limitations of the model.

Section snippets

Model description

The VSD model is designed to simulate the acidification (and recovery) of non-calcareous (unmanaged) soils. It consists of a set of mass balance equations, describing the soil input–output relationships, and equations describing rate-limited and equilibrium soil processes. Simulated soil solution chemistry depends solely on the net element input from the atmosphere (deposition), net uptake, net immobilisation and denitrification and the geochemical interaction in the soil (CO2 equilibria,

Functionality

Around the VSD core-model described in the previous chapter, a software package and graphical user interface (GUI) has been developed, called VSDStudio, which not only allows – for a single site at a time – simple forward simulations for given deposition scenarios, but also automatic (Bayesian) calibration, computation of steady-states (critical loads), target loads and damage/recovery delay times. In the following the functionalities of VSDStudio are described in detail.

Model evaluation and outlook

VSD has been tested on 182 intensively monitored forest sites in Europe. These sites are part of the European Commission/UNECE Intensive Monitoring Programme, also known as the Level II programme of the ICP Forests (De Vries et al., 2003b). Using measured soil solution data, aggregated to annual averages, VSD was calibrated using a Bayesian calibration technique employing a Markov Chain Monte Carlo approach to sample the parameter space (see above and Reinds et al., 2008). Results show that

Acknowledgements

Thanks to the users of earlier versions of VSDStudio who provided valuable suggestions for the improvement and extension of the interface. The preparation of this paper has been co-financed by the European Commission LIFE III programme within the framework of the European Consortium for Modelling Air Pollution and Climate Strategies (EC4MACS) and the trust fund for the partial funding of effect-oriented activities under the Convention on Long-range Transboundary Air Pollution.

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