Acidification models

Other modeling efforts include soil acidification models of the macroscopic type that account for the process of S04 sorption in different ways. These approaches, which assume equilibrium conditions to prevail, include the adsorption isotherm, solubility product, and anion exchange. Prenzel (1994) discussed the various limitations of the above approaches in their capability to account for changes in pH. Recently, Fumoto and Sverdrup (2000) used a constant capacitance approach to describe the pH dependency of S04 sorption isotherms in an andisol. Other modeling efforts of S04 isotherms were reported by Gustafsson (1995) in a spodosol. Such isotherm models are of the equilibrium type and include linear and Temkin types of models. 

Cosby B. J., Wright R. F., and Gjessing E. (1995) An acidification model (MAGIC) with organic acids evaluated using whole-catchment manipulations in Norway. J. Hydrol. 170, 101-122. 

Rose K. A., Cook R. B., Brenkert A. L., Gardner R. H., and Hettelingh J. P. (1991a) Systematic comparison of ILWAS, MAGIC, and ETD watershed acidification models 1. Mapping among model inputs and deterministic results. Water Resour. Res. Tl, 2577—2589. 

Schecher W. D. and Driscoll C. T. (1988) An evaluation of the equilibrium calculations within acidification models the effect of uncertainty in measured chemical components. Water Resour. Res. 24, 533-540. 

The assumption employed in models is that equilibrium chemistry is applicable in all relevant situations. This implies that the reaction of soil pH and other parameters to a change in input is virtually instantaneous and that processes such as diffusion can be neglected. Long-term, large-scale acidification models are difficult to calibrate and validate because of the paucity of sufficient long-term (>50 yr) observations. 

Kuylenstierna J. C. I. and Chadwick M. J. (1989) The relative sensitivity of ecosystems in Europe to the indirect effects of acidic depositions. In Regional Acidification Models (eds. J. Kamari, D. F. Brakke, A. Jenkins, and R. F. Wright). Springer, Berlin, pp. 3-22. 

Lange H., Hauhs M., and Schmidt S. (1995) Long-term sulfate dynamics at Lange Bramke (Harz) used for testing two acidification models. Water Air Soil Pollut. 79, 339-351. 

One clear prediction from the simple acidification model developed in the last section is the loss of alkalinity (as defined by equation 5.68) and base cations from soil solution by leaching. High the soil should naturally result in base cation loss... 

To date, emissions targets set in the United States and Europe have been met or exceeded. There are widespread decreases in surface water concentrations of sulfate and some waters are showing increases in ANC. Nevertheless, data suggest that these targets may not be sufficient to achieve the full recovery of sensitive ecosystems. In order to evaluate the extent to which historic and future emissions reductions will facilitate ecosystem recovery from acidic deposition, it is necessary to use acidification models to project the future relationship between emissions, deposition, and chemical recovery of acid-sensitive forest watersheds (see Box 3.5). 

Scientists have developed computer models that depict the physical, chemical and biological processes within forest watersheds. Watershed acidification models can be used as research and management tools to investigate factors responsible for the historical acidification of soil and water as well as the ecosystem response to anticipated future changes in acidic deposition. In order to effectively predict the pH, ANC and aluminum concentrations in streams, all major chemicals must be accurately simulated (e.g., sulfate, nitrate, calcium, magnesium). The acidification model PnET-BGC was used for this assessment because it has been rigorously tested at Hubbard Brook and other sites in the northeastern United States, and it allows the user of the model to consider the ecosystem response to multiple chemicals simultaneously. Other frequently used acidification models include MAGIC (Cosby et al. 2001), and NuCM (Lui et al. 1992). 

Figure 3.10. Time series of predictions with the acidification model PnET-BGC of changes in stream chemistry at Hubbard Brook to changes in past and potential future emissions of sulfur dioxide and nitrogen oxides, including the 1990 Amendments of the Clean Air Act and moderate and aggressive emission control scenarios. Shown are model-predicted stream concentrations of sulfate, nitrate, acid neutralizing capacity, pH and dissolved inorganic aluminum, and soil percent base saturation. Measured values are indicated for comparison...

Time series of predictions with the acidification model PnET-BGC of... 

Various process models have been used to estimate critical loads. These Include empirical models using process-oriented mechanisms in their structure, such as the MAGIC model (Cosby, Homberger and Galloway, 1984), the RAINS Lake Model (RLM) of Kamari and Posch (1987) and the Model to Assess a Critical Acid Load (MACAL) of de Vries (1988). More complex models such as the SteaSoil Chemistry Model (PROFILE) developed by Sverdrup and Warfvinge (1989) and RESAM, the Regional Soil Acidification Model (de Vries and Kiros, 1989) have been developed. 

Kamdri, J. and Posch, M. (1987). Regiotml application of a simple lake acidification model in northern Europe. In Systems Analysis in Water Quality Management (ed. by M.B. Beck), pp. 73-84. Pergamon, Oxford. 

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