Biogeochemical Models
305
the continuous production of cations by weathering and from the inputs in
precipitation, and the lack of a sink in the harvested biomass. This eventually
resulted in an increase in stream alkalinity (Fig. 21.3c).
In short, atmospheric deposition led to a moderate decrease in base
cations and alkalinity and had a stronger acidification effect than that caused
by a periodical forest management consisting of a mild harvest (removal of
33% of the stand biomass). The combined impact of forest management and
atmospheric deposition on streamwater quality was very small, unlike results
observed and modelled for more acidic systems in northern Europe, where
forest growth alone or in combination with atmospheric deposition had a
significant acidifying effect on soils and streamwaters (Jenkins et al. 1990;
Jenkins and Wright 1992).
21.3.2 Climate Change Scenarios
MAGIC was also used in ,lVlontseny to simulate the effects of climatic change
on the streamwater chemistry. To model climatic change, a temperature increase of 4 DC was considered, but, because of greater uncertainty regarding
future precipitation (Giorgi et al. 1992), two possible scenarios were devised:
a 10% precipitation increase and a 10% precipitation decrease, both with a
4 DC temperature increase. With these changed conditions of temperature
and precipitation, a different ratio between precipitation and runoff IS to be
expected, affecting the streamwater chemistry through simple dilution or
concentration mechanisms - the hydrological effect. However, in addition,
these climatic changes will lead to the modification of certain soil physical
variables and may well affect the weathering rates, which are highly dependent on soil temperature and soil water content (Sverdrup and Warfvinge
1993).
Here, the changes in streamwater chemistry were analyzed for hydrological effects alone and in combination with changes in the weathering rates.
The simulations were run with the MAGIC calibration in Montseny of steady
state for forest biomass and atmospheric deposition following historical sulphur emissions in Europe until 1985 and constant thereafter (Fig. 21.3a, scenario c).
The model's predictions for the streamwater Ca 2 + concentrations and alkalinityare shown in Figs. 21.4 and 21.5. The three scenarios are: b, the baseline scenario, with present-day temperature and precipitation; h, the scenario
accounting solely for the effects of the hydrological partition; and w, the scenario accounting for both the hydrological effects and the changes in weathering rates. The changes in the weathering rates as a consequence of changes
in the climatic conditions were obtained by applying PROFILE, a model
which calculates weathering rates at field conditions based on the rock and
soil mineralogy (Warfvinge and Sverdrup 1992; Sverdrup and Warfvinge
1993).
305
the continuous production of cations by weathering and from the inputs in
precipitation, and the lack of a sink in the harvested biomass. This eventually
resulted in an increase in stream alkalinity (Fig. 21.3c).
In short, atmospheric deposition led to a moderate decrease in base
cations and alkalinity and had a stronger acidification effect than that caused
by a periodical forest management consisting of a mild harvest (removal of
33% of the stand biomass). The combined impact of forest management and
atmospheric deposition on streamwater quality was very small, unlike results
observed and modelled for more acidic systems in northern Europe, where
forest growth alone or in combination with atmospheric deposition had a
significant acidifying effect on soils and streamwaters (Jenkins et al. 1990;
Jenkins and Wright 1992).
21.3.2 Climate Change Scenarios
MAGIC was also used in ,lVlontseny to simulate the effects of climatic change
on the streamwater chemistry. To model climatic change, a temperature increase of 4 DC was considered, but, because of greater uncertainty regarding
future precipitation (Giorgi et al. 1992), two possible scenarios were devised:
a 10% precipitation increase and a 10% precipitation decrease, both with a
4 DC temperature increase. With these changed conditions of temperature
and precipitation, a different ratio between precipitation and runoff IS to be
expected, affecting the streamwater chemistry through simple dilution or
concentration mechanisms - the hydrological effect. However, in addition,
these climatic changes will lead to the modification of certain soil physical
variables and may well affect the weathering rates, which are highly dependent on soil temperature and soil water content (Sverdrup and Warfvinge
1993).
Here, the changes in streamwater chemistry were analyzed for hydrological effects alone and in combination with changes in the weathering rates.
The simulations were run with the MAGIC calibration in Montseny of steady
state for forest biomass and atmospheric deposition following historical sulphur emissions in Europe until 1985 and constant thereafter (Fig. 21.3a, scenario c).
The model's predictions for the streamwater Ca 2 + concentrations and alkalinityare shown in Figs. 21.4 and 21.5. The three scenarios are: b, the baseline scenario, with present-day temperature and precipitation; h, the scenario
accounting solely for the effects of the hydrological partition; and w, the scenario accounting for both the hydrological effects and the changes in weathering rates. The changes in the weathering rates as a consequence of changes
in the climatic conditions were obtained by applying PROFILE, a model
which calculates weathering rates at field conditions based on the rock and
soil mineralogy (Warfvinge and Sverdrup 1992; Sverdrup and Warfvinge
1993).