Interaction of Climate Change and Acid Deposition - Climate Change Impacts on Freshwater Ecosystems - page 158

40
35
30
25
20
15
10
5
0

NO 3 concentration

2 =0.68

r

MAGIC
Obs

1-93 1-94 1-95 1-96 1-97 1-98 1-99 1-00 1-01 1-02 1-03 1-04

7

6 NO 3 flux

2 =0.88

r

5

4

MAGIC
Obs

3

2

1

0

1-93 1-94 1-95 1-96 1-97 1-98 1-99

Date (month-year)

1-00 1-01 1-02 1-03 1-04

Figure 7.4 Simulated (solid line) versus observed NO 3 concentrations (upper panel) and
fluxes (bottom panel) for the best set-up during calibration of MAGIC for Øygard.
Correlation coefficients ( r 2 ) between observed and modelled values are shown. (From
Sjøeng et al . 2009a.)

N and thus lower flux of NO 3 in winter and spring runoff. The results must be
treated with caution, however, because the projections are based on
extrapolations of regressions outside the range of observations because future
mean annual temperature will be much higher than the range of the year-to-
year variations in the 20-year record, 1986-2005. Also, the long-term fate of
stored N in the soil is unknown.
Process-orientated models offer another tool by which the effect of climate
change can be assessed. Sjøeng et al . (2009a, b) used the MAGIC model (Model
for Acidification of Groundwater in Catchments; Cosby et al . 1985a, b, 2001)
to project future NO 3 concentrations in streamwater at Øygard, a small catchment
in south-western Norway. MAGIC was first calibrated for monthly time-steps
to the 12-year data record and then driven by four climate scenarios
from PRUDENCE (A2, B2 with the Hadley and MPI models) under two different
'storylines' of assumptions involving future rates of plant processes. The
calibration of the model best fits the monthly observations when the variations in
precipitation and snowpack accumulation were taken into account (Fig. 7.4).
The calibrated model projected increases in future NO 3 concentrations under
the four climate scenarios, but the magnitude of the increase was dependent on
the assumptions made for future plant response to climate change (Fig. 7.5).
Under storyline 1, plant growth is assumed not to change, whereas under
storyline 2, the plant growth is assumed to increase (due to increased temperature)
and result in more C and N in litter input to the soil. Whereas under storyline
1 the C pool of the soil decreases over time as increased soil temperature results
in increased decomposition of soil organic matter, under storyline 2 the
increased litter input of C holds the C pool of the soil nearly unchanged (Sjøeng
et al . 2009b).