Modelling Catchment-Scale Responses to Climate Change - Climate Change Impacts on Freshwater Ecosystems - page 253

Observed and projected
climate data

T , P

T , P

HBV

Q

HER, SMD

R N

T , P

INCA-N MAGIC

NO 3

Q

T , P ,
Wind speed

FJORD

Figure 10.4 Data transfer scheme between models used to model the Bjerkreim River
in Norway. The flow of data is indicated by arrows. T , temperature; P , precipitation;
Q , water flow; SMD, soil moisture deficit; R N , nitrogen immobilization as a percentage of
input. Other abbreviations in Table 10.1. (From Kaste et al . 2006.)

nitrate concentrations and fluxes with the MPI scenario but a significant increase
in concentrations and a 40%-50% increase in fluxes with the Had scenario (Fig.
10.5). These results arose from balances between the effects of climate change on
nitrogen processes within the catchment. With the MPI scenario, the reduced N
deposition was largely compensated by a temperature-driven increase in N
mineralization (16%) such that the total available N in the system was nearly
constant. With the Had scenario, however, N mineralization increased by nearly
40% compared to the control period. This was partly compensated by reduced
N deposition and increased uptake by vegetation but counteracted by a reduction
in the basin's ability to retain N.
Among the N retention processes included in the INCA-N model, two opposing
factors operate at the same time. First, the long-term accumulation of N in the
system leads to a decreased C : N ratio in the organic soil layer, which increases
the risk of N leaching. Secondly, the increased temperature promotes vegetation
growth and hence uptake of N. The net effect of all these processes was that at
the Bjerkreim river outlet, the INCA-N model simulated a 4% decrease in mean
NO 3 − concentration but a 4% increase in the mean flux with the MPI scenario.
This is because the increased temperature accelerated aquatic N retention
processes and thus reduced NO 3 − concentrations, whilst increased precipitation
and thus stream water flow increased the total NO 3 − export from the basin. With
the Had scenario, the stream water NO 3 − concentrations and fluxes were
predicted to increase by approximately 50% and 40%, respectively. Here, the
predicted decrease in annual flow reduced the NO 3 − export potential relative to
the MPI scenario.
A possible consequence of increased nitrate concentrations is that the acidification
of the river could increase, thus offsetting ongoing recovery from acidification due