Modelling Catchment-Scale Responses to Climate Change - Climate Change Impacts on Freshwater Ecosystems - page 249

model HBV (Hydrologiska Byråns Vattenbalansavdelning), the INCA-N model
was used to understand which factors and processes control the flow and N
dynamics in different climate zones and to assess the relative inputs from diffuse
and point sources across the catchment. The simulations suggested that, in the
lowlands, seasonal patterns in the stream water nitrate concentrations were
dominated by diffuse agricultural and point-source effluents with an estimated
75% of the river load in the lowlands derived from arable farming. The model
proved able to simulate observed seasonal nitrate patterns at large spatial
(>300 km 2 ) and temporal (

≥

monthly) scales using available national data sets.
The model was equally good at simulating observations in the upper, mid and
lower reaches of the Garonne. This application of the linked HBV and INCA-N
models to a major European river system showed that it was possible to simulate
observed behaviour in a catchment commensurate with the largest basins to be
managed under the Water Framework Directive.
The success of the model in simulating observed nitrogen behaviour in
catchments allows its use to project changes in nitrogen fluxes in future as a result
of climate change. As described above, summer flow rates in the river Kennet in
SE England are likely to fall in the future as drought periods become more extreme.
Extending the modelling to simulate nitrate-N, Whitehead et al . (2006) used the
INCA-N model to show that the droughts might trigger a release of nitrate from
the soils and this nitrate would be exported to the river, as illustrated in Fig. 10.3.
With climate change predictions downscaled from the HadCM3 model and the
A2 emissions scenario, nitrate-N concentrations increased to values close to the
EU drinking water limit of 11.3 mg l −1 . Falling flow rates and rising nitrate levels
could affect water supply and put in doubt plans to improve the water quality and
ecology of such a sensitive chalk stream as the Kennet. A series of adaptation
strategies was investigated using the model to assess the effectiveness of potential
mitigation strategies. For example, reducing agricultural fertilizer use by 50% in
the catchment gave the biggest improvement, lowering nitrate concentrations to
levels not seen since the 1950s. Reducing atmospheric sources of oxidized and
reduced N by 50% reduced the nitrate by about 1 mg l −1 compared with the
baseline scenario. Constructing water meadows along the river (which is
parameterized in the model as an increase in the in-stream denitrification
coefficient) would be more beneficial, significantly slowing down the rising levels
of nitrate. A mixed strategy of a combination of all three approaches - reducing
fertilizer, reducing N deposition by 25% and constructing half the number of
wetland areas alongside the river system - also generated significant reductions in
nitrate. The realism of these simulations depends on how well the model represents
reality, and different approaches to the same system have yielded different results.
For example, other GCM simulations predicted an increase in flow rather than
the decrease shown in Fig. 10.3. Another study using the same climate and
emission scenario but different INCA parameterization predicted a decrease in
stream nitrate by 2050 rather than an increase (Skeffington 2008). Nevertheless,
the results give some impression of the likely effects of management options, and
also show the long response times before any improvement occurs. More explicit
modelling of groundwater transport in chalk catchments predicts even longer
response times (Jackson et al . 2007).