Application of MAGIC to Lake Redó (Central Pyrenees): an assessment of the effects of possible climate driven changes in atmospheric precipitation, base cation deposition, and weathering rates on lake water chemistry

The process-oriented catchment-scale model MAGIC was used to simulate water chemistry at Lake Redó, a high mountain lake in the Central Pyrenees, Spain. Data on lakewater and atmospheric deposition chemistry for the period 1984-1998 were used to calibrate the model, which was then used to reconstruct past and to provide forecasts for three hypothetical future scenarios of deposition. Forecast scenarios considered several combinations of changes in S and N deposition due to abatement strategies, and in base cation deposition due to climate-induced changes in air-mass trajectories from northern Africa. Scenario 1 assumed constant deposition of base cations at the present level plus the expected decrease in S and N deposition resulting from reduced emissions; scenario 2 (best case) assumed an increase in base cation deposition plus the same decrease in S and N deposition as in scenario 1; scenario 3 (worst case) assumed a decrease in base cation deposition plus no decrease in S and N deposition. The hindcast indicated that during the past 140-year period changes in lake water chemistry have been significant for a remote mountain catchment, although no substantial acidification has occurred. In this regard Lake Redó can be described as a "non-sensitive lake" maintaining a reference condition. The forecasts indicated changes that do not affect this status, but the trends, even if slight, were different between scenarios. A slight decline in the surface water ANC is predicted by Scenario 3. The N budget indicates an unusually low retention in the catchment, which may result in enhanced sensitivity to further increased N deposition. Some of the discrepancy between modelled and measured Ca2+ in lake water during 1984-98 could be explained by changes in rainfall amounts and by increased weathering rates due to increases in air temperature.