Contract EVK1-1999-00087 - RECOVER:2010

Part of the 'Sustainable Management and Quality of Water'

Ecosystem Functioning

Directorate General Research

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Long term changes in acidification and recovery at nine calibrated catchments in Norway, Sweden and Finland

F. Moldan1, R.F. Wright2, S. Löfgren3, M. Forsius 4, T. Ruoho-Airola5 and B.L. Skjelkvåle2
IVL Swedish Environmental Research Institute, Box 47086, S-40258 Gothenburg, Sweden
2Norwegian Institute for Water Research, Box 173 Kjelsas, N-0411 Oslo, Norway
3Institute for Environmental Assessment, Swedish University of Agricultural Sciences, Box 7050, S-75007 Uppsala, Sweden
4Finnish Environment Institute, Box 140, FIN-00251, Helsinki, Finland
5Finnish Meteorological Institute, Box 503, FIN -00101 Helsinki, Finland

Full Reference

Moldan, F., Wright, R.F., Löfgren, S., Forsius, M., Ruoho-Airola, T. and Skjelkvåle, B.L. (2001). Long term changes in acidification and recovery at nine calibrated catchments in Norway, Sweden and Finland. Hydrology and Earth System Sciences Vol. 5, No. 3, 339-349.


International agreements to reduce the emissions of acidifying pollutants have resulted in major changes in deposition of sulphur and nitrogen in southern Scandinavia over the past 25 years. Long-term monitoring of deposition and run-off chemistry over the past 12-25 years at nine small calibrated catchments in Finland, Norway and Sweden provide the basis for analysis of trends with special attention to recovery in response to decreased sulphur and nitrogen deposition in the 1980s and 1990s.

Fig.1.Map of Scandinavia showing location of catchments

The data used in this work are from national monitoring programmes in Finland, Norway and Sweden (Johannessen, 1995; Forsius et al., 1995; Wilander, 1998). The sampling and analytical methods are similar in the three countries. Site descriptions for the Norwegian catchments are from Lydersen (1994). Key catchment parameters are Summarised in Table 1.   Table 1 Catchment characteristics

Table(2) Catchment characteristics continued

Table 2. Average slopes in runoff trends by decade.

Sulphate and base cations (mostly calcium) concentrations declined and acid neutralising capacity increased. Occasional years with unusually high inputs of sea-salt confound the general trends.Trends at these catchments show the same general picture as that from small lakes in Scandinavia and in acid-sensitive waters elsewhere in Europe (Table 2).

Fig. 2 Volume weighted concentrations SO4 N Ca in precipitation

During the 1980s and 1990s sulphate deposition in the region decreased by 30 to 60 %, whereas inorganic nitrogen deposition showed very little change until the mid-1990s. Deposition of non-marine base cations (especially calcium) declined in the 1990s most markedly in southern Finland (Figure 2). Run-off response to these changes in deposition has been rapid and clear at the nine catchments.

Fig 3 Volume weighted concentrations of SO4 N and Ca in runoff

The long records from the three Norwegian catchments show that SO4* concentrations were high in the late 1970s and early 1980s (Figure 3). Non marine sulphate concentrations began to decline in the 1980s and the decline accelerated in the 1990s. This pattern was seen at all nine catchments. Nitrate accounted for the largest portion of inorganic N in runoff. Most of the inorganic N in deposition was retained by the catchments. No obvious trends in surface water NO3 concentrations were evident. This general picture is confounded in part by year-to-year variations in sea salts.


Forsius, M., Kleemola, S., Starr, M. and Ruoho-Airola, T., 1995. Ion mass budgets for small forested catchments in Finland. Water Air and Soil Pollution, 79, 19-38.

Johannessen, T. 1995. Acidification in Norway. The Norwegian monitoring programme from long range transported air pollutants. Water Air and Soil Pollution. 85, 617-621.

Lydersen, E. 1994. Long-term monitored catchments in Norway-A hydrological and chemical evaluation. Acid Rain Research Report34-A/1994, Norwegian Institute for Water Research , Oslo, Norway, 306 pp.

Wilander, A. 1998. Referanssjöarnas vattenkemi under 12 år; tillstånd och trender. Rapport 4652, Naturvårdsverket Förlag, Stickholm, Sweden, 79 pp.