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Climate change as a confounding factor in reversibility of acidification: RAIN and CLIMEX projects

R.F. Wright1 and A. Jenkins2
1Norwegian Institute for Water Research, P.O. Box 173, Kjelsås, N-0411 Oslo, Norway
2Centre for Ecology and Hydrology, Wallingford, Oxon OX10 8BB,UK

Full Reference

Wright, R.F. and Jenkins, A. (2001). Climate change as a confounding factor in reversibility of acidification: RAIN and CLIMEX projects. Hydrology and Earth System Sciences Vol. 5, No. 3, 379-390.


This paper presents the precipitation and runoff data for the entire 17 year record from the roof experiment at Risdalsheia, and evaluated the confounding effect of climate variationsin reversibility of water acidification. Previous results from these experiments have been presented after one year of treatment (Wright et al., 1986), two years of treatment (Wright et al 1987), four years of treatment (Wright et al 1988), eight years of treatment (Wright et al 1993), and after three years of CLIMEX treatment (Wright et al., 1998)

Table 1 Treatment at RisdalsheiaThe RAIN and CLIMEX experiments at Risdalsheia, southernmost Norway, together cover 17 years (1984-2000) of whole-catchment manipulation of acid deposition and climate. A 1200 m2roof placed over the forest canopy at KIM catchment excluded about 80% of ambient acid deposition; clean rain was sprinkled under the roof (Table 1). A climate change treatment (3.7oC increase in air temperature and increase in air carbon dioxide concentrations to 560 ppmv) was superimposed on the clean rain treatment for four years (1995-1998). Sea-salt inputs and temperature are climate-related factors that influence water chemistry and can confound long-term trends caused by changes in deposition of sulphur and nitrogen. The RAIN and CLIMEX experiments at Risdalsheia provided direct experimental data that allow quantitative assessment of these factors.

Summary of results

Runoff chemistry responded rapidly to the decreased acid deposition. Sulphate concentrations decreased by 50% within three years (Figure 1); nitrate and ammonium concentrations decreased to new steady-state levels within the first year (Figure 3). Acid neutralising capacity increased and hydrogen ion aluminium decreased (Figure 4). Similar recovery from acidification was also observed at the reference catchment ROLF, in response to the general 50% reduction in sulphate deposition over southern Norway in the late 1980's and 1990s. Variations in sea-salt deposition caused large variations in run-off chemistry at the reference catchment ROLF and the year-to-year noise in acid neutralising capacity was as large as the overall trend over the period. These variations were absent at KIM catchment because the sea-salt inputs were held constant over the entire 17 years of the clean rain treatment. The climate change experiment at KIM catchment resulted in increased leaching of inorganic nitrogen, probably due to increased mineralisation and nitrification rates in the soils.

Fig 1 Flux of SO4 in deposition runoff netflux cumulative net fluxInput-output budgets for SO4 show that KIM catchment released an average of c. 140 meq m-2 yr-1 of stored SO4 from the soil pool during the period 1985-1991 as the catchment adjusted to the new low level of SO4 inputs, this continued until the treatment stopped in 2000 (Figure 1).

Net release of SO4 also occurred at ROLF catchment in response to the decrease in ambient SO4 deposition during the 1990s. Cumulative loss was 60 meq m-2 yr-1 by 1994, about one-half the amount lost at KIM catchment.










Fig 2 Concentration of NH4 SBC TOC in runoff




Ammonia concentrations in run-off at KIM catchment also decreased in response to the clean rain treatment and by 1990, concentrationswere less than 5 µeql-1. At ROLF catchment, NH4 concentrations showed relatively large year-to-year variations but no significant trend in NH4 deposition during this period

 Fig 3 Flux of Inorg N in deposition and runoff

Under ambient conditions, before the clean rain treatment, KIM catchment retained about 50% of the incoming total N flux (Figure 3). Following the decrease in N deposition in 1984, the catchment continued to leak a similar proportion. At ROLF catchment, the net flux throughout the experimental period was very variable with a mean of c.60% retention. In 1995, with the start of the climate treatment at KIM catchment, both NO3 and NH4 concentrations in runoff increased relative to previous years, despite the fact that N deposition decreased

Figure 4 Concentrations of ANC H inorg Al SBC TOC in runoff

The decreased concentrations of the strong acid anions SO4 and NO3 in runoff at KIM catchment through to 1994 was compensated only slightly by a decrease in the sum of base cations (SBC) (Figure 2) and mainly by an increase in ANC (Figure 4). Acid neutralising capacity showed a rapid increase over the first five years, from c. -100 µeql-1 in 1984 to c. -20 µeql-1 in 1989, followed by a slower increase to c. -5 µeql-1 in 1994. At ROLF catchment ANC also increased from c. -90 µeql-1 in the late 1980s to c. -40 µeql-1 in 2000.

At KIM catchment, concentrations of of Al decreased from c. 10 to 2 µeql-1, while at ROLF catchment the decrease was 5 µeql-1(Figure 4).


Wright, R.F., 1987. Rain Project: Results after 2 years of treatment. In: Reversibility of Acidification (H. Barth (Ed.)), 14-49. Elsevier Applied Science, London.

Wright, R.F., 1998. Effect of increased CO2 and temperature on run-off chemistry at a forested catchment in southern Norway (CLIMEX project). Ecosystems, 1, 216-225.

Wright, R.F., Gjessing, E., Christophersen, N., Lotse, E., Seip, H.M., Semb, A., Sletaune, B., Storhaug, R. and Wedum, K., 1986. Project RAIN: changing acid deposition to whole catchments. The first year of treatment. Water Air Soil Pollut., 30, 47-63.

Wright, R.F., Lotse, E. and Semb, A., 1988. Reversibility of acidification shown by whole-catchment experiments. Nature, 334, 670-675. Wright, R.F., Lotse, E. and Semb, A., 1993. RAIN project: results after 8 years of experimentally reduced acid deposition. Can. J. Fisheries Aquat. Sci., 50, 258-268.