The decreasing importance of acidification episodes with recovery from acidification: an analysis of the 30-year record from Birkenes, Norway

The 30-year record 1975–2004 of weekly samples of streamwater chemistry from Birkenes, Norway, shows 106 acid episodes below the threshold of ANC−50 µeq l−1. The frequency, severity and duration of episodes have diminished since about 1990 due to chemical recovery following reduced deposition of sulphur. In particular SO4-driven episodes in the first runoff following drought have become less intense and less frequent, whereas episodes driven by climate (wind, high flow) continue. The data show significant empirical relationships between strength of the driver, degree of chemical recovery, and severity of ANC depression.