The effects of forestry on low flows in Scotland and Northern Ireland

Considerable uncertainty in the UK water industry surrounds the effects of forestry on low flows. In Scotland and Northern Ireland no previous study has specifically addressed the problem but the need for an improved understanding has never been greater. This paper reports on the main results from a study aimed at identifying changes in low river flows due to forestry. Data from 43 gauging stations in catchments with more than 10% forest showed 11 records with a significant downward trend in specific Q , an upward trend in two and no trend in eight. The remaining 23 stations had too short a record to determine whether any trend existed. When the specific Q was analysed against the percentage afforestation in the catchment the low flows were found to decrease with increasing forest cover. A review of the existing literature on the subject of forestry and low river flows showed that similar results were reported from different stages in the forest cycle: pre-planting drainage increased the low flows if more than 25% of the catchment is drained; forest growth produces a decline in low flows starting some six years after planting; and clear felling results in an initial increase in low flows (15-350%) depending on percentage area felled.