Report SR 94 (01) F
EFFECTS OF FORESTRY ON LOW FLOWS IN SCOTLAND AND NORTHERN IRELAND
Considerable uncertainty in the 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. The principal concern of this report is the identification of changes in low river flows due to forestry. The approach was to focus on the extensive database of river flows available from the National River Flow Archive. Data from 43 gauging stations were used in the record which included 23 recommended for investigation by the measuring authorities.
A review of the existing literature on the subject of forestry and low river flows found that similar results were reported from different stages in the forest cycle:
The seven River Purification Boards in Scotland and the Northern Ireland Department of the Environment were consulted to decide on the most useful low flow statistics to use in the analysis. They unanimously agreed that the Q95 flow was the most commonlv used statistic and for comparisons between catchments the summer specific Q95 (Q95 divided by catchment area) should be used. In the analysis of the 43 stations a significant downward trend in specific Q95 was detected in 11 records, an upward trend in 2 and no significant change in 8. The remaining 23 stations had too short a record to determine whether any trend existed. Little information was gained from the analysis of base flow indices, it is likely that if any forestry effects existed they were masked by annual variations produced by climatic variations.
The Woodburn catchments in Northern Ireland were used as a case study in which the low flows from three forested catchments and one grassland catchment were compared. Dry weather baseflows in one of the forested catchments showed a reduction over a 10 year period compared to the grassland control catchment. No change was detected in the shape of the low flow recession curves but when flows were compared from time when soil moisture deficits were the same then the recession curves were found to be lower after 10 years in moderate low flows. This indicated that the moderate low flows include drainage from shallow sources which are affected by the forest growth but the very low flows are from deep sources which are controlled by the solid or drift geology.
Recommendations from this work are:
Low flows; Scotland; Northern Ireland; forestry; Q95; base flow index
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