Report SR 94 (01) F

March 1996

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:

1. Pre-planting drainage - Most cases showed an increase in the low flows if more than 25% of the catchment is drained;
2. Forest growth - Decline in low flows(15-60%) starting some 6 years after planting, once the trees have sufficient growth;
3. Clearfell - Most catchments have initial increase in low flows (15-350%) depending on percentage area felled and the seasonal rainfall pattern, followed by an unsteady decline depending on the rate and type of revegetation. An exception was coastal catchments which can show a reduction in low flows if fog occurs frequently;
4. Clearfell: snow - Possible alteration in the timing of snow melt to enhance late spring low flows.

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:

1. Forest planting should be deterred in catchments with known pressure on water resource systems, where dry weather flows are critical for the maintenance of efffluent dilution ratios or where sensitive aquatic ecosystems might be threatened;
2. Concentrated planting in the catchment of any one river should be discouraged;
3. Where forest development must be concentrated within a geographical area, staggered planting times should be considered;
4. Forest planting should be designed to avoid important shallow soil water reserves such as marshy areas immediately adjacent to stream channels;
5. The introduction of forest clearings can provide more gradual snowmelt in higher altitude catchments, and thus support higher low flows for longer periods during spring than would otherwise occur;
6. Spot gauging programmes during prolonged periods of dry weather would allow measuring authorities to assess the importance of individual sub-catchments to water resource questions in larger rivers;
7. The establishment of some continuous flow gauges in small, upland catchments would be of great benefit in characterising flows in the type of catchment of greatest interest when considering the effects of forestry on low flows.

Low flows; Scotland; Northern Ireland; forestry; Q₉₅; base flow index

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