Graham Hall is a Dolgellau based scientist who has conducted extensive local research on forest hydrology and water cachement, among other things. The extracts below are from various research papers.

Referring to Waen Y Griafolen blanket bog (at the head of the river Mawddach)

"The older [humified] peat was found to have an unexpectedly small buffering effect, with the water table rising by 1cm per hour and the peat reaching saturation within a single storm event. Watertable decline during a dry period is at a slower rate of about 1cm per day." p.5

"A theoretical model is being developed for the hydrology of the blanket bog, which attributes the main controlling influence to the zones of active sphagnum peat. Storm runoff from the older peat is rapid, but water then passes slowly through the sphagnum zones which have a significant buffering capacity." p.5

2. Extracts from

The extract below refers to three sites that Graham studied, all located at Hermon, as follows

"Runoff production is significantly higher from the clear felled hillslope in comparison to the mature conifer plantation, with permanent grassland giving an intermediate response. It is possible to link this result directly to the soil profiles developed beneath the sites.

Conifers at site 1 were planted during the 1950’s and have developed beyond commercial maturity. This has allowed a progressive rising and opening of the forest canopy, promoting the growth of ground vegetation. The combined effects of trapping slope wash sediment and addition of organic material over a 50 year period has led to the development of around 140cm of forest brown earth.

At site 2, clear felling was accompanied by the die back of mosses and ferns through reduced humidity...Within one year of clear felling, severe erosion led to the removal of over a metre of forest brown earth to leave only 30cm of matted humus as a relatively impermeable hillslope cover above periglacial deposits.

Soil development beneath the grassland has reached a stable profile depth of around 90cm. The limited depth in comparison to the conifer plantation may be due to less efficient trapping of downslope sediment wash by grasses." p.10

"Mature conifer and broadleaf woodlands lying within the high rainfall zone are both able to develop the prolific moss and ground vegetation seen at Hermon. Moss growth maintains the humid conditions within the forest. Thus, the ecological system becomes self-sustaining and soil accumulation is promoted. Clear felling destabilises the moss-fern association, leading to soil erosion. The greatly altered nature of the hill surface produces a substantial increase in surface runoff during storm events." p.12-13

"Expansion of forestry in the high rainfall zone of Rhobell Fawr and the Aran mountains should lead to further growth of hillslope moss-fern associations, with consequent soil deepening and improved runoff interception properties. The most immediate benefits would be derived from the planting of fast growing conifers and birches, with a gradual progression to semi-natural oak woodlands as a desirable long term objective.

Continuous cover forestry practices should be employed, to avoid soil erosion from clear felled hillslopes. Trees need to grow beyond normal commercial maturity to ensure the open canopy conditions favourable for high productivity of ground vegetation." p.20-21