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Project title

Forest operations - quantification and management of erosion and siltation

Primary Funding Agency

Environmental Protection Agency

Co-Funding Organisation(s)

n/a

Lead Organisation

National University of Ireland Galway (NUIG)

Lead Applicant

Michael Rodgers

Project Abstract

The effects of clearfelling and harvesting lodgepole pine trees on the quality of receiving waters in a productive salmonid blanket peat catchment in Burrishoole Co. Mayo were investigated in this research study. The research was carried out in the context of examining and making recommendations on the Irish Forest Service Forestry and Water Quality Guidelines. The trees were planted in 1971 and the annual rainfall in the catchment exceeds 2000 mm. The site preparation for the original tree-planting comprised double mould board ploughing at right angles to the contours and the installation of collector drains. The study site was contained within a 20 hectare area and was drained by a first order stream that received the flows from the ploughed furrows mainly via the collector drains. The stream waters discharged directly to the salmonid Shrahrevagh River and did not flow through a buffer zone. The study site was divided in two parts. The downstream part of 10.5 hectares was clearfelled in early Autumn 2005 and the upstream part of 7.2 hectares was left intact. Two instrumented stations were established i- one just upstream (US) and the other just downstream (DS) of the area to be clearfelled. The instrumentation at each station included a flume water-level recorder automatic water sampler and probes for measuring physico-chemical water variables; 2 rain gauges were installed at the downstream station. The rainfall flows and water quality variables at the two stations were examined every five minutes for about 1 year pre-clearfelling during clearfelling and harvesting and for 19 months post-clearfelling. During flood events water samples were taken hourly at the US and DS stations for suspended sediment (SS) and phosphorus (P) testing; the SS sampling was carried out for one year and the P sampling for one month before clearfelling. Also hourly water samples were taken in base flow conditions over a 24 hour period weekly throughout the study. The study site was clearfelled by Coillte Teoranta and their contractors in accordance with the Forest Service Guidelines with no on-site work being carried out during wet weather. The SS concentrations in the stream water were statistically significantly higher than the pre-clearfelling concentrations for less than 1 year after the clearfelling and harvesting operations were complete. One month after the clearfelling and harvesting operations were completed the mean SS concentrations at the US and DS stations in a flood event on Nov. 2nd 2005 were 11 mg/l and 42.2 mg/l respectively. From August 2005 to April 2006 there were significant differences between the peak US and DS SS concentrations in flood events. From May 2006 to May 2007 the SS concentrations at the US and DS stations were similar indicating that the effect of the clearfelling and harvesting operations on the SS concentrations in the study stream had ceased. The amount of additional solids (100 kg/ha.year) - above normal losses - that left the harvested area during that first year after harvesting was less than the equivalent of 0.1 mm of peat over the area. The mean daily phosphorus (P) concentration at the DS station increased from about 6 g total reactive phosphorus (TRP)/l pre-clearfelling to 429 g TRP/l in August 2006 and had reduced to 100 g TRP/l by April 2007. More than 80% of the phosphorus was in soluble form. These results indicate that the effects of clearfelling and harvesting are longer lasting on the P mass balance in the catchment than on the soil mass balance. However in this catchment the dilution available in the Shrahrevagh River maintained the average P concentrations in the water downstream of the confluence of the river and study stream at less than 10g TRP/l. The dilution ratio in this study was about 24 i- based on the ratio of the area of the upstream river catchment to the clearfelled area. This indicates that knowledge of areas flows and nutrient concentrations at strategic locations in a catchment could provide a rational and quantitative basis for sizing felling areas that would have minimal effects on receiving waters. In summary the Forest Service Guidelines provide good guidance for sustainable forestry. The results demonstrate that the current guidelines and measures implemented in the catchment were effective at protecting water quality. Further detailed studies should be carried out to: (i) provide quantitative data on the removal of solids and nutrients in buffer zones dilution effects and erodibility indices for Irish soils; (ii) develop further P loss models; (iii) identify the fate of exported material and nutrients; and (iv) identify the concentrations of water physico-chemical parameters that are important for stream organisms.

Grant Approved

�158,919.74

Research Hub

Natural Environment

Research Theme

Improve our Knowledge on the State of our Water Resources and Pressures

Start Date

01/12/2004

Initial Projected Completion Date

n/a