Hardwood harvesting and water quality in the Otways.  I. West Barham Catchment (water supply for Apollo Bay).  P. W. Farrell and P. M. Novotny.  November 1985.  65 pp. (unpubl.)

SUMMARY

The relationship between water quality and hardwood harvesting was investigated for the West Barham catchment, which is the water supply catchment for Apollo Bay, South-western Victoria. This catchment is predominantly within the Aire Land System, and is south of the main ridge traversing the Otways in a high rainfall zone (average annual rainfall of 1910 mm during the study period). Harvesting was found to have had a moderate effect on water quality in sub-catchments that had been harvested between 1976 and 1983. During this time harvesting prescriptions were significantly upgraded, with the introduction in late 1981 of prescriptions specifically formulated for the Otways State Forest.

In streamwater from harvested sub-catchments, turbidity exceeded the maximum desirable level of 5 NTU^† in 24% of samples compared with 9% of samples from undisturbed sub-catchments. Likewise, the concentration of filterable solids was significantly higher in streamwater from two of the harvested sub catchments studied. Poor drainage from compacted areas such as roads, snig tracks and log landings was identified as the key factor responsible for these effects. In contrast, colour and electrical conductivity were both related to intrinsic catchment characteristics, rather than to harvesting. Phosphorus levels in streamwater were generally low and largely unaffected by harvesting, with annual exports unlikely to have any effect on site fertility.

An intense storm in March 1993, with an average recurrence interval of 10-17 years, produced significant effects on in the West Barham catchment. Landslips occurred in areas that had been disturbed by logging, roading and wildfire. Although planning for meteorological events is difficult, care needs to be taken when locating roads and snig tracks near drainage lines. A number of streambank slumps also occurred; these mostly appear to have been caused by exceptionally high streamflows.

Harvesting of a small area under the most recently introduced prescriptions had no impact on water quality. This is largely attributable to such requirements in the current prescriptions as: better delineation of logging coupe boundaries in relation to streams and filter strips; specification of distances required between cut-off bars; and clearer definition of activities permissible in, and adjacent to, buffer and filter strips. Nonetheless, roads are still a significant hazard, with greater attention required to drainage and the design and construction of stream crossings. Approaches to stream crossings should be flat, with the road surface gravelled, and bridges are preferable to fills and culverts.

In areas economic for sawlog harvesting, harvesting of waste material for pulpwood is unlikely to have any additional effect on water quality in this catchment. In contrast, harvesting of additional areas specifically for pulpwood potentially involves a greater hazard to water quality, particularly if further road construction and stream crossings are required.

Strict adherence to the current harvesting prescriptions, and improved roading practices, are essential to ensure that hardwood harvesting has a minimal effect on water quality in the West Barham catchment. Anything less than this could well result in a decline in water quality, as has been the case in the past. Although these findings are specific to the West Barham catchment, they probably apply to high rainfall catchments on similar land systems south of the main ridge traversing the Otways.

^† Nephelometric turbidity units.