Hardwood harvesting and water quality in the Otways. II. West Barwon catchment (a major water supply for the Geelong Region).  P. W. Farrell and P. M. Novotny.  September 1986.  43pp. (unpubl.)

SUMMARY

The relationship between water quality and hardwood harvesting was investigated for the West Barwon catchment, which is a major water supply catchment for the Geelong region in South-western Victoria. This catchment is predominantly within the Aire Land System, and is north of the main ridge traversing the Otways in a high rainfall zone. During 1983 rainfall measured in the catchment was 1502 mm.

Hardwood harvesting was found to have had a moderate effect on turbidity in small sub-catchments (32 to 158 ha) which had been disturbed by logging in the previous two years. In contrast, the effect of harvesting on turbidity in a larger sub-catchment (1003 ha) was not significant. This was attributed in part to dilution effects and the presence of well established regeneration (up to 8 years old) on many of the previously disturbed areas. The effect on filterable solids was not clear-cut, due to lack of data during high rainfall events. In streamwater from harvested sub-catchments, turbidity exceeded the maximum desirable level of 5 NTU^† in 16% of samples compared with 8% of samples from undisturbed sub-catchments. Poor drainage from disturbed and compacted areas such as logging coupes and snig tracks was identified as a key factor responsible for these effects. In general, major roads are not considered to be a significant problem in this catchment as most are located on ridges and spurs. However, some of the minor tracks around the 'Fork Paddocks' area are a potential hazard due to their condition and close proximity to streams.

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.

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. Irrespective of whether pulpwood is harvested, strict adherence to the current harvesting prescriptions is essential. Harvesting of additional areas specifically for pulpwood potentially involves a greater hazard to water quality, particularly if any further roading involves major side-cuts and stream crossings in the catchment headwaters. Prescriptions which can minimise hazards due to roading are discussed.

These findings and prescriptions are relevant to high rainfall catchments on similar land systems in the Otways.

^† Nephelometric turbidity units.