THE IMPACT OF FOREST HARVESTING ON WATER YIELD - MODELING HYDROLOGICAL CHANGES DETECTED BY POLLEN ANALYSIS

Forest harvesting practices have been shown to have a hydrological imp act that is akin to bushfire regeneration. Fine resolution fossil poll en studies in the upper Delegate River basin, East Gippsland, Australi a, have revealed large changes in the reproductive activity of aquatic plants synchronous with the onset of harvesting activities. This pape r seeks to establish an explanation for these changes by applying a 2- parameter regional bushfire yield trend model to the wet eucalypt fore sts above the pollen core site. Using histories of piecemeal coupe (ha rvesting plot) development, the model was used to reconstruct annual r unoff yields between 1970 and 1991 assuming zero harvesting. The resul ts indicated a progressive reduction in simulated annual yields (relat ive to the zero disturbance scenario) attaining between 20 and 55 % lo ss by 1987/1988. Given the historic rate of harvesting, over a 50 % re duction of water yield is expected to occur by 2005 due to factors inc luding enhanced forest transpiration. Such hydrological changes would have significant implications for downstream fauna and flora and for s tream dynamics.