WATER YIELD ISSUES IN THE JARRAH FOREST OF SOUTH-WESTERN AUSTRALIA

The jarrah forest of south-western Australia produces little streamflo w from moderate rainfall. Water yield from water supply catchments for Perth, Western Australia, are low, averaging 71 mm (7% of annual rain fall). The low water yields are attributed to the large soil water sto rage available for continuous use by the forest vegetation. A number o f water yield studies in south-western Australia have examined the imp act on water yield of land use practices including clearing for agricu ltural development, forest harvesting and regeneration, forest thinnin g and bauxite mining. A permanent reduction in forest cover by clearin g for agriculture led to permanent increases of water yield of approxi mately 28% of annual rainfall in a high rainfall catchment. Thinning o f a high rainfall catchment led to an increase in water yield of 20% o f annual rainfall. However, it is not clear for how long the increased water yield will persist. Forest harvesting and regeneration have led to water yield increases of 16% of annual rainfall. The subsequent re covery of vegetation cover has led to water yields returning to pre-di sturbance levels after an estimated 12-15 years. Bauxite mining of a h igh rainfall catchment led to a water yield increase of 8% of annual r ainfall, followed by a return to pre-disturbance water yield after 12 years. The magnitude of specific streamflow generation mechanisms in s mall catchments subject to forest disturbance vary considerably, typic ally in a number of distinct stages. The presence of a permanent groun dwater discharge area was shown to be instrumental in determining the magnitude of the streamflow response after forest disturbance. The lon g-term prognosis for water yield from areas subject to forest thinning , harvesting and regeneration, and bauxite mining are uncertain, owing to the complex interrelationship between vegetation cover, tree heigh t and age, and catchment evapotranspiration. Management of the forest for water yield needs to acknowledge this complexity and evaluate fore st management strategies both at the large catchment scale and at long time-scales. The extensive network of small catchment experiments, re gional studies, process studies and catchment modelling at both the sm all and large scale, which are carried out in the jarrah forest, are a ll considered as integral components of the research to develop these management strategies to optimise water yield from the jarrah forest, without forfeiting other forest values.