Hydrological impact of two intensities of timber harvest and associated silviculture in the jarrah forest in south-western Australia

Abstract

Summary The hydrological impact of two different intensities of timber harvest and associated silviculture, one standard and the other more intensive, was investigated using a paired-catchment study in jarrah forest in south-western Australia. This study was undertaken during a period when average annual rainfall was below the long-term average and deep groundwater levels were declining. Following treatment, recharge increased and slowed the decline in deep groundwater levels in proportion to the magnitude of the initial reduction in vegetation density. However, neither treatment was sufficiently intense to reverse the continued decline in groundwater levels over the course of the study. Annual stream salinity did not increase in response to either treatment because saline deep groundwater did not rise following the treatments. Annual streamflow did not increase in either catchment for three reasons. Firstly, there was little additional net precipitation to the intermittent shallow perched groundwater system in the streamzone because the area remained untreated. Secondly, there was minimal additional throughflow to the perched groundwater system in the streamzone from upslope areas because the increased net precipitation in hillslope areas following treatment was used in replenishing the progressively increasing soil moisture deficit. Thirdly, because groundwater levels did not rise and hence there was little prospect for an increased discharge of saline groundwater or of an expanded deep groundwater discharge zone. This study has demonstrated that the measures that were implemented to timber harvest and silvicultural methods to reduce the magnitude of the groundwater response and hence the risk of a transient increase in stream salinity are effective. If annual rainfall remains relatively low and deep groundwater levels remain at current levels or decline further, then the risk of an increase in stream salinity from either the standard or the more intensive harvest and silviculture will be low.