Resource Conflict Across Melbourne’s Largest Domestic Water Supply Catchment

Abstract

Quantifying the effects of competition for natural resources between different sectors and interests is a key part of natural resource management globally. A major form of land use conflict in natural forests is between water production and timber production. Here we explore trade-offs in water yield resulting from logging in the forested water catchments north-east of Melbourne – the second largest urban settlement in Australia with a current population of five million. It has long been understood that logging significantly decreases water yields in Melbourne’s water catchments. However, the extent of losses of water yield from past logging have rarely been documented. Here, we model changes in water yield in Melbourne’s largest single catchment, the Thomson Catchment, resulting from: (1) past forest management activities (especially clearfell logging), and (2) future forest management scenarios. Our particular focus was on the effects of logging on water yields from ash-type eucalypt forests. This is because these areas have the greatest impact on water runoff due to them receiving the most rainfall and being the forest types subject to the most intensive and extensive industrial logging. We modelled four key scenarios:  Scenario (1) Historical logging of the Thomson Catchment with continued logging in the future (current reality/status quo);  Scenario (2) If there had been no logging and none was planned (past, present or future) in the Thomson Catchment;  Scenario (3) Logging ceasing in 1967 (as specified under the first Wood Pulp Agreement Act 1936 – but which never occurred); and  Scenario (4) Impacts of the past logging, but with cessation of logging in 2018. Our initial spatial analysis revealed that 42% of the ash-type eucalypt forests in the Thomson Catchment have been logged. Moreover, there are 4,000 hectares of Ash forest assigned for logging in the next 5 years under the existing Timber Release Plan for the Central Highlands region. Our analyses revealed that the current (in 2018) reduction in water yield due to historical logging of the ash forests across the Thomson Catchment exceeds 15,000 ML annually. This loss is projected to increase to nearly 35,156 ML by 2050. Under Scenario (3), where logging would have ceased in 1967 if the first Wood Pulp Agreement 1936 was implemented, the loss in water yield by 2018 was projected to be 1,079 ML, annually. This loss is a result of logging occurring prior to 1967. This was modelled to remain constant through to 2050. Under Scenario (4), where logging ceases in 2018, we projected that approximately 20,149 ML would have been returned to the Thomson Catchment by 2050 compared with Scenario (2) of no historical logging. Losses in water yield as a result of logging correspond to 9%-20% of the ash forest catchment water yield for 2018 and 2050, respectively. Based on an estimated consumption of 161 litres of water per person per day, the loss in water yield resulting from logging would equate to the lost water for nearly 600,000 people by 2050. Given the strategic importance of water from the Thomson Catchment, our analyses suggest that native forest logging should be excluded from this catchment, particularly in the context of increasing human consumption of water and decreasing stream inflows from the catchments. Previous work has shown that the economic value of the water across all of Melbourne’s Water Catchments, including the Thomson Catchment, is 25.5 times greater than the economic value of the timber produced from the all native forests, based on integrated economic and environmental accounting (e.g. under the System of Environmental and Economic Accounting [SEEA] developed by the United Nations). It is not the difference in value between water and timber that is important, it is the change due to the use of an ecosystem service, resulting in the reduction of water yield. Therefore, we suggest that ongoing logging of the Thomson Catchment, when it is known to reduce water yields, is a questionable natural resource management policy.