Accounting for the groundwater impacts of plantation forests in the South East of South Australia

Author: Darryl Harvey

  • Thesis download: available for open access on 29 Jul 2020.

Harvey, Darryl, 2018 Accounting for the groundwater impacts of plantation forests in the South East of South Australia, Flinders University, College of Science and Engineering

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Abstract

In the South East of South Australia groundwater management now incorporates 145,000 ha of plantation forest as a licensed water user; accounting for 30 per cent of all licensed water allocations administered under a statutory water allocation plan. This is the first time in Australia (and possibly the world) that plantation forests have been required to hold a water licence to offset their hydrological impacts and this has been acknowledged by the United Nations Association with an Excellence in Water Management Award in 2016.

It is impractical to measure plantation impacts on groundwater resources at a commercial plantation scale, whether in terms of impacts on groundwater recharge, or extraction from shallow water-tables. Therefore, there is a need for a robust model to account for hydrological impacts of plantation forests at a groundwater management area scale. The 2006 forest water accounting model was developed to account for the hydrological impacts of plantation forests on regional groundwater resources. The model is based on biophysical principles and scientific observations available at that time, however, until this study, no attempt has been made to validate the model outputs and to establish that the model is fit for the intended purpose.

The forest water accounting model was tested by calculating a net annual water-mass-balance for softwood and hardwood plantation sites against 30-years of observed changes in groundwater storage, as indicated by the changes in groundwater level. Five sites, each of 5000 ha, where plantation forests are the main land use, were assessed using an age profile of the plantation forest estate at those sites. The forest age and area profile is assembled from industry data provided when seeking their initial grant of forest water licenses for the plantation forest estate. The granted licenses are for offsetting of the forest impacts on groundwater recharge, and extraction where the plantation forest overlay shallow water tables, which is considered to be 6 metres, or less, below ground level.

The calculated annual net changes in groundwater storage compare well with the actual observed changes in groundwater storage. These results indicate that the adopted water accounting method can adequately estimate the annual net impacts of plantation forests on groundwater resources, concluding that it is fit for the intended purpose of estimating the hydrological impacts of the plantation forest estate at a groundwater management area scale. However, during the study it was found that existing plantation forests are extracting groundwater up to 10 metres below ground level.

The investigation also compared grassland land use impacts on groundwater resources, over the same time period, to further confirm the significance of plantation forest hydrological impacts on groundwater resources, relative to the land uses being replaced by plantation forest as the forest industry expands its activities.

Due to the uniqueness of requiring the regional plantation forest to be accountable for its hydrological impacts with licensed groundwater allocations, this thesis also includes a background of the groundwater policy development and the associated models forest water accounting models.

This thesis also includes some recommendations to refine the forest water accounting model and its application to provide a higher level of confidence in its predictions.

Keywords: plantation forest as a licensed water user, plantation forest impacts on groundwater recharge, plantaion forest extract groundwater, forestwater accounting model

Subject: Hydrology thesis, Water Management thesis

Thesis type: Masters
Completed: 2018
School: College of Science and Engineering
Supervisor: Prof Craig Simmons