In the South East of South Australia groundwater management now incorporates 150,000 ha of plantation forestry as a licensed water user; accounting for 30 per cent of all licensed allocations. This is the first time that forestry has been required to hold a water licence to offset its 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 at a commercial scale, whether in terms of recharge, or extraction from shallow water-tables. Therefore, there is a need for a robust model to account for forest impacts at a sub-regional scale. In this study, we evaluated the water accounting methodology adopted by the regional water allocation plan. The model accounts for forest impacts on groundwater recharge, and extraction where the water-table is shallow, using biophysical principles and scientific observations.
The forest groundwater accounting methodology was tested by calculating an annual water-mass-balance for softwood and hardwood plantations, with various depths to the water table, and comparing this calculation against 40-years of observed changes in groundwater storage, indicated by the changes in groundwater level. Sites, each of 5000 ha, where plantation forestry is the main land use were assessed for the study using a profile of the plantation forest estate from industry data.
The calculated annual net changes in groundwater storage compared well with the actual observed changes in groundwater storage. These results indicate that the adopted water accounting method can accurately estimate the annual net impacts of plantation forests on groundwater resources, concluding that the forest water accounting model is fit for the intended purpose of estimating, for management purposes, the impacts of the regional plantation forest estate on groundwater resources.