Groundwater management in the South East of South Australia incorporates plantation forestry as a licensed water user. Forest water accounting methods incorporate rates of groundwater recharge interception by forestry, and rates of groundwater extraction where the watertable is shallow (<6 m below ground). However, the method adopted in policy has not been tested against long term observations. Furthermore, hardwood (Tasmanian blue gum) plantations have not been managed as originally intended, as plantations which extract groundwater have remained in place for much longer than policy accounted for. Hence there is a need to revisit plantation forest water accounting methods, and their potential impact on groundwater resources.
An existing numerical groundwater model was revised and re-calibrated to assess whether current policy assumptions on recharge and forestry impacts are consistent with groundwater observations. The model assumes policy adopted rates of groundwater recharge, recharge interception by plantation forestry, and groundwater extraction by plantation forestry. The model was calibrated by varying hydraulic parameters with PEST. Model parameter uncertainty was then investigated using an alternative model calibrated allowing both hydraulic conductivity and adopted recharge rates to vary in parameter estimation.
The model calibrated well to existing policy adopted rates of recharge and forestry impacts, and the hydraulic parameters match our conceptual understanding. The forest water accounting methods can be considered broadly accurate, with a few exceptions. The modelling work will help guide future iterations of water allocation plans in the region, particularly in relation to managing forestry impacts on groundwater. The additional calibration work also helps water planners understand uncertainty in groundwater management numbers.