Integrated science informs forest and water allocation policies in the South East of Australia

Abstract

Groundwater-dependent ecosystems are particularly vulnerable to groundwater decline. The South East of South Australia has experienced considerable hydrological change due to drainage, clearance of native vegetation, irrigated agriculture, and forestry. Recent hardwood plantings in the Limestone Coast Prescribed Wells Area (PWA) have increased considerably since 2000, coincident with groundwater decline; however, the introduction of hardwood plantations coincided with a period of lower than average rainfall. Given the economic implications of modifying water allocation policy based on apparent anthropogenic causes, establishing the contributions to the observed declines in groundwater level was necessary. The water balance shows a net gain in water across the entire Lower Limestone Coast Prescribed Wells Area, yet sites with forestry land use show the water table has been declining at a rate up to 0.7 m yr⁻¹ since 2004, whereas no long-term change has occurred in the water table depth at adjacent pasture sites. Forested areas have 4050 wetlands at risk with groundwater decline >0.3 m yr⁻¹, covering a total wetland area of of 212 km². A series of 15 policy scenarios were simulated using a groundwater flow model to examine how climate, land use, and particularly forestry would impact groundwater, with an imperative to enact policy to protect remaining wetlands. Water allocation policy accounted for groundwater extraction for irrigated agriculture, but other activities, such as forestry, were not considered in water allocation even though they were found to be a major water user. Following this study, the South Australian Government amended its Natural Resources Management act to stipulate that forestry must have a water licence and a water allocation to account for water used by the forest.