Impacts of urbanisation, climate change and management on groundwater recharge beneath Perth, Western Australia

Urbanisation has profound effects on environmental water fluxes and storages, which can be further compounded by climate change and management responses. This study shows how climate change influences the type and amount of recharge in urban areas with different housing densities under Perth, Western Australia. We demonstrate that consequent water conservation measures aimed to reduce irrigation may be having unintended negative consequences by increasing urban heating. Groundwater recharge was measured under Perth, Western Australia, in 1980-81 and again in 2017-18, a 38-year climate period which saw annual rainfall fall by 150 mm; months with heavy rain reduce by half; maximum annual temperatures increase by more than 1 degrees C and potential evaporation increase by 15%. The consequent reduction in direct recharge through soils due to climate change was enhanced by regulated reductions in sprinkler irrigation, whilst interception losses via tree canopies increased as rain fell as lighter showers rather than prolonged events. Indirect recharge off roofs and roads, however, was less affected and now dominates recharge to the shallow, unconfined aquifer. The primary management response to falling groundwater levels has been to reduce extraction from the aquifer. We propose an additional viable strategy would be managed aquifer recharge, using either water from main drains that remove groundwater, or secondary-treated wastewater, which is currently pumped to the ocean. Increasing, or at least maintaining, local irrigation could provide multiple positive benefits, including evaporative cooling over large urban areas to help reverse urban heating. Increased urbanisation worldwide generally increases urban runoff and generates urban heat islands. In cities, such as Perth, built over transmissive sand aquifers, however, increasing local recharge can provide valuable non-potable water and help urban cooling. Identifying and installing additional recharge systems, however, is difficult for managers as it requires innovation and additional resourcing compared with purely restricting licensed extraction.