Riparian restoration benefits across space and time in subtropical Australia: From drinking water quality to wider society

Riparian restoration can improve water quality, reduce sources of risk and pathogen load in raw drinking water, sequester carbon, and increase biodiversity. However, there is uncertainty around the return on investment (ROI) in riparian restoration for decision-makers, as studies that estimate the spatial and temporal aspects of flood related and non-flood related monetary benefits to specific human populations are rare. We established the baseline cost and risk to the drinking water of Brisbane (population ∼ 1 million) due to variation in turbidity during floods with a range of magnitudes. Subsequently, the merits of 95 km of targeted riparian restoration were explored from the perspective of: 1. A ROI for the water supply utility that included avoided treatment costs and revenue from sale of carbon sequestration credits with biodiversity co-benefits; 2. A risk analysis for the water supply utility using reduced variability of treatment costs and disaster risk following riparian restoration; 3. A ROI for the region that included avoided population disease burden; and 4. A ROI for society that included avoided social cost of carbon. Substantial time was required before saplings delivered maximum water quality benefits. However, carbon sequestration and biodiversity enhancement benefits were delivered while trees were growing, and their inclusion allowed a modest positive ROI to the utility based on avoided operating costs alone. Including benefits from avoided population disease burden and avoided social cost of carbon increased the ROI further. Results demonstrated that if riparian restoration was appropriately targeted, it could address complex water challenges and contribute to global sustainability goals.