EDITOR'S NOTE: This article is part of the special series "Ecological consequences of wildfires." The series documents the impacts of large-scale wildfires in many areas of the globe on biodiversity and ecosystem condition in both terrestrial and aquatic ecosystems, the capacity for systems to recover and management practices needed to prevent such destruction in the future.

The Australian Black Summer wildfires between September 2019 and January 2020 burnt many parts of eastern Australia including major forests within the Sydney drinking water catchment (SDWC) area, almost 16.000 km2. There was great concern on post-fire erosion and water quality hazards to Sydney’s drinking water supply, especially after the heavy rainfall events in February 2020. We developed a rapid and innovative approach to estimate post-fire hillslope erosion using weather radar, remote sensing, Google Earth Engine (GEE), Geographical Information Systems (GIS), and the Revised Universal Soil Loss Equation (RUSLE). The event-based rainfall erosivity was estimated from radar-derived rainfall accumulations for all storm events after the wildfires. Satellite data including Sentinel-2, Landsat-8, and Moderate Resolution Imaging Spectroradiometer (MODIS) were used to estimate the fractional vegetation covers and the RUSLE cover-management factor. The study reveals that the average post-fire erosion rate over SDWC in February 2020 was 4.9 Mg ha−1 month−1, about 30 times higher than the pre-fire erosion and 10 times higher than the average erosion rate at the same period because of the intense storm events and rainfall erosivity with a return period over 40 years. The high post-fire erosion risk areas (up to 23.8 Mg ha−1 month−1) were at sub-catchments near Warragamba Dam which forms Lake Burragorang and supplies drinking water to more than four million people in Sydney. These findings assist in the timely assessment of post-fire erosion and water quality risks and help develop cost-effective fire incident management and mitigation actions for such an area with both significant ecological and drinking water assets. The methodology developed from this study is potentially applicable elsewhere for similar studies as the input datasets (satellite and radar data) and computing platforms (GEE, GIS) are available and accessible worldwide.

<p>The 2019/20 Australian wildfires burned the largest forested area in Australia´s recorded history, with major environmental and socio-economic consequences. These included extensive ash and soil erosion events which threatened water quality in parts on eastern Australia.  </p><p>The second largest fire was the 280,000 ha Green Wattle Creek Fire, which burned large forested areas of the Warragamba catchment. This protected catchment provides critical ecosystem services for Lake Burragorang, Australia’s largest urban supply reservoir delivering ~85 % of the water used in Greater Sydney. </p><p>The fire caused major challenges for maintaining the supply of clean water to the Greater Sydney region (> 5 million consumers). Shortly after the fire was contained, an extreme rainfall event (up to ~276 mm in 72 h), caused extensive ash and sediment delivery into the reservoir.</p><p>Here we (i) summarise the effects of this unpreceded fire seasons on erosion and water quality in general and (ii) report on the interactions between science and land management to quantify predict and mitigate the risk to water supply for the Greater Sydney region. The latter included assessments of fire severity, ash quantities and their pollutant content using remote sensing and ground measurements, as well as the application and further development of a new modelling tool,  WEPPcloud-Wildfire Ash Transport and Risk tool (WEPPcloud-WATAR - https://wepp.cloud -). This tool allows predicting probabilities for sediment, ash and contaminant transport for different rainfall scenarios, and aided the identification of risk hotspots to focus post-fire erosion mitigation measures.  </p><p>Risk modelling, on-ground monitoring and operational mitigation measures ensured the continuity of safe water supply to Greater Sydney. This collaborative interaction between scientists and water managers, that also allowed the refinement of the model capabilities and its outputs, exemplifies the successful outcomes that can be achieved through the close collaboration between science and end-users.   </p>