Abstract
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.
Highlights
Because of its mostly hot, dry, and erratic climate, wildfires in Australia and many parts of the world are frequently occurring events during the hotter months
The Sydney drinking water catchment (SDWC) area includes five main catchments and 204 sub-catchments. It extends from north of Lithgow in the upper Blue Mountains, to the source of the Shoalhaven River near Cooma in the south, and from Woronora in the east to the source of the Wollondilly River west of Crookwell. These catchments cover an area of almost 16,000 km2, about seven times larger than the Australia Capital Territory (ACT) to its southwest (Figure 1)
This study focuses on the recent advancements in weather radar rainfall estimation, satellite estimation of FVC and Google Earth Engine (GEE) technology which enable accurate and rapid estimation of some Revised Universal Soil Loss Equation (RUSLE) factors, for the rainfall erosivity (EI30i) and cover factors (Ci)
Summary
Because of its mostly hot, dry, and erratic climate, wildfires in Australia and many parts of the world are frequently occurring events during the hotter months. Between 2017 and 2019, severe drought developed across much of eastern and inland Australia including Queensland, New South Wales (NSW), and Victoria, extending into parts of South Australia and Western Australia. As at late 2019, many regions of Australia were still in significant drought, contributing to water restrictions and extreme fire conditions [1]. In NSW, the three-year (2017–2019) severe drought had left forests tinder dry, facilitating rapid expansion of fires across the state. Sydney—the nation’s most populated city (about 5 million residents)—had been under water restrictions since late 2019, when its dams fell below 45% capacity. The NSW Rural Fire Service (RFS) reported that the 2019–2020 fires burnt 5.4 million hectares (including ~11,264 bush or grass fires across 6.7% of the State) and destroyed 2439 homes [3]
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