Introduction

In late May, major storms brought record rainfall to Eastern Australia. Weeks later, land was still submerged, five fatalities were reported, and recovery efforts continued. As the region responded to the disaster, NASA provided near real-time (NRT) satellite data to help affected communities understand the scope of the storm’s impacts and identify areas that were in the greatest need of aid and resources.  

One community, Taree, a city in northeastern Australia and part of New South Wales, was inundated with over 24 inches (2 ft.) of rain from May 20 to 24. Taree is primarily an agricultural area, known for its dairy, timber, fishing, and vegetable growing industries. With many farms filled with water, farmers were faced with determining the damage to their livestock and crops, while others in the community assessed the impacts to their businesses and homes.

Flood Maps

The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra and Aqua satellites provide daily global coverage, which is used to create the MODIS NRT Global Flood Product (MCDWD) derived from the NRT MODIS Surface Reflectance (MOD09) datasets available from NASA’s Level-1 and Atmosphere Archive and Distribution System Distributed Active Archive Center (LAADS DAAC) and Land Processes DAAC (LP DAAC). This product provides a daily global map of flooding, where red indicates areas of flooding and light blue indicates consistent areas of surface water. 

For each map, the Flood Product uses a 2-day period, current day and prior day, to create the images from NASA Worldview. For example, the May 24 images also include data from May 23. The 1-day flood product is not currently available in Worldview, but is available in the full HDF product file, and as separate GeoTIFF files from Land, Atmosphere Near real-time Capability for Earth observation (LANCE).

The flood map does have some limitations. Clouds, buildings, tree canopies, and cloud-shadows can obscure flood detection, while shadows from clouds or terrain can lead to false-positive flood detections. To ensure that the detection of flooding is accurate, data users are advised to compare the flood product against MODIS corrected reflectance imagery. Where MODIS corrected reflectance data indicates cloud shadow, there may be a false reading. In the NASA Worldview images from May 24 above, areas of water appear as dark blue and areas of vegetation as bright green. Areas that were red in the Flood Product also appear dark blue, validating that the Flood Product is accurate. 

The maps created from the MODIS NRT Global Flood Product (MCDWD) maps are available to aid organizations and also contribute to long term data records of flooding worldwide. The flood maps can also be referenced against population data to identify areas where people may be impacted.

While Taree is not a bustling city like Sydney, population maps based on data acquired in 2020 as part of the Gridded Population of the World (GPW) collection show that Taree has areas where over 1,000 people reside per square kilometer. Just outside the city, where farms sprawl over the landscape, there are fewer people, but the impacts to this area can have far-reaching effects on the people that rely on the food that this rural area produces.

Coupling flood maps with population data can help incident responders better understand how best to allocate resources to populated areas and land managers to quickly assess what types of agriculture are impacted.