Automatic near real-time flood detection using Suomi-NPP/VIIRS data
Introduction
As the costliest natural disasters worldwide, most climate change forecasts predict that floods will become increasingly frequent (Milly et al., 2002, Hirabayashi et al., 2008, Lehner et al., 2006). At high latitudes, floods are caused by ice jams and snow melt during almost every break-up season. Floods caused by intense rainfall also threaten the safety of human lives and property. Near real-time satellite-derived flood maps are invaluable to river forecasters and decision-makers for disaster monitoring and relief efforts.
Flood detection has a history in satellite remote sensing that dates back to the 1970s. Imagery from the NOAA (National Oceanic and Atmospheric Administration) VHRR (Very High Resolution Radiometer) and AVHRR (Advanced Very High Resolution Radiometer) served as the main data sources for flood/standing water detection prior to the development of the MODIS (Moderate Resolution Imaging Spectroradiometer) system. Many case studies have been conducted to analyze severe flood events all over the world. These studies laid a foundation for the methods and approaches of flood detection with coarse-to-moderate-resolution satellite data (Wiesnet et al., 1974, Barton and Bathols, 1989, Ali, 1989, Sheng and Xiao, 1994, Sheng et al., 1998, Sheng and Gong, 2001). With coarse 1-km spatial resolutions, however, VHRR and AVHRR data could only show the macro flood distributions of select major floods and failed to address any inundation details. To resolve this issue, Landsat imagery with a 30-m spatial resolution is widely used as an alternative in flood detection, disaster assessment and flood pattern analysis (Gupta and Bodechtel, 1982, Gupta and Banerji, 1985, Wang et al., 2002, Mueller et al., 2016, Fisher et al., 2016, Tulbure et al., 2016). Although VHRR, AVHRR and Landsat imagery play effective roles in flood mapping, the flood detection capabilities of these optical sensors can be severely affected by cloud cover during flood periods. To derive flood information under cloud cover, radar remote sensing satellites and imaging systems such as Radarsat, SAR, TerraSAR-X and Sentinel-1 are becoming more popular in flood monitoring and analysis. Their high spatial resolution and capability to penetrate cloud cover make radar data very popular in hydrological fields for multiple-scale flood mapping, flood management and disaster relief (Brakenridge et al., 1994, Matgen et al., 2007, Schumann et al., 2007, Martinis et al., 2009, Matgen et al., 2011, Pulvirenti et al., 2011, Martinis et al., 2013).
Although Landsat and radar imagery have excellent capabilities for flood mapping, the narrow swath widths and long revisit periods of their sensors are major drawbacks. Because most floods are short-term events, it is not realistic to completely rely upon these images for flood mapping and management purposes. In comparison, moderate-spatial-resolution satellites provide steadier and lower-cost data sources for near real-time flood mapping. After the EOS (Earth Observing System) flagship Terra was launched in 1999, MODIS has gradually become the preferred satellite instrument for flood detection because of its daily global coverage and higher spatial resolution of the visible, near infrared (250 m) and shortwave infrared (500 m) channels compared to the 1-km resolution channels with the AVHRR (Gumley and King, 1995, Brakenridge and Anderson, 2006). Newer algorithms such as the decision-tree approach and the open water likelihood method have used MODIS to more accurately detect flooding and standing water (Sun et al., 2012, Ticehurst et al., 2014, Ticehurst et al., 2015). The continuous observations from MODIS also make it possible to analyze flood inundation dynamics and generate global water masks from multiple-year detected results (Carroll et al., 2009, Andrimont et al., 2012, Huang et al., 2014). In 2011, an experimental global flood detection system using MODIS imagery was released by NASA (National Aeronautics and Space Administration) (http://oas.gsfc.nasa.gov/floodmap). This system processes near real-time MODIS data and generates 1-day, 2-day, 3-day and 14-day composite global flood products for 10° × 10° tiles from the MODIS instrumentation aboard the Terra and Aqua satellites (Brakenridge, 2011). The system also provides systematic datasets with a robust interface to access the products. The multiple-day composition process is applied mainly in order to filter out cloud shadows and terrain shadows, and it produces multiple-day composite flood maps rather than near real-time ones. The problem with the multiple-day composition process is that some real floodwater data may be lost in the composition process, and the process introduces a bias in the experimental MODIS flood maps. Even after the composition process has finished, cloud shadows can persist in the MODIS flood products, especially at high latitudes. More recently, the HAND (height above nearest drainage) algorithm has been applied to MODIS flood detection attempts with a better removal of terrain shadows. The accuracy of MODIS flood products are still susceptible to deep terrain shadows that cannot be filtered either through multiple-day compositions or the HAND algorithm (Brakenridge, 2011, Liu et al., 2016).
With the launch of the Suomi-NPP in 2012, the VIIRS sensor has exhibited many advantages over MODIS data in environmental and natural disaster monitoring and analysis. SNPP/VIIRS imagery has a moderate spatial resolution of 375 m in the shortwave IR bands, a swath coverage width of 3000 km, and a relatively constant resolution across the scan. These new features make SNPP/VIIRS data an excellent source for near real-time flood detection. With the support of the JPSS/PGRR program since 2013, VNG Flood V1.0 has been developed using SNPP/VIIRS imagery to derive near real-time flood maps for the National Weather Service (NWS) River Forecast Centers (RFC) in the USA. A series of algorithms have been developed in the software, including those for water detection, cloud shadow removal, terrain shadow removal, minor flood detection, water fraction retrieval, and floodwater determination. The successful development of the cloud shadow and terrain shadow removal algorithms promises consistent results and makes the detection of near real-time flooding feasible and operational using moderate-resolution satellite data. This paper presents a comprehensive introduction to the software, describes the required datasets, introduces the algorithms, presents the results, and concludes with a summary discussion.
Section snippets
Data used
The main datasets used for flood detection with the VIIRS imagery are the SNPP/VIIRS SDR (sensor data record) data in imager bands 1 (600–680 nm), 2 (850–880 nm), 3 (1610 nm) and 5 (1050–1240 nm) with nominal resolutions of 375 m and I-band terrain-corrected geolocation data, which includes longitude, latitude, solar zenith angles, solar azimuth angles, sensor zenith angles and sensor azimuth angles (GITCO). The SNPP/VIIRS 750-m resolution cloud mask intermediate product (IICMO) and M-band
Physical basis
Water detection with vegetation and bare land background conditions using optical satellite data is primarily based on the spectral differences between water features and other land cover types in the visible (Vis, VIIRS I1 band: 600–680 nm), near infrared (NIR, VIIRS I2 band: 850–880 nm) and shortwave infrared (SWIR, VIIRS I3 band: 1580–1640 nm) channels (Wiesnet et al., 1974, Barton and Bathols, 1989, Sheng and Xiao, 1994). As shown in Fig. 1, water has a higher reflectance in the Vis channel
Applications
During a demonstration project operated by the JPSS PGRR Program since 2014, the developed VNG Flood V1.0 has been running routinely for five river forecast centers in the USA at two locations that process VIIRS direct broadcast data in near real-time: the Space Science and Engineering Center at the University of Wisconsin-Madison (SSEC/UW-Madison) and the Geographic Information Network of Alaska at the University of Alaska-Fairbanks (GINA/UAF). The flood maps are distributed via the Unidata
Discussion
With the support from JPSS/PGRR Program, VNG Flood V1.0 has been developed for automatic near real-time flood detection using SNPP/VIIRS data. Algorithms include water detection, cloud shadow removal, terrain shadow removal, minor flood detection, water fraction retrieval, and flood determination. With a demonstration project initialized by JPSS/PGRR Program, the software has been running routinely using direct broadcast VIIRS data in near real-time flood detection for five river forecast
Conclusion
This study presents a comprehensive introduction to VNG Flood V1.0, and can be summarized as follows:
- 1.
The VIIRS NOAA/GMU Flood Version 1.0 software has been developed for automatic near real-time flood detection using SNPP/VIIRS imagery. Floods are divided into two types: supra-veg/bare soil floods and supra-snow/ice floods. A series of algorithms, including water detection, cloud shadow removal, terrain shadow removal, minor flood detection, water fraction retrieval, and floodwater
Acknowledgement
This work was supported by NOAA grant #NA14NES4400007. We thank the five river forecast centers: NCRFC, MBRFC, NERFC, WGRFC and APRFC in the USA for the great efforts they have made to improve this software. The manuscript's contents are solely the opinions of the authors and do not constitute a statement of policy, decision, or position on behalf of NOAA or the U. S.A.
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