# Nationwide Flood Monitoring for Disaster Risk Reduction Using Multiple Satellite Data
## 1 Introduction  
Future disaster risk and climate change are fundamental threats to sustainable development of all countries, including advanced countries [1]. Since the early 21 st century, increases in natural disaster risk have been globally recognized; in particular, large-scale floods have been more frequent in both developing and advanced countries. Even after the adoption of the Sustainable Development Goals (SDGs), natural disasters continue to undermine efforts to implement national and international developments. In response, the mainstreaming of Disaster Risk Reduction (DRR) has been strengthened to reduce the number of people killed or affected by natural disasters including water-related disasters at local, national, regional, and global levels in priority areas defined by UNISDR [2]. The importance of understanding disaster risk was mentioned as the first priority with evidence-based policy making and practices for disaster risk management. The significance of strengthening disaster risk governance also was highlighted to manage disaster risk as the next priority. In 2015, the third UN World Conference on DRR (WCDRR) was held in Sendai, Japan, and emphasized the importance of \"geospatial and space-based technologies and related services and maintaining and strengthening in situ and remotely-sensed earth and climate observations\" to support national measures for understanding disaster risk and successful disaster risk communication [2]. In the context of the UN framework on climate change, the Paris agreement (COP21) was adopted by UN member nations as a new climate agreement to promote coherence and mutual reinforcement of disaster risk reduction after the Sendai Conference [1; 3; 4]. In the remote sensing application related to disaster risk management, multiple satellite-based flood mapping and monitoring are an imperative process and an essential part of risk assessment so that authorities and stakeholders can carry out appropriate disaster response and relief activities to achieve disaster risk reduction and mitigation under the emergency aid strategy at the early stage. Multiple satellite constellations, such as European Sentinel and COSMO-SkyMed (Constellation of Small Satellites for Mediterranean basin Observation, conceived by Agenzia Spazialie Italiana, and funded by the Italian Ministry of Research and the Italian Ministry of Defense, Rome, Italy. ), and frequent revisiting time steps with wide area coverage are useful in providing valuable risk information to determine damage distribution for recovery activities and restoration planning. Recently, remote sensing technology has been rapidly developed with big data and cloud-computing technology to bring innovation opportunities with advanced computer technology, e.g., web-based tools, data storage, data processing, and data sharing [5; 6]. For example, the Google Earth Engine is an advanced cloud-based geo-spatial processing platform using globally available open big data including remotely sensed imagery and geospatial datasets via petabyte-scale archives on a global scale [7; 8]. During the past few decades, earth observation satellite (EOS) imagery has become one of the most credible data sources to find evidence of the earth's surface changes. Land-cover change detection focusing on flood detection is one of the traditional themes of satellite-based observation utilizing optical and synthetic aperture radar (SAR) sensors. For example, optical sensing-based index algorithms have been designed and developed to detect surface water, relying mainly on the calculation of spectral band, such as the Normalized Difference Water Index (NDWI) [9; 10] and the Land Surface Water Index (LSWI) [11; 12]. The Moderate Resolution Imaging Spectroradiometer (MODIS), one of the main contributors to global mapping and monitoring, is able to generate near-real-time global flood maps on a daily basis [5; 13; 14; 15; 16].