# Drought: A Common Environmental Disaster
## 4 Remote Sensing and Drought Monitoring  
Remote sensing information can be used to assess and monitor drought features because it provides real-time geographic observations of many atmospheric and land surface variables that can be utilised to estimate precipitation, evapotranspiration, soil moisture and vegetation conditions [59; 60; 61]. Traditional drought monitoring and evaluation approaches rely on rainfall data, which are scarce in some regions, frequently erroneous, and, most crucially, impossible to gather in near-real time [62]. The data from satellite sensors, on the other hand, are always available and may be used to predict the beginning of drought, as well as its duration and magnitude [63]. NOAA's National Environmental Satellite Data and Information System (NESDIS) uses the Advanced Very High-Resolution Radiometer (AVHRR) data to report on global vegetation conditions regularly for North America using the Evaporative Stress Index (ESI). Other national and global watch centres for drought exist across various platforms all over the world using a variety of indices to estimate drought conditions and provide real-time updates based on the design of the system. Data from these space-based platforms have played an increasingly important role in drought studies over the last decade, with the geographical and temporal advantages that remote sensing can provide [64]. Furthermore, the increase in space missions in Europe, America, China and Japan, to mention a few, developments in algorithm, as well as the advent of space/cloud-based processing and storage capacity, have substantially increasedthe potential of remote sensing for drought studies [65; 66] in the last two decades. Apart from providing an independent observational capability, remote-sensing data allow for the reduction of uncertainty and constraining of drought prediction modelling attempts [67]. With all of these advancements, growing numbers of research on drought monitoring and impacts have been published [68; 69]. While there has been significant and important research on drought monitoring and its various impacts, with several of these studies emphasising the importance of integrated drought monitoring [70], there has been no systematic review focusing on some of the recent advances in multi-sensor remote sensing for drought studies, and how these might further advance the field [10]. Drought has complicated environmental effects and can affect multiple ecosystem components at the same time, as is well known [70; 71]. A single drought index, when used alone, is unlikely to convey the complexities of process interactions and different drought impacts, whereas multisensor platforms, aided by multivariate retrievals, may better depict the extent and severity of drought conditions [70]. Drought may currently be explored from a variety of aspects using current remote sensing products, such as precipitation, air and land surface temperature, soil moisture, evaporation, total water storage and vegetation health [72]. Some remote-sensing platforms provide continuous long-term drought-related information for usage at a large scale, including the Advanced Very High-Resolution Radiometer (AVHRR) from the National Oceanic and Atmospheric Administration (NOAA) satellites that provide global coverage from 1979 to the present [73], and the Moderate Resolution Imaging Spectroradiometer (MODIS) by National Aeronautics and Space Administration (NASA) [73]. Only multi-sensor/multi-platform and remote-sensing data fusion can provide such long-term coverage. Prior to now, a single satellite sensor was unable to provide real-time drought information with high spatiotemporal resolution until the recent addition of MODIS and aperture radar [74; 75], as traditional remote-sensing approaches generally require a compromise between spatial resolution and temporal frequency [76].