# Reviewing the Potential of Sentinel-2 in Assessing the Drought
## 3 Progress in Remote Sensing of Drought from a Climatological and Ecological Perspective
### Remote Sensing of Surface Water and Wetland Analysis in Drought  
The impacts of drought on surface water, especially its seasonal changes in quantity and quality, are well documented. Spatio-temporal analysis of surface water is the main objective in most drought studies. Drought is a phenomenon that occurs in an area, due to extreme climate or lack of water. They are among the most harmful and expensive natural disasters in the world [178; 179; 180; 181]. Droughs are becoming more common around the world in varying frequencies and intensities and are affecting developed and underdeveloped countries [181; 182]. They have a large impact on agriculture, economy, water resources, environment, and society [183; 184; 185]. The impacts of drought on wetland and surface water can be monitored effectively by analyzing meteorological, hydrological, and hydrogeological information. Drought brings new challenges to the wetland ecosystem as its distribution and functions are subjected to the impact of drought. It is estimated that wetland stores 39% of the terrestrial carbon, though it only covers 9% of the total Earth's surface. Wetlands are a fragile ecosystem, unique and natural landscape features with valuable functions such as protecting the environment, wildlife habitat, water quality, storing floodwater, and maintaining the water levels in the dry season [186]. Inversion of the PROSAIL radiative transfer model was applied to retrieve the LAI of the wetlands ecosystem using Sentinel-2 and RapidEye data. The derived LAI was validated with the field measurement and its statistical relations were assessed. The existing vegetation map was used to mask out regions apart from the region of interest and was used for the inversion process. The research concluded that the Sentinel-2 derived LAI has a higher accuracy rate compared to the RapidEye, irrespective of its fine spatial resolution. Considering climatic factors such as temperature, rain, and evaporation can directly or indirectly affect wetland ecosystems, it can be concluded that there is a certain connection between the change of the wetland ecosystem and the occurrence of drought. The onset of drought is mainly due to lack of rain, the presence of high temperatures, and evaporation. These phenomena can lead to changes in wetland ecosystems, such as wetland drainage and wetland reduction, as well as water supply, which directly leads to long periods of drought, and thus disturbs the balance of ecosystems [187]. Numerous studies have predicted that droughts will be more frequent and much more severe primarily due to climate change and human activities [188; 189; 190; 181]. Surface waters and wetlands are very important ecosystems on planet Earth because they provide a habitat for many plant and animal species. Wetland habitats are defined as ecotones, i.e., transitional habitats between the dry land and the water body [191]. That is why it is very important to monitor these ecosystems economically and efficiently because in that way we protect wetlands. By monitoring and mapping changes in wetlands, we monitor drought and reduce the risk of drought events. Satellite data offers great potential for wetland monitoring and has been the subject of many studies [192]. The use of multi-temporal images increases the accuracy of classification results [193] and allows the monitoring of changes in wetlands over one or more seasons. In observing water and wet surfaces on satellite images, strong absorption was observed in the near-infrared (NIR) and shortwave infrared (SVIR) spectrum, while in contrast, dry soil and vegetation have weaker absorption in these spectral ranges [192]. Therefore, these spectral bands and the vegetation indices derived from them, represent great potential for measuring certain parameters necessary for wetland detection and as such are good indicators for mapping wetlands and the demarcation of water bodies [194; 195; 196; 197].