## 4 Discussion
### EO Studies per Thematic Area: Limitations, Research Gaps, and Future Directions  
A general limitation on EO research which was identified in all thematic areas is the lack of in situ (ground truth) data. This issue has many implications such as on the validation of the EO outputs and on the further development of algorithms and models. The increasing number of countries and organizations adopting open data policies will partially resolve this issue, but inevitably there is a need for research infrastructure investments, especially in data-scarce regions. Our review revealed a wide range of EO applications in the thematic area of atmosphere. Similar to the findings of [PERSON] et al. [30], we found that many studies in the EMMENA focused on dust (PM\({}_{10}\)/PM\({}_{2.5}\)), as this is a common problem among these countries. Also, most of the EMMENA countries are situated in water-scarce semi-arid and arid climate zones. Thus, as expected, EO research on precipitation estimates has a high research impact. In addition to this, the rise and peak of the coronavirus pandemic occurred within the period selected for this review, thus many studies related to the virus outbreak received high research interest. Future studies on the thematic area of atmosphere should focus on the development of EO methods for improved monitoring of air quality [30]. Figure 8: Number of EO publications per year for the EMMENA region (blue line) and the world (red line). Most studies in the water thematic area focus on groundwater research, as shown both by the article review and the results of the citation topics. However, there is a range of topics where EO applications can contribute such as monitoring the water level of lakes, dams, and rivers with satellite radar altimeters and identifying the presence of different substances in water based on their unique spectral signatures, which can be measured by satellites with thermal and optical sensors and provide further information on water quality parameters (e.g., chlorophyll-a, total suspended sediments, colored dissolved organic matter) [212]. In addition to this, the management of transboundary rivers at a watershed level will be an emerging issue as water resources continue to diminish in the EMMENA region. Thus, EO applications may become a necessity in the emerging topic of hydrodiplomacy. EO research in the agriculture thematic area covered a wide range of applications. According to the World Bank, due to population growth agricultural production will need to expand by approximately 70% by 2050 to cover the food need. Thus, it is important to develop sustainable farming practices, such as precision (smart) irrigation through EO. Such studies are having a great impact on agriculture, especially in water-scarce regions such as EMMENA. However, our review did not reveal high-impact studies on this topic. Also, there were no studies on the emerging subject of the photosynthetic activity of plants based on solar-induced fluorescence (SIF) which can be obtained by the Sentinel-5 satellite mission [213]. Furthermore, research on pest and disease detection, especially in the early stages, was limited. There is a great potential to advance EO research in this direction. The thematic area of disaster risk reduction can be considered the most impactful in terms of research interest, as it has on average a higher number of citations than the other thematic areas. This was expected, as this thematic area deals with human casualties and economic losses. However, our review identified research carried out only on landslides and floods. Unexpectedly, forest fires were not included among the top 20 articles. The choice of the proper parameters needed and the variety of algorithms available (machine learning, artificial intelligence, hybrid) along with the rapid development in the field of big Earth data have created confusion on which approach is more suitable for landslide and flood mapping.