Research development, current hotspots, and future directions of water research based on MODIS images: a critical review with a bibliometric analysis.

Water is essential for life as it provides drinking water and food for humans and animals. Additionally, the water environment provides habitats for numerous species and plays an important role in hydrological, nutrient, and carbon cycles. Among the existing natural resources on Earth's surface, water is the most extensive as it covers more than 70% of the Earth. To gather a comprehensive understanding of the focus of past, present, and future directions of remote sensing water research, we provide an alternative perspective on water research using moderate resolution imaging spectroradiometer (MODIS) imagery by conducting a comparative quantitative and qualitative analysis of research development, current hotspots, and future directions using a bibliometric analysis. Our study suggests that there has been a rapid growth in the scientific outputs of water research using MODIS imagery over the past 15 years compared to other popular satellites around the world. The analysis indicated that Remote Sensing of Environment was the most active journal, and "remote sensing," "imaging science photographic technology," "environmental sciences ecology," "meteorology atmospheric sciences," and "geology" are the top 5 most popular subject categories. The Chinese Academy of Sciences was the most productive institution with a total of 477 papers, and Hu CM (Chinese) was the most productive author with 76 papers. A keyword analysis indicated that "vegetation index," "evapotranspiration," and "phytoplankton" were the most active research topics throughout the study period. In addition, it is predicted that more attention will be paid to research on climate change and phenology in the future. Based on the keyword analysis and in consideration of current environmental problems, more studies should focus on the following three aspects: (1) develop methods suitable for data assimilation to fully explain climate or phenological phenomena at continental or global scales rather than at local scales; (2) accurately predict the effect of global change and human activities on evapotranspiration and the water cycle; and (3) determine the evolutionary process of the water environment (i.e., water quality, macrophytes, cyanobacteria, etc.), ascertaining its dominant factors and driving mechanisms. By focusing on these three aspects, researchers will be able to provide timely monitoring and evaluation of water quality and its response to global change and human activities.