The 21st century is often called "the century of water." Water is an essential element of the Earth's environment and is indispensable for our life and economic activities. Many places in the world now face water problems, such as water shortages and floods, which can cause food shortages, epidemic diseases, and so on. In addition to these problems, global warming and climate change affect the global water cycle and result in abnormal weather, such as frequent heavy rains and droughts. Phenomena range from short-term, local events like heavy torrential rains to long-term, large-scale events like El Niño.

  In order to solve these problems, we urgently need to accurately determine the rainfall distribution, which is the input to water resources, and to improve the techniques of predicting and preparing for abnormal weather. For this purpose, it is imperative to monitor rainfall changes in every corner of the Earth and to recognize that we have to share the limited water resources. A comprehensive international satellite observation plan for understanding and predicting the water distribution in realtime is one way to address these problems.

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