Earth

as a single complex system

is changing; these changes affect every aspect of life on the planet and include profound implications on society. Studying the Earth system is essential to understanding the causes and consequences of climate change and other environmental concerns. Space-based observation systems explore interactions among the atmosphere

oceans

land surface interior

ice sheets

biology

and life itself under global coverage and a daily timescale. The present development trend of Earth system study focuses on the important circulation processes（such as energy cycle

water cycle

and biochemical cycle） that drive the Earth system. The water cycle is the most active cycling process and is thus the core foundation of scientific issues in the Earth system. To improve Earth system science and global change research

accurate observations of various spatial-temporal processes and key parameters of the global water cycle should be derived. Moreover

significantly improving the accuracy and synchronization of key water cycle parameter measurements necessitate the development of innovative theory and techniques for observation and retrieval.