Freshwater availability on Earth is continually influenced by the movement of water through the global water cycle. Precipitation delivers water to land surfaces, where it collects in soil, rivers, and lakes, supporting ecosystems and human needs such as drinking, watering crops, and power generation.

Understanding where and how much rain or snow falls globally is critical for monitoring freshwater availability, managing agricultural needs, and responding to natural disasters. As water moves through Earth's four spheres—the hydrosphere, geosphere, atmosphere, and biosphere—it continuously evaporates, condenses, precipitates, and flows, driving the distribution of accessible freshwater across the planet.