Since the water cycle is a closed system, it should be possible to account for how much water moves in and out of Earth’s different water stores over time. Unfortunately, scientists cannot yet confidently account for where all the water is, the ‘water budget’. A review led by scientists from ESA’s Climate Change Initiative and published recently in the Bulletin of the American Meteorological Society explores why.

“We’ve made a lot of advances, especially in space-based observations of the water cycle. The Copernicus programme is proving vital for long-term, consistent satellite observations,” says lead author Wouter Dorigo, who is based at Vienna University of Technology.

The authors point to the Climate Change Initiative as it supports the development of global, long-term satellite datasets that cover many key components of the water cycle, including glaciers, ice sheets, sea level, soil moisture, lakes, snow, water vapour and permafrost.

While these data have helped improve the picture, there are still components and fluxes of the water cycle that satellites cannot monitor.

Dr Dorigo commented, “In particular, we don’t have accurate knowledge of the fluxes for groundwater use, recharge and natural discharge, which makes it difficult to project future sustainable use.”

Groundwater is, by far, Earth’s largest store of liquid freshwater, but decades of extraction has led to depleted aquifers in many regions around the globe.