Global Lake and Reservoir Storage

Surface water bodies sustain diverse, complex societies and ecosystems. Knowing the extent of human influence on these surface water bodies is essential for the sustainability of freshwater resources on Earth. However, tracking global surface water variability is challenging, particularly for small water bodies. Water level data for reservoirs are often proprietary, inaccessible, or provided in idiosyncratic formats that hinder global-scale analyses. At the same time, few natural water bodies are gauged, especially in remote areas and in less industrialized countries. Although radar altimeters are capable of tracking reservoir storage from space, their coarse resolution limits their applicability to only a couple of hundred of the world’s largest water bodies.

The launch of NASA’s ICESat-2 satellite in October 2018 opened up new possibilities for observing surface water from space by providing very high resolution and highly accurate observations of water level. By merging ICESat-2 retrievals with a global water mask and two global reservoir databases, we produced a first global dataset of seasonal water storage variations in 227,386 individual water bodies. We found that seasonal variability in human managed reservoirs averages 0.86 m, whereas natural water bodies vary by only 0.22 m. Natural variability in surface water storage is greatest in tropical basins, whereas human-managed variability is greatest in the Middle East, southern Africa and the western USA. Overall, we found that 57% of the Earth’s seasonal surface water storage variability occurs in human-managed reservoirs. This percentage is even higher south of 45°N and in many arid and semi-arid regions such as the western US, Middle East, Australia and Southern and Western Africa, where it rises to 90% or higher.

This work only represents one step in what is a massive growth in our capacity to observe surface water from space. I am currently working on fusing ICESat-2 with other types of satellite data to improve the temporal resolution of ICESat-2 height retrievals and therefore enable higher resolution tracking of lake and reservoir storage variability. Furthermore, with the upcoming launches of NISAR (in 2021) and SWOT (in 2022), the ability to observe both fluctuations in water area and in water level will significantly increase in the coming years, therefore enabling unprecedented tracking of the global water cycle.

Relevant Publications

Cooley, S.W., J.C. Ryan and L.C. Smith (2021), Human alteration of global water storage variability, Nature

Ryan, J.C., L.C. Smith, S.W. Cooley, L.H. Pitcher, and T.M. Pavelsky (2020), Toward global characterization of inland water reservoirs using ICESat-2 altimetry and climate reanalysis, Geophysical Research Letters