Earth’s freshwater cycle out of stable state

For the past century, humans have been pushing the Earth’s freshwater system far beyond the stable conditions that prevailed before industrialization. A new study published in the journal Nature Water shows that human pressures, such as dam construction, large-scale irrigation and global warming, have altered freshwater resources to such an extent that their capacity to regulate vital ecological and climatic processes is at risk.

The study revealed an increase in the frequency of exceptionally dry or wet conditions –deviations in streamflow and soil moisture. Overall, the global land area experiencing deviations has nearly doubled compared with pre-industrial conditions. Because the analysis was done at a high spatial and temporal resolution, the researchers could explore geographical differences in the deviations. Exceptionally dry streamflow and soil moisture conditions became more frequent in many tropical and subtropical regions, while many boreal and temperate regions saw an increase in exceptionally wet conditions, especially in terms of soil moisture.

The authors calculated monthly streamflow and soil moisture at a spatial resolution of roughly 50x50 kilometers using data from hydrological models that combine all major human impacts on the freshwater cycle. As a baseline, they determined the conditions during the pre-industrial period (1661-1860) and compared it with the industrial period (1861-2005). This is the first time that global water cycle change has been assessed over such a long timescale with an appropriate reference baseline.

With this comprehensive view of the changes in streamflow and soil moisture, researchers are better equipped to investigate the causes and consequences of the changes in the freshwater cycle. The findings can function as a guide for mitigation policies and underline the urgency of decreasing human-driven pressures on freshwater systems.