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An analysis of the relative effects of transpiration and evaporation, which can be distinguished by how they affect isotope ratios in water, shows that transpiration is by far the largest water flux from Earth’s continents, representing 80 to 90 per cent of terrestrial evapotranspiration and using half of all solar energy absorbed by land surfaces.

Direct observations of 4.2 million wells across the USA indicate that many streams are potentially losing water to underlying aquifers.

Groundwater that predates the Holocene is commonly assumed to be unaffected by modern contamination. A global analysis of fossil groundwater suggests that modern contaminants are present in deep wells that tap fossil aquifers.

Analysis of about 170,000 monitoring wells and 1,693 aquifer systems worldwide shows that extensive and often accelerating groundwater declines are widespread in the twenty-first century, but that groundwater levels are recovering in some cases.

Most Brazilian rivers are found to have the potential for stream water losses into underlying aquifers, especially in drier climates, thicker aquifers, and regions with extensive groundwater pumping.

How groundwater recharge changes with global warming is not well constrained. Here, the authors use an empirical relationship to show that groundwater recharge is more sensitive to aridity changes than expected, implying a strong response of water resources to climate change.

Decades-old groundwater is vulnerable to pollution from land uses. This work shows that decades old groundwater flows to deep depths where groundwater pumping is more intense, implying that groundwater pumping can endanger deep groundwater quality.

The authors here investigate in the susceptibility of coastal aquifers to seawater intrusion. Based on 20 years’ worth of observational data, the study finds that 15% of the US coastline is affected by landward hydraulic gradients conducive to seawater intrusion.

Groundwater supplies about 59% of global river flow, suggesting a larger contribution of groundwater to the global water cycle than currently appreciated, according to an analysis integrating estimates from models and observations.

Plant transpiration is the largest continental water flux. Research now shows that climate and water availability projections are highly sensitive to the ways that plant responses to changing atmospheric conditions are represented.

Streamflow is a mixture of precipitation of various ages. Oxygen isotope data suggests that a third of global river discharge is sourced from rainfall within the past few months, which accounts for less than 0.1% of global groundwater.

At depths of over 500 m, deep groundwater has long residence times and likely contributes less than 0.1% to global streamflow and only sporadically connects with the surface terrestrial water cycle on geological timescales, according to estimates derived from the chloride mass balance approach.

Fossil groundwater has been under the ground for more than ~12 thousand years. Here the authors show that many wells in the United States tap fossil groundwater resources, and that the frequency that wells are drilled into fossil aquifers is increasing, highlighting the importance of safeguarding fossil groundwater quality and quantity to meet present and future water demands.

Groundwater wells in the United States are under more stress than ever before due to drought conditions and rising demand, but the extensive nature of deeper drilling has been unreported. This analysis compiles nearly 12 million groundwater wells across the United States to determine water vulnerability and sustainability.

Groundwater recharged less than 50 years ago is vulnerable to contamination and land-use changes. Data and simulations suggest that up to 6% of continental groundwater is modern—forming the largest component of the active hydrologic cycle.

Soil water is usually assumed to be equally available for all purposes, supplying plant transpiration as well as groundwater and streamflow; however, a study of hydrogen and oxygen isotopes from 47 globally distributed sites shows that in fact the water used by plants tends to be isotopically distinct from the water that feeds streamflow.

Irrigation accounts for a substantial proportion of global water usage and can have biophysical and biogeochemical impacts on Earth systems. This Review outlines key irrigation–Earth system interactions, and discusses the effect of future climate and socioeconomic changes on irrigation patterns and their interaction.