IAEA Develops Method to Assess River Flow Contributions Using Isotope Hydrology

This analysis has enabled the determination of the "young water fraction"—the portion of river water that is less than two to three months old.

The International Atomic Energy Agency (IAEA) has developed a groundbreaking method to evaluate the contributions of different water sources to river flow, detailed in a new publication set to be released on July 17 in Nature Water. By utilizing isotope hydrology techniques, the IAEA has analyzed the stable isotopes of oxygen and hydrogen in water molecules from 136 perennial rivers and 45 large catchments worldwide. This analysis has enabled the determination of the "young water fraction"—the portion of river water that is less than two to three months old.

Additionally, the study introduces a dynamic water retention indicator to describe river flow dynamics. High dynamic water retention suggests that water moves slowly through the catchment, resulting in a slower response to hydroclimate events. Conversely, low dynamic water retention indicates rapid water movement and a faster response to such events. Key factors influencing dynamic water retention include land-use changes (e.g., crop and forest cover) and climate changes (e.g., air temperature and precipitation).

Why Do Rivers Flow When It’s Not Raining?

Rivers are vital to our planet, supporting ecosystems, providing nutrients to marine environments, offering transport routes, generating hydroelectric power, and providing recreational opportunities. They transport life-sustaining water from one region to another, even in the absence of rain, due to contributions from sources such as direct precipitation, surface runoff, interflow through soil, and baseflow from groundwater. The relative proportions of these sources vary greatly depending on the river's catchment area. The transport time for these water components ranges from immediate (direct precipitation) to significantly older (groundwater that has resided in the catchment for more than a year).

Understanding River Dynamics

Climate change and land-use changes are significantly altering water movement through river catchments, affecting the "age" of water in rivers. This process, akin to river aging, has profound impacts on the services rivers provide. For instance, during the European summer of 2023, the Rhine River's levels dropped so low that it disrupted the flow of goods and services.

Implications for Hydrological Risk Assessment

Catchments with low dynamic water retention are predicted to be more susceptible to hydrological extremes such as droughts and floods. Therefore, dynamic water retention can serve as a crucial indicator for hydrological risk assessment, helping to predict and mitigate the impacts of climate and land-use changes on river systems.

By understanding and monitoring these dynamics, we can better adapt to and manage the challenges posed by a changing climate and evolving land-use patterns, ensuring that rivers continue to provide their essential services to ecosystems and human societies alike.