Biogeosciences Editors' Vox

Tracing Water Through the Critical Zone

The authors of a recent paper in Reviews of Geophysics describe how isotope hydrology offers new insights into interactions at the interface between soil, vegetation, and the atmosphere.

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The “critical zone”, the Earth’s surface and near-surface environment comprising air, living organisms, water, soil and rock, is now widely recognized in an integrated, holistic way. Researchers from across disciplines are studying the complex interactions between different elements. A recent review article in Reviews of Geophysics explored water in the soil-vegetation-atmosphere interface. The editors asked two of the authors some questions about methodological developments in this field that are improving our understanding of hydrological processes and vegetation response to climate change.

Why is the soil-vegetation-atmosphere interface important from a hydrological point of view?

The soil-vegetation-atmosphere interface plays a crucial role in the water cycle, because this is where rainwater gets divided into groundwater recharge, soil water storage, evaporation and transpiration. A proper understanding of the interactions of these processes is therefore pivotal to evaluate the impact of human activities on water availability and to correctly represent water fluxes in hydrological models. Because of environmental alterations due to land-use and climate change, quantifying the feedbacks between soil, vegetation and the atmosphere is needed to assess the current state of the critical zone and to be able to estimate potential future changes to this system.

What are the advantages of an interdisciplinary approach in this field of study?

While further research is needed for a better understanding of the processes taking place within each hydrological compartment (e.g., mineral-water interactions, water mixing in soil pores and plant tissues), the fluxes between the compartments are of main interest. In this context, interdisciplinary collaborations will foster the understanding of soil-vegetation-atmosphere interactions. For example, an intense exchange between soil hydrologists and tree physiologists will provide an integrated view on current ecohydrological problems. This interdisciplinary approach applies to experimental efforts to observe the water fluxes between soils, vegetation and the atmosphere, but also to model developments on scales ranging from the plot to the global scale.

What kind of methods or measurements are used to understand hydrological processes at the soil-vegetation-atmosphere interface?

One effective way to track water fluxes within the soil-vegetation-atmosphere interface is the use of stable isotopes of water (18O and 2H). Being part of the water molecule itself, this natural tracer has provided valuable insights into the fate of the water in the hydrological cycle over the last six decades. With technical developments in the last 15 years, easier sampling of stable isotope data has allowed a wider application of water stable isotopes as an environmental tracer. As new sampling strategies are being developed (e.g. in-situ measurements of 18O and 2H), we see new opportunities for isotope hydrology. We can conduct stable isotope measurements in soils, vegetation, and the atmosphere with an improved sampling coverage (to cover the variation in space and have replicate sampling) and higher sampling frequency (variation in time). However, we also face new challenges as it is currently not fully understood how well the water is mixed in the soil pore space or plant tissues.

How can these methods contribute to a better understanding of vegetation response to climate change?

There is an immense prospect in the application of water stable isotopes to understand how vegetation uses water from the soil, as shown in our review. With better understanding about from where in the subsurface plants take up water and how this water uptake changes over time, we will be able to improve estimates on how vegetation will respond to future alterations of plant water availability (e.g. droughts or extreme precipitation) due to climate change. Experimental data has revealed new insights into the soil-water-atmosphere interactions. Including such findings into hydrological models—on scales ranging from a soil profile to continents—will result in an improved representation of the physical processes, which leads to a more robust simulation of hydrological responses to climate change scenarios.

What are some of the major unsolved or unresolved questions in this research area?

Water in soil can undergo evaporation, transpiration, mixing and percolation leading to great subsurface variability over space and time. Thus, we need a better understanding of the variation in the stable isotopic composition of water in the soil. However, different methods of soil water isotope analysis have generated different results. Therefore, the interpretation of data from different sampling methods also needs to be better understood. These current efforts and developments of stable isotope hydrology in the context of the soil-vegetation-atmosphere interface are promising to further improve our understanding of the processes taking place within the critical zone, that sustains nearly all terrestrial life.

—Matthias Sprenger, Northern Rivers Institute, School of Geosciences, University of Aberdeen; email: [email protected]; and Markus Weiler, Faculty of Environment and Natural Resources, University of Freiburg