Editors’ Vox is a blog from AGU’s Publications Department.
The effects of topography on the distribution and movement of water on Earth have been observed throughout history. And while many scientific advances are well-documented, there are no summaries of our current knowledge from a large-scale perspective.
A new article in Reviews of Geophysics seeks to close this gap by exploring the influence of topography on the global terrestrial water cycle, from the atmosphere down to the groundwater. Here, we asked the authors about early observations of topography’s influence, what their review covers, and what open questions remain.
What are some of the earliest recorded observations of the relationship between topography and the terrestrial water cycle?
The fact that mountains influence climate, weather, and the flow of water across Earth’s surface was recognized thousands of years ago by ancient scholars from around the world, such as Aristotle (4th century BC) or Wang Chong (1st century AD). The idea of a “water cycle” also has a long history, and early sketches of the water cycle already included mountains, like the 1641 water cycle diagram by Kircher (Figure 1). In Kircher’s water cycle, water flows underground from whirlpools in the sea up into the mountains, where it emerges as springs and rivers that transport the water back to the sea.
Based on today’s scientific understanding, to what extent were these early observations accurate or inaccurate?
The fact that topography influences the water cycle still holds true today, but we now know of course, that Kircher’s picture was inaccurate. In some parts of the world, a more accurate representation of the terrestrial water cycle existed much earlier, while in Europe, Perrault’s De l’origine des fontaines (1674) paved the way for a quantitative understanding of the water cycle. Perrault’s work demonstrated that rainfall provides enough water to sustain rivers, and that the water is not supplied via subsurface pathways fed by whirlpools but via the atmosphere.
Although the subsurface does not deliver water from sea to summit, it does play an important role in slowly delivering water back to the sea or to distant regions where it may be taken up by plants (see Figure 2). The importance of subsurface flow paths has become increasingly well understood, but a complete global picture of groundwater discharge to streams, neighboring catchments, and the sea remains elusive to this day.
Why do you think there is still more to discover on this topic?
Many aspects of the Earth system are complex, and we often cannot (or at least not directly) observe the variables that interest us. This is especially true for the “frontier beneath our feet”, which continues to remain largely invisible. We thus anticipate observational and conceptual breakthroughs and surprising new insights especially when it comes to the subsurface. In addition, many of the changes in climate and land use we currently experience are unprecedented, at least since the start of the Holocene approximately 11,700 years ago. These changes possibly lead to shifts in the relationship between topography and the water cycle that we are just beginning to see.
What perspectives does your review article add to previous literature on this topic?
One central aspect of our review is its broad scope…bringing an inherently interdisciplinary perspective to this topic.
One central aspect of our review is its broad scope, spanning atmosphere, land surface, and subsurface, thus bringing an inherently interdisciplinary perspective to this topic. To unite these disciplines in a common framework, we decided to focus our discussion on gradients and contrasts. Topography-related gradients often strongly control water fluxes and stores across different landscapes – think of precipitation gradients or contrasts between sunny and shady slopes. We think that this is a useful way of thinking which connects not only scientific disciplines, but also observations and models. As our review takes a global perspective, we also tried to bring in balanced examples from around the globe, which is of course not always easy because research activities are biased, for instance, towards high-income countries.
Your review explores the topic as a 3-part vertical cross-section: above, at, and below the Earth’s surface. What are the benefits and drawbacks of studying these as separate sections?
It seemed logical and intuitive to us to organize the review as a vertical cross-section. In this way, we could break down how topography influences water fluxes in the atmosphere, on land, and underground. This helps the reader to navigate this expansive topic, as topography influences the water cycle over a wide range of processes and on very different spatial and temporal scales. But of course, these boundaries are fuzzy: plant roots can tap into groundwater; rivers can lose or gain water from aquifers; and runoff can occur as overland flow, in the unsaturated zone or as groundwater discharge (i.e., at, just below, and deep below the surface). It is important to study these different sections and their interactions to gain a holistic understanding of the Earth system.
How is climate change expected to influence the relationship between the water cycle and topography?
The effects of climate change are still not entirely clear and require continuous measurements, data analysis, and modeling efforts.
Climate change will influence the relationship between the water cycle and topography in a variety of ways. Some examples include: a weakening of orographic precipitation gradients, possibly due to a slowing of the jet stream; changes in the climatic water balance (precipitation minus potential evaporation), which will lead to a shift in land surface fluxes and vegetation zones along elevation gradients; changes in the mountain water cycle and the flow regime of mountain rivers due to dwindling meltwater contributions. However, the effects of climate change are still not entirely clear and require continuous measurements, data analysis, and modeling efforts.
What are some of the remaining challenges where additional data or research efforts are needed?
A major challenge will be the efficient and continuous collection of observational data, especially in regions that are difficult to access, such as high mountains. Existing datasets often provide an imbalanced picture of Earth’s water fluxes and stores, as they are biased towards certain landforms and climates (see Figure 3).
Ideally, we would want to take both long-term measurements at specific locations and at many locations that cover the Earth and its spheres in a representative manner. However, this requires large collaborative efforts and may be difficult to reconcile with existing funding structures, which are usually at national level.
In addition, a better understanding of the interactions between changes in atmospheric conditions (water, energy, carbon), the land surface, and the subsurface requires both interdisciplinary collaboration and the training of a new generation of interdisciplinary Earth scientists.
—Sebastian Gnann (sebastian.gnann@hydrology.uni-freiburg.de, 
0000-0002-9797-5204), University of Potsdam and University of Freiburg, Germany; Jane W. Baldwin (0000-0002-4174-2743), University of California Irvine and Columbia University, United States; Mark O. Cuthbert (0000-0001-6721-022X), Cardiff University, United Kingdom; Tom Gleeson (0000-0001-9493-7707), University of Victoria, Canada; Wolfgang Schwanghart (0000-0001-6907-6474), University of Potsdam, Germany; and Thorsten Wagener (0000-0003-3881-5849), University of Potsdam, Germany
Editor’s Note: It is the policy of AGU Publications to invite the authors of articles published in Reviews of Geophysics to write a summary for Eos Editors’ Vox.