The hidden physics of watersheds: Why some are more sensitive to climate variability than others

Water is everywhere, from the snowpack in the mountains to the tap in our kitchens. But while we often think about rainfall and snow as the main drivers of our water supply, it turns out that something we rarely see has just as much influence: the underground structure of the landscape itself.

As a hydrologist, I spend my time trying to understand how water moves through landscapes. In our latest study, published in Water Resources Research, my master student Mahbod Taherian and I discovered that not all watersheds respond equally to climate variability—and the reason lies deep below the surface.

The puzzle

We focused on a process called snowmelt partitioning—what happens to snowmelt water when it reenters the hydrologic system each spring? Does it run off quickly into rivers? Infiltrate slowly into the ground to recharge aquifers? Or evaporate back into the atmosphere through plants?

Where that water goes matters, because it determines whether a watershed floods, stores water or dries out during the summer. Understanding snowmelt behavior is critical for communities that depend on mountain water for drinking, agriculture, ecosystems and hydroelectricity.

But here’s the puzzle: We know that the amount of snow or rain in a given year affects these outcomes, yet watersheds with similar climates often behave very differently. What makes one watershed resilient to a dry autumn, while another swings wildly in its behavior from year to year? That question drove our research.

Going underground

To answer that, we applied a physics-based hydrologic model to both real and virtual snow-dominated watersheds across North America and Europe. The virtual catchments allowed us to control for everything above ground—like rainfall, slope, and vegetation—and vary only the subsurface structure. We specifically looked at two key features: how permeable the soil and rock layers are, and how that permeability varies horizontally (from hill to hill) and vertically (from surface to bedrock).

Watersheds with laterally homogeneous underground structure—that is, where permeability is relatively uniform laterally across the landscape, but decays vertically—tended to route snowmelt in the same way every year, regardless of whether the preceding autumn was wet or dry. In contrast, heterogeneous watersheds with patchy subsurface permeability behaved very differently.

After dry autumns, spring snowmelt and summer rainfall would mostly run off quickly through shallow lateral paths, with high vulnerability to flooding. After wet autumns, the same watersheds would activate deeper pathways—almost like flipping a switch—routing far more snowmelt to long-term underground storage. In one of our natural experiments, the fraction of snowmelt reaching long-term storage jumped from 4% after a dry autumn to 25% after a wet one.

The mechanism becomes clear once you see it. A wet autumn fills the deeper zones of the subsurface, priming the system for vertical water movement. When snowmelt arrives the following spring, it encounters a subsurface that is already connected—pathways to deep storage are open, and water moves downward rather than laterally to streams. But in a watershed with a laterally uniform underground structure, these deeper pathways remain inactive regardless of how wet the previous autumn was. The subsurface behaves the same way year after year.

Climate amplifiers

This means that subsurface heterogeneity acts like a climate amplifier. If a watershed has variable underground structure, climate variability has more room to change how water moves through it. And that makes these watersheds more sensitive to climate swings. A single wet or dry season can fundamentally alter the watershed’s behavior for months to come.

It also means that many models we use today to predict water supply and flood risk under climate change may be missing a critical piece. Most large-scale climate impact assessments rely on surface indicators—like slope, vegetation, and land cover. But those features alone cannot explain the dramatic differences we observed. What’s missing is the invisible architecture underground.

Why this matters

These findings are particularly relevant in a changing climate. As precipitation patterns become less predictable, knowing which watersheds are more vulnerable to swings in rainfall and snowmelt becomes essential. For instance, in snow-fed regions like western North America, water managers are grappling with how to store water from increasingly erratic snowmelt to avoid both floods and shortages. Our work suggests that some of those watersheds may be inherently more variable than others—and that has everything to do with what lies below the ground.

The implications extend beyond water supply to water quality. In watersheds where snowmelt takes deep pathways, it may pick up—or leave behind—dissolved nutrients, contaminants, and legacy pollutants stored in soils and aquifers. If climate variability can switch these pathways on and off, it can also alter water quality outcomes in ways we have not fully anticipated.

Looking ahead

One challenge, of course, is that the subsurface is hard to see. We cannot easily map underground permeability at large scales with high resolution. But this is precisely why studies like ours matter: by identifying which structural features control climate sensitivity, we can better target where detailed subsurface characterization is most needed. Future work combining physics-based models with emerging geophysical and remote sensing technologies could help us see underground in ways that were not possible before.

Ultimately, what excites me most is that this work opens new doors for both science and society. For scientists, it highlights the importance of looking beyond surface features to the hidden controls on hydrologic behavior. For water managers and policymakers, it offers a roadmap for anticipating which watersheds are most vulnerable in a changing climate.

Water may be invisible underground, but its pathways shape the world we live in. To manage our resources wisely, we need to understand not just what falls from the sky, but where it goes once it hits the ground. And for that, the physics beneath our feet may matter more than we ever realized.

This story is part of Science X Dialog, where researchers can report findings from their published research articles. Visit this page for information about Science X Dialog and how to participate.

Ali Ameli is an Assistant Professor in the Department of Earth, Ocean and Atmospheric Sciences at the University of British Columbia. His research sits at the intersection of hydrology, hydrogeology, and data science, focusing on how the physical structure of landscapes governs water movement from rainfall to streamflow. He combines mechanistic modeling with machine learning and remote sensing to improve predictions of water availability, flood risk, and water quality in both gauged and ungauged basins. His work spans scales from individual hillslopes to continents, with applications to climate change impact assessment, wetland conservation, and groundwater management. He collaborates with government agencies and conservation organizations to translate scientific insights into practical tools for water resource decision-making. Ameli is a recipient of the Canadian Geophysical Union Early Career Award, recognizing outstanding contributions to Geophysics.