Revealing the Secrets of Forest Water Cycles 

Forest soils may appear drier but store significant amounts of older water from previous months and seasons. A hydrologist revealing the mysteries along the forest water cycle. 

Dr. Marius Florciancic is a hydrologist and senior scientist at ETH Zürich, where he studies the mysterious dynamics of water in forest ecosystems. Based at the Department of Civil, Environmental and Geomatic Engineering, his research integrates fieldwork and large-scale data analysis to uncover how water moves through soil, vegetation, and the atmosphere. He is also the scientific lead of “WaldLab – Experimental Forest Site,” an outdoor forest laboratory within the city of Zurich, where long-term experiments explore the complexities of ecohydrology. 

In this conversation, he explains the puzzling “old water paradox,” why forest soils hold older water than expected, and how do-it-yourself science tools can transform our understanding of nature. 

For a long time, the dominant assumption in hydrology was quite intuitive. You see rainfall, and shortly after, a flood forms in a nearby stream or river. The logical conclusion was that the water in the flood must be the same rainfall that had just fallen. It made sense observationally, but ignored a crucial dynamic. 

What we discovered through tracer studies is that the water leaving a catchment during a storm is often much older than the rain that triggered the flow. That is the paradox. Rainfall does not directly become streamflow. Instead, the new rain infiltrates into the ground and displaces older water that has been stored in the subsurface, sometimes for months or even decades. It is that older water that dominates the runoff. This radically changes how we think about flow generation, storage, and the resilience or vulnerability of a catchment is to drought and flood extremes. 

WaldLab is special because of both its accessibility and the type of forest we study. It is a managed beech-spruce forest that reflects the typical Central European woodland. But what really gives us an edge is proximity. It is just a short walk from our offices, which means we can collect high-resolution data, maintain long-term experiments, and monitor changes across seasons with minimal logistical barriers. 

This kind of closeness allows us to do things that would be too resource-intensive at remote sites. For example, we can monitor soil moisture and water chemistry every day of the year, including in winter, when most field studies typically shut down. Another distinctive element is public engagement. We regularly open the site to schools, local communities, and practitioners like foresters. This two-way exchange ensures our research is not only academic but also practical and socially relevant. 

This was an unexpected result that carries serious implications. Most people assume shallow soil water is “new”,  that is, rain from the last few days or weeks. But what we found is that much of this water originates from winter precipitation, sometimes several months old. This is especially interesting because Zurich and many similar regions in Central Europe receive most of their rain in the summer. 

However, soil water recharge is far more effective in winter. In cooler months, plants are dormant, evaporation is low, and precipitation can fully infiltrate the ground. In summer, by contrast, much of the rain gets intercepted by tree canopies, evaporates, or is trapped in the forest floor’s organic matter before it can reach the soil. This has major implications for climate resilience. If warming winters bring less snowfall more erratic rainfall, and higher evaporation, forests may struggle to store enough water for the growing season ahead. 

Commercial sensors work well for standard parameters like rainfall or groundwater level. But the more nuanced processes we study often require us to invent new tools or modify existing ones. With the rise of affordable microcontrollers and open-source software, we have started building our own measurement devices tailored to our research needs. 

For instance, we repurposed camelback-style hiking bladders and fitted them with pressure sensors. Then we placed dead wood on top to create a kind of weighing scale that tracks how much water the wood absorbs and releases. We discovered that dead wood takes up water from dew and fog during the night, then releases it throughout the day. This creates a subtle but steady moisture exchange that helps maintain a humid forest microclimate. These DIY tools are relatively cheap, adaptable, and allow us to study micro-processes that commercial instruments simply were not designed to capture. 

Yes, our measurements suggest that forests receive significantly less effective rainfall than open land. About 20 percent of annual precipitation never reaches the soil because it evaporates from the canopy. Another 18 percent is intercepted by and evaporated from the litter layer, the leaves, dead wood, and organic debris on the forest floor. That is nearly 40 percent of rainfall lost before it even touches the soil. 

In Zurich, this means forests may receive only about 600 millimetres of effective precipitation per year. That is very little, especially in light of climate change with increasing evaporative demand. Compounding the issue, trees themselves are large consumers of water, further depleting the available water the soil. All of this paints a more fragile picture of forest hydrology than previously assumed. If forests are getting significantly less water than we thought, their vulnerability to heatwaves, droughts, and long dry spells increases accordingly. 

One of the most powerful levers in forest resilience is soil health, which is closely tied to organic matter. Rich organic soils are like sponges. They store more water, retain nutrients, and support microbial communities that aid in decomposition and nutrient cycling. This organic layer is built up over decades and must be actively preserved if we want long-term ecosystem functioning. 

We also found that closed canopies and high humidity beneath the trees reduce atmospheric demand for water. That makes trees less stressed during heatwaves. As a hydrologist, I always advocate for species diversity not just for ecological reasons, but also for water dynamics. Different trees have different rooting depths.  Some pull water from shallow layers, while others pull water from deep underground. When these species are combined, it reduces competition and improves the overall water-use efficiency of the forest. That is a key strategy in climate-smart forest design. 

Most of our experiments are set up with long timescales in mind. We have accepted that the true value of this research will unfold over decades. Personally, I will not see many of the final results from what we are starting now. But that is the point. Climate change is a long-term phenomenon. If we want to understand how forests evolve, adapt, or degrade under changing conditions, we need equally long-term datasets. 

Our hope is that in the future scientists can build on what we have started, refining our models and exploring new questions we have not even thought to ask yet. With continuous data collection over a century, WaldLab can serve as a living observatory for the water cycle, biodiversity, and forest-climate interactions. It is our contribution to a future where science is proactive, not just reactive.