Old Water, New Insights

At the ETH Zurich forest lab, environmental engineers are investigating the so-called "old water paradox." Their findings are not only scientifically intriguing but also highly relevant for addressing climate change and forest conservation. The research employs a range of sensors placed in the soil, on trees and in streams – including many devices from Decentlab.

"Old water paradox"
Even during heavy rain, most of the water in the soil and streams comes from older sources — sometimes months or even years old. Only a small portion comes from recent rainfall.

Data brings clarity
Thousands of water samples show that even at just 10 cm depth, about two-thirds of the water is more than three weeks old.

Winter is crucial
Soil primarily stores water during the leafless winter season. Dry winters are therefore especially critical for water availability in summer.

Litter Layer’s Big Role
About 18% of rainfall is retained in leaves, needles, and deadwood — much more than previously thought. Only 60% of the rain actually infiltrates the soil.

Different tree strategies
Beeches use water more lavishly than spruces. The latter conserve water earlier — at the expense of photosynthesis.

Practical relevance
These findings support flood risk assessment, climate-resilient forest management, and tree species selection.

Soil quality matters
Humus-rich soil with plenty of deadwood stores significantly more water. Biodiversity also increases a forest’s resilience to climate stress.

The continuous measurements provide valuable data for the entire research community. The complete datasets are planned to be published and made freely accessible in 2025.

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