To explore this hidden water cycle, Hoeg drew on an unusual treasure trove of data: four years of daily samples of rain, snow, and stream water collected in a mountain valley near Einsiedeln in central Switzerland. By measuring concentrations of a heavy form of oxygen, he could tell whether the water was from recent rainfall, a few weeks old, or had been stored for much longer. This allowed him to watch how the proportions of old, young and new water shifted throughout the year.

One important finding is the role of forests. In areas with more tree cover, streams carried more of the older, stored water. Forest soils and roots act like sponges, holding water back and releasing it gradually. This is important, because older water helps to keep rivers flowing steadily, especially during dry periods.

However, Hoeg’s study also revealed a weakness. In summer, when rain is scarce and evaporation is high, streams usually rely more on recent rainfall and snowmelt than on older reserves. In fact, during hot, dry months, new and young water often outweighs old water in streams and rivers. This makes rivers more vulnerable if the weather stays dry during summer.

Hoeg also discovered that the timing of water flow changes with the seasons. In autumn and winter, streams often carry water that has been stored for weeks or months. In spring and summer, snowmelt and sudden rainstorms send much younger water rushing through the system in just days.

With climate change bringing drier summers and less reliable winter snowfall, these patterns raise concerns. If streams depend more on recent rainfall, dry spells could hit harder, leading to more frequent and severe water shortages. Hoeg’s work shows that protecting forests and soils will be crucial to maintaining water reserves into the future.