The majority of Alpine river discharge is older than one month

A recent study published in the journal "Hydrology and Earth System Sciences" provides new insights into the age composition of Swiss and Austrian Alpine rivers. The researchers used stable water isotopes – the fingerprint of water – to precisely trace the origin of river water.

Data series of monthly isotope measurements were collected over several years to decades to investigate the temporal and spatial variability in 32 catchments in the Alps. The stable water isotopes in precipitation and runoff were used to analyse the proportion of 'new water', i.e. from recent precipitation. The study external page Monthly new water fractions and their relationships to climate and catchment properties across Alpine rivers shows that the majority (on average about 93%) of runoff water in Alpine rivers is much older than one month.

We also analysed how climate and landscape characteristics affect the age distribution in the rivers: we find more old water in higher catchment areas, in steep areas with large differences in altitude and correspondingly larger storage volumes. In winter and during dry periods, the proportion of recent precipitation in the runoff is particularly low. In wetter periods there is more recent precipitation in the rivers, but even then well over 70% of the runoff is still older than one month. This means that even during floods, only a small proportion of the runoff consists of recent precipitation.

The old water paradox

The 'old water paradox' in hydrology describes the surprising observation that during rainfall or snowmelt, most of the water in rivers and streams does not come from recent precipitation, but from 'old water' that has been stored in the catchment for weeks, months or even years. This is paradoxical, as one would expect that it is mainly the recently available water (from precipitation and snowmelt) that ends up in runoff. However, this is not the case, as we know from observations that new water seeps away and old water already stored in the soil or groundwater is displaced from the subsurface and then enters the rivers. The new water often takes longer to travel across the landscape, while the old water, which is already stored closer to the river, is mobilised more quickly, leading to flooding during heavy rainfall events. These findings are not new, but the new study is the first to systematically analyse and demonstrate these processes for the major rivers in Switzerland and Austria.

Implications for water Management

The results of the study are particularly important for water managers and planners in the Alpine region. The lead author of the study, Marius Floriancic from the ETH-Department of Civil, Environmental and Geomatic Engineering explains: “Our research highlights the importance of both climatic and site-specific characteristics for the age composition of Alpine rivers. Understanding the dynamics of new water is crucial for predicting how water availability and quality might change under different climate scenarios and land uses.”

Professor Christine Stumpp from BOKU University emphasises the importance of water isotopes as natural tracers: 'Water isotopes give us crucial information about the age of the water, its origin and hydrological processes. For example, we have been able to show that much of the water in Alpine rivers comes from underground sources, i.e. water stored in the soil and groundwater. “

This knowledge can help in the development of adaptation strategies to ensure a reliable water supply in times of climate change. Sufficient underground storage of precipitation is therefore particularly important. Soil and groundwater act as flood buffers and water can only be stored temporarily if there is sufficient storage volume.