Technical Note: Revisiting the Procedure for Quantification of the Young
Water Fraction Based on Seasonal Tracer Cycles
Abstract
The transit time (TT) of streamflow encapsulates information about how
catchments store and release water and solutes of different ages. The
young water fraction (Fyw), the fraction of streamflow that is younger
than a certain age (normally 2–3 months), has been increasingly used as
an alternative metric to the commonly used mean TT (mTT). In the
commonly used (‘traditional’) procedure presented by Kirchner (2016),
the age threshold (τyw) of Fyw separating young from old water is not
pre-defined and differs from catchment to catchment depending on the
shape of the (gamma) transit time distribution. However, it can be
argued that it is important to use the same pre-defined τyw for
inter-catchment comparison of Fyw. In this study, we propose an
alternative (‘proposed’) procedure for the estimation of Fyw with any
pre-defined τyw. This allows us to also compare the effects of data
sampling frequencies on the results of Fyw estimation using the same
τyw. We applied the traditional and proposed procedures using daily
oxygen isotope (δ18O) data in the Alp and Erlenbach catchments,
Switzerland. We found that our proposed and the traditional procedure
can give very different Fyw values. With the proposed procedure, the
estimated Fyw significantly increases when the sampling frequency
changes from sub-monthly to monthly time steps. Overall, our study
highlights the importance of the selection of τyw and the sampling
frequency in Fyw estimation, which should be given more attention.