Fractions of Different Young Water Ages are Sensitive to Discharge and
Land Use - an Integrated Analysis of Water Age Metrics under Varying
Hydrological Conditions for Contrasting Sub-Catchments in Central
Germany
Abstract
With ongoing climate change and more frequent high flows and droughts,
it becomes inevitable to understand potentially altered catchment
processes under changing climatic conditions. Water age metrics such as
median transit times and young water fractions are useful variables to
understand the process dynamics of catchments and the release of solutes
to the streams. This study, based on extensive high-frequency stable
isotope data, unravels the changing contribution of different water ages
to stream water in six heterogeneous catchments, located in the Harz
mountains and the adjacent northern lowlands in Central Germany.
Fractions of water up to 7 days old (Fyw7), comparable with water from
recent precipitation events, and fractions of water up to 60 days old
(Fyw60) were simulated by the tran-SAS model. As Fyw7 and Fyw60 were
sensitive to discharge, an integrated analysis of high and low flows was
conducted. This revealed an increasing contribution of young water for
increasing discharge, with larger contributions of young water during
wet spells compared to dry spells. Considering the seasons, young water
fractions increased in summer and autumn, which indicates higher
contributions of young water after prolonged dry conditions. Moreover,
the relationship between catchment characteristics and the water age
metrics revealed an increasing amount of young water with increasing
agricultural area, while the amount of young water decreased with
increasing grassland proportion. By combining transit time modelling
with high-frequency isotopic signatures in contrasting sub-catchments in
Central Germany, our study extends the understanding of hydrological
processes under high and low flow conditions.