Lessons Learned from the Spatiotemporal Analysis of Long‐term and Time‐variable Young Water Fractions of Large Central European River Basins

The travel time of precipitation entering a catchment and leaving it as streamflow varies according to the flow paths it takes, with fast travel times posing a high risk to river water quality. However, investigating influences on travel times is challenging due to the complex water flow through heterogeneous landscapes. Recent studies investigated the fraction of streamflow younger than approximately three months (Fyw) using multi-year data (long-term Fyw) or one-year calculation windows to investigate its time-variability (time-variable Fyw). Nonetheless, the influences on time-variable Fyw and the demarcation between long-term and time-variable are not clear yet. Here, we investigated the long-term, the time-variable, and the Fyw derived from an exponential TTD model of nine major catchments in Central Europe and compared them to catchment characteristics and hydrometeorological variables. Additionally, one- to eight-year calculation windows were used and its impact on the variability of time-variable Fyw was investigated. All three methods of estimating Fyw led to similar results, indicating spatial organization of water flow in Central Europe. Spatial analysis further indicated a negative relationship between Fyw and catchment altitude. Contradicting and lacking spatiotemporal relationships to other investigated variables pointed to possibly unknown, region-specific influences on Fyw. With increasing calculation window size, the variability of time-variable Fyw results decreased. Long-term Fyw depended on the method used to define “long-term”, and many time series related factors, beside the actual target of investigation, i.e., catchment water flow, impacted Fyw. This finding points to difficulties in comparability of studies and catchments when using different window sizes, and we thus recommend future studies to calculate long-term Fyw using all data and one- to several-year time-variable Fyw to facilitate comparability.