Reduced fraction of young water in Alpine catchments with increased seasonal snow cover

Effective water resource management can benefit from estimations of when water entered the catchment and how long it takes to flow to the outlet. In this context, the so-called young water fraction (F_yw) based on seasonal input and output tracer cycles is becoming increasingly used as robust tool to compare the hydrological function of catchments. In seasonally cold environments, this F_yw estimation is complicated by the fact that a large part of the precipitation will be in the form of snow, will be stored before melting and becoming available as water, resulting in a distinct winter low flow and summer high flow season. Nevertheless, F_yw might enclose extremely interesting information in such environments since they incorporate the relationship between late summer and autumn flow and the previous winter’s snow input.  However, most currently available methods for F_yw estimation do not explicitly account for the seasonal shift of water input from snow. Therefore, we propose a novel framework to explicitly account for this “snowmelt” delay in F_yw and explore related uncertainties using experimental data from three high-elevation Alpine catchments, the Vallon de Nant in Switzerland, and the Noce Bianco at Pian Venezia and the Bridge Creek Catchments in Italy. Experimental data from these environments expose some limitations of existing methods in accounting for unavoidable sampling inconsistencies. Using our method that explicitly accounts for snowmelt, we found extremely low F_yw in these three Alpine catchments: 6%, 13%, and 31%. In this contribution, we will present our method in detail and highlight emerging challenges and implications of the F_yw estimation.