Snowmelt drives a large portion of streamflow in many mountain areas of the world. However, the water pathways since snow melts until water reaches the streams, and its associated transit time is still largely unknown. Such processes are important for drawing conclusions about the hydrological role of the upstream snowpack after melting. This work analyzes for first time the influence of snowmelt on spring streamflow in years of different snow accumulation and duration, in an alpine catchment of the central Spanish Pyrenees. A multi-approach research was performed, by combining the analysis of climatic, snow, streamflow, piezometric levels, water temperature, electrical conductivity and isotopic (δ 18O) data. Results show that snow played a preeminent role on the hydrological response of the catchment during spring. Liquid precipitation during the melting period also determined the shape of the spring hydrographs. When snow cover disappeared from the catchment, soil water storage and streamflow showed a sharp decline. Consequently, streamflow electrical conductivity, temperature and δ 18O showed a marked tipping point towards higher values. The fast hydrological response of the catchment to snow and meteorological fluctuations, as well as the marked diel fluctuations of streamflow δ 18O during the melting period, strongly suggests soil storage was small, leading to short meltwater transit times. As a consequence of this hydrological behavior, independently of the amount of snow accumulated and of melting date, summer streamflow remained always low, with small runoff peaks driven by rainfall events. The expected reduction of snow accumulation and duration in the area in a next future will bring an earlier snowmelt and rise of stream water temperature. However, given the low storage capacity of the catchment and the contribution of rainfall events to spring runoff, the annual water balance and the runoff seasonality of the catchment would not change drastically.
