Runoff and Sediment Transport during the Snowmelt Period in a Mediterranean High-Mountain Catchment

The hydrological and geomorphic functioning of high-mountain catchments is heavily influenced by snow accumulation and melt processes, which condition the timing and characteristics of discharges, solute outputs, and suspended sediment and bedload transport. We report here the transport of suspended sediment and solutes during the snowmelt period in a small experimental catchment in the subalpine belt of the Central Spanish Pyrenees. The seasonality of hydrological and sediment responses throughout the year was investigated using daily data of discharge, suspended sediment transport and solute outputs of the hydrological years 2003/2004 and 2005/2006. The study demonstrated the importance of the snowmelt period in terms of runoff production, and solute and suspended sediment yield: whereas precipitation during the snowmelt period (2-2.5 months) represented 10-13% of annual precipitation, discharge and suspended sediment transport accounted for up to 50% and 60%, respectively, and solute output approximately 40-50%. Solute transport dominated throughout the snowmelt period, whereas suspended sediment transport mostly occurred during the second phase of the snowmelt period (June), when an expanding area of the catchment was free from snow. The moderate daily increases in discharge, which were related to day night temperature fluctuations, were insufficient to transport bedload material. Hourly data were used for preliminary assessment of the relationships among discharge, suspended sediment, and solute concentration, which provided insights into sediment sources and delivery mechanisms. Thus, during snowmelt-related events, the sediment mobilized was most probably derived from areas near or within the channel. In contrast, during events involving both snowmelt and rainfall, the gully system near the divide contributed to sediment load. The solute concentration was inversely related to water discharge, with higher concentrations during the first half of the snowmelt period (May) than during the second half (June). The results of this study demonstrate the key role of snow accumulation and melting processes in controlling the hydrological dynamics and patterns of particulate and solute mobilization in high-mountain environments. Future changes in snow volume and duration will affect the timing of snowmelt-related spring high flows, as well as soil erosion and transport.