Frequency–magnitude relationships for precipitation, stream flow and sediment load events in a small Mediterranean basin (Vallcebre basin, Eastern Pyrenees)

Precipitation, discharge and suspended sediment concentrations were continuously measured during 10 years (1995–2004), at the Ca l'Isard sub-basin (1.32 km2) of the Vallcebre experimental catchments. Daily precipitation record is 22 years long. When the ranks of the events obtained with the diverse variables considered (precipitation depth, peak discharge, runoff depth, suspended sediment concentration and sediment load) were compared, the results confirmed the complexity of the response of the catchment and demonstrated that precipitation is an irregular criterion for ranking the main sediment transporting events. When the partial duration series corresponding to the 10% major events were analysed, daily precipitation, peak discharge and sediment concentration series showed good fits with log-normal distributions, but event precipitation, runoff and sediment load series needed bi-modal log-normal distributions. This behaviour may be attributed to the fact that event mass magnitudes depend not only on event intensity but also on its duration.