Debris flow hazard in Alpine environment: the cases of T. di Fiernaz (AO) and T. di Valle Materlo (SO), Northern Italy

During a storm it is well known that the total discharge of a stream contains event (new) and pre-event (old) waters. Many different studies on this subject have revealed that the old water, stored from previous rainfalls, volumetrically dominates the streamflow response to storm rainfall (Kennedy et al., 1986). It has also been found that, in some cases, changes in water composition lag behind change in streamflow (Glover and Johnson, 1974; Walling and Foster, 1975; Nolan and Hill, 1990) even though field observations indicate that much of the storm runoff must be composed of new water. On the countrary, in other cases, it has been documented that the chemograph trough may precede the stream flow peak, expecially for those solute species that consistently exhibit a decrease in concentration during a storm event (Walling and Foster, 1975).