Inferences from catchment-scale tracer circulation experiments

In this paper the mechanisms determining the mobilization and transport of 11 solutes driven by rainfall through runoff pathways at catchment scales are 12 investigated through the analysis of tracer experiments. The hydro-chemical 13 response of a small catchment in Northern Italy has been monitored in con14 tinuous during four weeks by properly measuring rainfall rates, streamflows 15 and stream flux concentrations. The chemical response has been analyzed 16 by employing two different tracers: nitrates from diffuse agricultural sources (NO− 17 3 ) and lithium from a point injection (Li+). A modelling exercise sim18 ulating the observed hydro-chemical response of the test catchment has also 19 been carried out. Inferences from the comparative analyses prove instructive, 20 in particular concerning the scaling of mobilization processes and the age of 21 runoff water. Indeed, the interactions between old and new water were found 22 to be central to understand the mechanisms driving the transfer of solutes 23 and pollutants from soil to stream water. The modeling exercises also evi24 denced the noteworthy potential of the formulation of transport by residence 25 time distributions to describe large scale solute transport processes.