Using time domain and geographic source tracers to conceptualize streamflow generation processes in lumped rainfall-runoff models

The temporal dynamics and geographical sources of streamflow were conceptualized in a lumped rainfall-runoff model (isoSAM(dyn)) using isotopic and geochemical tracers derived from a field study in a 3.6 km(2) upland catchment in Scotland. High-resolution (daily) sampling of stable isotopes in precipitation and streamflow over a hydrological year, supplemented by fortnightly sampling of groundwater and riparian saturation zones, allowed hypotheses of runoff generation processes to be tested. These were conceptualized in a previously developed model (SAM(dyn)) based only on geochemically defined geographic source tracers, which showed that the nonlinear dynamic expansion and contraction of riparian saturation areas is the dominant mechanism for storm runoff generation in the catchment. While SAM(dyn) resulted in reasonable simulation of streamflows and alkalinity (as a source tracer), it was unable to reproduce the rainfall-runoff dynamics of deuterium (delta H-2). Using the model in a learning framework, incorporation of parameters for passive storage in catchment hillslopes and groundwater mixing in riparian saturation zones, along with associated isotopic fractionation, improved delta H-2 simulations in stream water and the major catchment water stores. The resulting model provided a conceptualization of rainfall-runoff processes that broadly reconciled hydrometric data and geochemical and isotopic signals. However, in this particular catchment, fractionation of water in surface saturation zones appears to be a complex process that prevents the simulation of short-term isotope dynamics in the stream during the summer period.