SPECTRAL ANALYSIS OF TRACER TIME SERIES: A WINDOW INTO CATCHMENT PROCESSES
By comparing the spectral signatures of rainfall-derived reactive and passive tracers (like sodium and chloride in seasalt), one can measure chemical retardation of reactive compounds at whole-catchment scale (Feng et al., J. Hydrol., 2004). This whole-catchment chemical retardation factor is much smaller than one would estimate from chemical analyses of catchment soils. This implies that catchment flowpaths are chemically isolated from most of the subsurface.
One can also use spectral methods to analyze long-term hydrologic (water flux) time series. Hydrologic time series measure the downslope propagation of the hydraulic potentials that mobilize runoff, whereas chemical tracer time series clock the propagation of water itself through the catchment. Spectral analyses of water fluxes imply that hydrologic signals are transmitted downslope more rapidly, and with much less dispersion, than chemical tracer signals are. Thus small upland catchments transmit hydraulic potentials (which drive runoff) much less dispersively than they transport water itself. These observations provide important constraints for models of catchment processes.