SPECTRAL ANALYSIS OF TRACER TIME SERIES: A WINDOW INTO CATCHMENT PROCESSES

Spectral analyses of tracer time series can be used to probe the internal workings of catchments. Spectral analyses have shown that catchments act as fractal filters for inert chemical tracers like chloride, converting "white noise" rainfall chemistry inputs into fractal "1/f noise" runoff chemistry time series (Kirchner et al., Nature, 2000). This implies that catchments have long-tailed travel-time distributions, and thus retain soluble contaminants for unexpectedly long timespans. Long-term monitoring data from North America, Britain, and Scandinavia show that this behavior is found in many diverse geologic settings. This fractal phenomenon has a prosaic explanation: advection and dispersion of spatially distributed rainfall tracer inputs will generate fractal tracer time series, as long as the flow system is highly dispersive (Kirchner et al., J. Hydrol., 2001). This implies that subsurface flow in small catchments is dominated by large conductivity contrasts, such as arise from macropores and fracture networks.

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.