Hydrologic Response of Catchments to Precipitation: Quantification of Mechanical Carriers and Origins of Water

The hydrological response of a catchment to precipitation (e.g. precipitation-induced overland, vadose zone and groundwater flow and solute mixing processes) is simulated using a fully-integrated flow and transport model to analyze the temporal and spatial origins of water in a stream. The calculated hydrological response within the hill slope region of a typical catchment indicates that subsurface flow can be significantly enhanced during a precipitation event in the shallow saturated zone below a stream and in the capillary zone adjacent to the stream (especially in the presence of macropore flow), but that this component tends to be insignificant relative to the strong pre-event water discharge often observed in the stream as the precipitation becomes more intense. In cases where tracer signals in the stream are dominated by irreversible mixing processes, including by molecular diffusion, the tracer signals measured in the stream are not likely to be correlated to the mechanical flow components contributing to stream flow. It is therefore concluded that it is not straightforward to decompose tracer signals measured in stream water samples to infer the different spatial origins of the water, and that strong pre-event signals in the samples does not necessarily reflect enhanced mechanical groundwater inputs.