Factors influencing the residence time of catchment waters:: A virtual experiment approach

[1] Estimates of mean residence time (MRT) are increasingly used as simple summary descriptors of the hydrological processes involving storage and mixing of water within catchment systems. Current understanding of the physical controls on MRT remains limited, and various hypotheses have been proposed to explain its variability between catchments. We present a series of virtual experiments to investigate different hypotheses regarding the significance of different hydrological processes and geographical controls in determining the MRT of catchment waters. The experiments were undertaken using a semidistributed conceptual hydrological model, applied to the Maimai experimental catchment in New Zealand. Our results show that in this small steep catchment, with largely impermeable bedrock, the primary control on the stream water mean residence time is storage within the unsaturated zone. The physical location on the hillslope had only a small influence on soil water residence time. Stream water mean residence time was very sensitive to small additional amounts of deep groundwater in the model. Overall, our results suggest that stream water MRT is additive. The component residence times of stream water MRT appear relatable to characteristic properties of the catchment. Through this mechanism there is future potential for extrapolating MRT data from experimental catchments to other areas.