Factors controlling inter-catchment variation of mean transit time with consideration of temporal variability

Abstract

17The catchment transit time, a lumped descriptor reflecting both time scale and spatial structure of 18 catchment hydrology can provide useful insights into chemical/nuclear pollution risks within a catchment. 19Despite its importance, factors controlling spatial variation of mean transit time (MTT) are not yet well 20 understood. In this study, we estimated time-variant MTTs for about ten years (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012) in five 212 / 42 mesoscale sub-catchments of the Fuji River catchment, central Japan, to establish the factors controlling 22 their inter-catchment variation with consideration of temporal variability. For this purpose, we employed a 23 lumped hydrological model that was calibrated and validated by hydrometric and isotopic tracer 24 observations. Temporal variation patterns of estimated MTT were similar in all sub-catchments, but with 25 differing amplitudes. Inter-catchment variation of MTT was greater in dry periods than wet periods, 26 suggesting spatial variation of MTT is controlled by water 'stock' rather than by 'flow'. Although the 27 long-term average MTT (LAMTT) in each catchment was correlated with mean slope, coverage of forest (or 28 conversely, other land use types), coverage of sand-shale conglomerate, and groundwater storage, the 29 multiple linear regression revealed that inter-catchment variation of LAMTT is principally controlled by the 30 amount of groundwater storage. This is smaller in mountainous areas covered mostly by forests and greater 31 in plain areas with less forest coverage and smaller slope. This study highlights the topographic control of 32 MTT via groundwater storage, which might be a more important factor in mesoscale catchments, including 33 both mountains and plains, rather than in smaller catchments dominated by mountainous topography. 34 35

Mean transit times in headwater catchments: insights from the Otway Ranges, Australia

“…Consequently, groundwater discharge does not have a discrete age but rather has a distribution of transit times. MTTs are commonly estimated using lumped parameter models (LPMs) that describe the distribution of water with different ages or tracer concentrations in simplified aquifer geometries (Maloszewski andZuber, 1982, 1996;Maloszewski et al, 1983;Cook and Bohlke, 2000;Maloszewski, 2000;Zuber et al, 2005). LPMs represent a viable and commonly used alternative to estimating MTTs using numerical groundwater models that rely upon hydraulic parameters that are seldom known with certainty and which vary spatially.…”

“…Large groundwater storage volumes will likely also result in long MTTs (e.g. Ma and Yamanaka, 2016). Groundwater flow is likely to be more rapid through steeper catchments due to the higher hydraulic gradients, resulting in shorter MTTs (e.g.…”

“…Nonetheless, it has been successfully used to predict tracer concentrations over time in a number of flow systems (e.g. Maloszewski, 2000). Utilisation of this model requires defining a dispersion parameter, D p , which represents the ratio of dispersion to advection.…”

Using isotopes to understand the evolution of water ages in disturbed mixed land‐use catchments

“…As such, this can help provide insights into potential impact of human activities on water partitioning in the landscape (Banks, Simmons, Love, & Shand, ). Previous studies have shown that controls on water ages are complex and can depend on the internal structure of the catchment and can be variously dominated by physiographical characteristics, including drainage density (Hrachowitz, Soulsby, Tetzlaff, Malcolm, & Schoups, ), topography (McGuire et al, ; Mosquera et al, ), soil cover (Hrachowitz et al, 2009), soil hydraulic properties (Heidbüchel, Troch, & Lyon, ; Muñoz‐Villers, Geissert, Holwerda, & McDonnell, ), land cover (Ma & Yamanaka, ), and their spatial variation. However, several of these characteristics can be affected by land management (e.g., Soulsby, Birkel, Geris, & Tetzlaff, ), which makes MTT a useful indicator to evaluate land use impacts on hydrological processes (Jacobs et al, ).…”

Effectivity of dissolved SF6 tracer for clarification of rainfall–runoff processes in a forested headwater catchment

“…As mentioned above, rainfall–runoff processes have been analysed and examined by multiple researchers based on spatial‐ and component‐based information. However, unfortunately, there are few studies incorporating the timescale (age information) in the groundwater component, including the bedrock groundwater component of the rainfall–runoff water during rainstorms, even though the stream water age has been analysed in previous studies (e.g., Klaus et al, ; Ma & Yamanaka, ). The stable isotope‐based convolution integral methods have been known as traditional and effective dating methods of discharge water (reviewed in McGuire & McDonnell, ).…”