Exploring the Dynamics of Transit Times and Subsurface Mixing in a Small Agricultural Catchment

Abstract

The analysis of transit/residence time distributions (TTDs and RTDs) provides important insights into the dynamics of stream‐water ages and subsurface mixing. These insights have significant implications for water quality. For a small agricultural catchment in central Germany, we use a 3D fully coupled surface‐subsurface hydrological model to simulate water flow and perform particle tracking to determine flow paths and transit times. The TTDs of discharge, RTDs of storage and fractional StorAge Selection (fSAS) functions are computed and analyzed on daily basis for a period of 10 years. Results show strong seasonal fluctuations of the median transit time of discharge and the median residence time, with the former being strongly related to the catchment wetness. Computed fSAS functions suggest systematic shifts of the discharge selection preference over four main periods: In the wet period, the youngest water in storage is preferentially selected, and this preference shifts gradually toward older ages of stored water when the catchment transitions into the drying, dry and wetting periods. These changes are driven by distinct shifts in the dominance of deeper flow paths and fast shallow flow paths. Changes in the shape of the fSAS functions can be captured by changes in the two parameters of the approximating Beta distributions, allowing the generation of continuous fSAS functions representing the general catchment behavior. These results improve our understanding of the seasonal dynamics of TTDs and fSAS functions for a complex real‐world catchment and are important for interpreting solute export to the stream in a spatially implicit manner.

The Demographics of Water: A Review of Water Ages in the Critical Zone

“…Spatially distributed, continuum-based hydrological models (Hrachowitz & Clark, 2017) are also being increasingly used to simulate time-varying TTDs by tracking the age of particles of water as they flow through the catchment (Davies et al, 2013;Maxwell et al, 2016;Danesh-Yazdi et al, 2018;Remondi et al, 2018;Yang et al, 2018;Figure 3d. In these models, mixing hypotheses can be formulated at smaller scales.…”

“…This recharge to groundwater can thus contain noticeable fractions of "old" water. The interactions of saturated and unsaturated zones are generally responsible for hysteresis in the stream water ages when plotted against catchment wetness (Benettin, Bailey, et al, 2017;Hrachowitz et al, 2013;Rodriguez et al, 2018;Yang et al, 2018).…”

Regionalization of Groundwater Residence Time Using Metamodeling

“…Many studies have investigated the link between landscape characteristics and catchment TTD using tracer data from precipitation and streams (Godsey et al, 2010;Hrachowitz et al, 2009Hrachowitz et al, , 2010Hrachowitz et al, , 2011Kirchner et al, 2001;McGuire et al, 2005;Peralta-Tapia et al, 2016;Soulsby & Tetzlaff, 2008;Tetzlaff et al, 2009) or numerical models (Ameli et al, 2016;Fiori et al, 2009;Fiori & Russo, 2008;Maxwell et al, 2016;Yang et al, 2018). In either of these two approaches, TTDs can be expressed using various distributions, including exponential (Hrachowitz et al, 2009;Kirchner et al, 2001), power law (Ameli et al, 2016;Fiori et al, 2009;Fiori & Russo, 2008;McGuire et al, 2005), gamma (Ameli et al, 2016;Berghuijs & Kirchner, 2017;Fiori et al, 2009;Godsey et al, 2010;Hrachowitz et al, 2009Hrachowitz et al, , 2010Hrachowitz et al, , 2011Kirchner et al, 2001;McGuire et al, 2005;Peralta-Tapia et al, 2016), and Weibull (Ameli et al, 2016).…”

From Points to Patterns: Using Groundwater Time Series Clustering to Investigate Subsurface Hydrological Connectivity and Runoff Source Area Dynamics

“…Hydrological models can also be used to obtain information on the likely subsurface source areas. Even though there is a growing number of conceptual or physically based modeling studies that investigate the spatial and temporal source area dynamics (Hopp & McDonnell, 2009;Weiler & McDonnell, 2004;Yang et al, 2018), validation of the simulated spatial patterns in groundwater levels remains limited (Loague & Ebel, 2016). Field studies on catchment-scale patterns of runoff sources are still rare (Burt & McDonnell, 2015), and the data to test (and/or parameterize) the models are thus often not available.…”

“…Mapping runoff source areas and their connectivity to the stream network and understanding the processes and factors that affect their dynamics improve our understanding of catchment‐scale streamflow generation processes (Jencso et al, ; McGlynn & Seibert, ; Nippgen et al, ; Tetzlaff, Soulsby, Waldron, et al, ). The spatial and temporal change in the extent of runoff source areas and their hydrologic connectivity to the stream are also of key importance to understand the temporal and spatial variability in nutrient transport (Creed et al, ; Dick et al, ; Thompson et al, ; Vidon & Hill, ), stream solute concentrations (Ali et al, ; Fischer et al, ; McGlynn & McDonnell, ), mean transit times (McGlynn, ; McGuire, ; Tetzlaff et al, ; Yang et al, ), and aquatic habitats (Soulsby et al, ; Tetzlaff, Soulsby, Bacon, et al, ).…”