Following tracer through the unsaturated zone using a multiple interacting pathways model: Implications from laboratory experiments

Abstract

Models must effectively represent velocities and celerities if they are to address the old water paradox. Celerity information is recorded indirectly in hydrograph observations, whereas velocity information is more difficult to measure and simulate effectively, requiring additional assumptions and parameters. Velocity information can be obtained from tracer experiments, but we often lack information on the influence of soil properties on tracer mobility. This study features a combined experimental and modelling approach geared towards the evaluation of different structures in the multiple interacting pathways (MIPs) model and validates the representation of velocities with laboratory tracer experiments using an undisturbed soil column. Results indicate that the soil microstructure was modified during the experiment. Soil water velocities were represented using MIPs, testing how the (a) shape of the velocity distribution, (b) transition probability matrices (TPMs), (c) presence of immobile storage, and (d) nonstationary field capacity influence the model's performance. In MIPs, the TPM controls exhanges of water between pathways. In our experiment, MIPs were able to provide a good representation of the pattern of outflow. The results show that the connectedness of the faster pathways is important for controlling the percolation of water and tracer through the soil. The best model performance was obtained with the inclusion of immobile storage, but simulations were poor under the assumption of stationary parameters. The entire experiment was adequately simulated once a time‐variable field capacity parameter was introduced, supporting the need for including the effects of soil microstructure changes observed during the experiment.

Testing the ‘two water worlds’ hypothesis under variable preferential flow conditions

“…This outcome suggests that higher quantities of preferential flow require continuity with the soil matrix, especially under extreme rainfall. More preferential flow corresponds to greater physical separation between fast and slow hydrological domains (Cueto‐Felgueroso et al, 2020; Scaini et al, 2019; Worthington, 2019), but some amount of exchange between domains is necessary to initiate and sustain macropore flow (Klaus et al, 2013; Weiler & Naef, 2003). The concept of matrix‐derived preferential flow further highlights the importance of domain exchange and disputes the concept of two distinct ‘water worlds’ in soils.…”

“…Subsurface pore topology (e.g., soil structure, macroporosity) is considered to be a primary source of flow heterogeneity in the vadose zone, with rainfall intensity acting as an important factor affecting the proportion of water moving through more conductive pores (Flury et al, 1994; Stewart, 2019). Additionally, as soils approach saturation, the two flow domains can become more distinct (Cueto‐Felgueroso et al, 2020; Scaini et al, 2019) and drainage can select for younger water (Rodriguez et al, 2018; Rodriguez et al, 2020; Sprenger, Stumpp, et al, 2019). Under such conditions, it can be expected that structured soils with high macroporosity (e.g., intact soil columns) will exhibit faster dual domain flow compared to soils with limited structural pore space (e.g., repacked soil material), dominated by slower single domain flow through the soil matrix (Gerke, 2006; Köhne et al, 2009a; Köhne et al, 2009b).…”

Rapid-Response Unsaturated Zone Hydrology: Small-Scale Data, Small-Scale Theory, Big Problems

“…More recent alternative theoretical developments based on concepts such as films, waves, particle-tracking, or transfer functions (e.g., Jury, 1982;Dragila and Wheatcraft, 2001;Tsutsumi et al, 2005;Hincapié and Germann, 2009;Davies et al, 2013;Nimmo and Mitchell, 2013;Scaini et al, 2019), have been motivated by the incompatibility of diffusive-flow concepts with preferential flow. Many of these incorporate promising innovations, but none is yet in widespread use.…”

Multimodal water age distributions and the challenge of complex hydrological landscapes

“…For instance, in heterogeneous media, dispersion causes the distribution of v to broaden and flatten with increasing L (Le Borgne, de Dreuzy, Davy, & Bour, 2007). This can be interpreted as an increased probability with time that the advective velocity of a water particle has changed along its transport (e.g., as in Davies et al, 2013; Scaini, Amvrosiadi, Hissler, Pfister, & Beven, 2019; Zehe & Jackisch, 2016). Therefore, L and v are probably not independent, making the TTD, as the pdf of their ratio, difficult to estimate a priori (Curtiss, 1941).…”