Model‐based exploration of hydrological connectivity and solute transport in a forested hillslope

Abstract

Preferential flowpaths control hydrological connectivity in forested hillslopes. However, current understanding on the nature, extent, and impacts of hydrological connectivity within hillslopes is still limited. Distributed 2‐pore domain models of subsurface water flow and solute transport in and between the preferential flowpaths and the soil matrix provide applicable means to investigate the connectivity. We identified possible (dis)connections of preferential flowpaths in a forested hillslope section from dye tracer data and incorporated them in a 3D 2‐pore domain model; the model was run and evaluated against ion tracer data. The objectives were to explore hydrological connectivity within the hillslope and quantify its impacts on solute transport. The main control for capturing the internal concentration dynamics of the ion‐tracer plume with the model was an explicit description of local disconnections of preferential flowpaths in the form of local reductions in their hydraulic conductivity. Continuous preferential flowpaths were formed by rooting activities, erosion caused by subsurface flow, freezing–thawing cycles, and soil fauna, but they were locally disconnected by, for example, cemented soil material. As a result, the dominant lateral flowpath was tortuous, and tracer transport within this pathway was highly preferential: Compared to earlier models without flowpath disconnections, increase in the average lateral tracer load was 1.7‐fold and in the maximum load 7.9‐fold. The results indicate that the volume and hydraulic conductivity of the actual, connected preferential flowpaths control, in combination with linkages between preferential flowpaths and soil matrix, pollutant transport in forested hillslopes.

Connectivity in hydrology and sediment dynamics

“…There are studies looking at land‐use effects, such as grazing (Martínez‐Murillo, Hueso‐González, & Ruiz‐Sinoga, 2018), revegetation (Lizaga, Quijano, Palazón, Gaspar, & Navas, 2018), land abandonment (Calsamiglia et al, 2018), urbanization (Ferreira, Walsh, Steenhuis, ∓ Ferreira, 2018), and effects of wild fire (Martínez‐Murillo, & López‐Vicente, 2018), and others focus on characterization of specific landscape types, including alpine (Rainato et al, 2018), badlands (Moreno‐de las Herras et al, 2019; Caraballo‐Arias, Di Stefano, & Ferro, 2018), and gullies (Conoscenti, Agnesi, Cama, Caraballo‐Arias, & Rotigliano, 2018; Zegeye et al, 2018). Also, some studies address only hydrological connectivity (e.g., Laine‐Kaulio and Koivusalo, 2018) or sediment connectivity (e.g., Porto, Walling, & Callegari, 2018), while others look at both types of connectivity (Ricci, Girolamo, Abdelwahab, & Gentile, 2018). Other authors have focused on the temporal scale of connectivity, such as short times scales (e.g., Laine‐Kaulio and Koivusalo, 2018) and landscape evolutionary time scales (e.g., van der Waal and Rowntree, 2018).…”

“…Also, some studies address only hydrological connectivity (e.g., Laine‐Kaulio and Koivusalo, 2018) or sediment connectivity (e.g., Porto, Walling, & Callegari, 2018), while others look at both types of connectivity (Ricci, Girolamo, Abdelwahab, & Gentile, 2018). Other authors have focused on the temporal scale of connectivity, such as short times scales (e.g., Laine‐Kaulio and Koivusalo, 2018) and landscape evolutionary time scales (e.g., van der Waal and Rowntree, 2018). Finally, some authors also investigated the landscape as a snapshot to calculate the potential connectivity, which is also described as the structural connectivity (e.g., Martinez‐Murillo and López‐Vicente, 2018; Calsamiglia et al, 2018).…”

A Field Experiment for Tracing Lateral Subsurface Flow in a Post-Glacial Hummocky Arable Soil Landscape

“…Field tracer studies, where a conservative solute such as salt, is added to the soil water solution, might give insight into travel times and transport pathways for nutrient and pesticide transport in sloping agricultural soils. Hereby, lateral flow between the B-horizon and a till layer [15], along the organic-mineral boundary [16], or along tree roots [17] was identified in forested landscapes. In agricultural soil, Bero et al [18] applied a potassium bromide solution to observe LSF in loamy sand soil.…”

The relevance of preferential flow in catchment scale simulations: Calibrating a 3D dual‐permeability model using DREAM

“…Flow in the macropore domain may be described using also the Darcy–Richards equation or some other equation, such as, for example, a gravity‐driven kinematic wave formulation (e.g., the MACRO model, Jarvis & Larsbo, ). Such preferential flow models have been implemented successfully to simulate water flow and/or solute transport at the plot scale (Arora, Mohanty, & McGuire, ; Köhne et al, ; Köhne & Mohanty, ; Larsbo, Roulier, Stenemo, Kasteel, & Jarvis, ; Roulier et al, ) and at the hillslope and catchment scale (Christiansen, Thorsen, Clausen, Hansen, & Refsgaard, ; Laine‐Kaulio, Backnäs, Karvonen, Koivusalo, & McDonnell, ; Laine‐Kaulio & Koivusalo, ; Roulier et al, ; Schaik et al, ; Shao et al, ; Shao et al, ; Yu, Duffy, Baldwin, & Lin, ;Zehe et al, ). Working with bi‐ or multimodal soil hydraulic functions in a single‐domain setup is another possibility to represent flow in heterogeneous pore systems (e.g., Durner, ; Othmer, Diekkrüger, & Kutilek, ; Ross & Smettem, ).…”