Hillslope dynamics modeled with increasing complexity

Virtual experiments: a new approach for improving process conceptualization in hillslope hydrology

Agricultural hillslopes present particular challenges for estimating vadose zone dynamics due to a variety of processes, such as surface runoff, vertical flow, erosion, subsurface preferential flow affected by soil structure and layering, non-linear chemical behaviour, evapotranspiration, etc. To investigate these processes and complexity, the SUPREHILL critical zone observatory (CZO) was started in 2020, at vineyard hillslope site in Croatia. The observatory is extensively equipped for the soil-water regime and agrochemical fluxes monitoring, and includes an extensive sensor network, lysimeters (weighing and passive wick), suction probes, surface and subsurface flow and precipitation collection instruments. The main objective of the SUPREHILL observatory is to quantify subsurface lateral and local scale preferential flow processes. Local-scale nonlinear processes in eroded agricultural hillslope sites have large significance on water and solute behaviour within the critical zone and thus need to be researched in depth using combined methods and various approaches. First results from the sensor and lysimeter network, soil-water regime monitoring, isotope analysis, and agrochemical concentrations in 2021 supported the hypothesis of the observatory, that the subsurface flow plays a relevant part in the hillslope soil-water dynamics. In the wick lysimeter network, although the highest cumulative outflow values were found at the hilltop, the highest individual measurements were found at the footslope. During high-intensity rainfall events, there were differences in weighing lysimeters, possibly related to subsurface lateral flow. Based on the isotope analysis, wick lysimeters exhibit a greater variation of d-excess values than suction probes. Agrochemical fluxes confirmed the sloping effect on their transport in soil and demonstrated the favourability of Cu transport by subsurface flow. Using the comprehensive database presented herein, future analyses of this hypothesis will be carried out in more detail using model-based analyses.  

<p>Subsurface flow in small first-order catchments is dominated by both, precipitation patterns and subsurface structure. We report on a series of repeated tracer experiments under transient conditions in a small forested first-order catchment (F4, 2.3 ha) at Gårdsjön in SW Sweden. Podsols are the dominant soil types, soil thickness varies strongly (0-50 cm) and bedrock outcrops are partly visible at the surface. A small wetland is situated directly upstream of the runoff weir. A hillslope of the catchment is equipped with a sprinkler system and can be irrigated at around 38-45 m<sup>3</sup> day<sup>-1</sup>. Depending on the meteorological conditions in the respective year of the experiment, natural rainfall comes in addition.</p><p>A bromide tracer solution was injected into groundwater at a single location about 40 m upstream the weir over a period of approximately an hour, and was monitored using a set of groundwater tubes and the weir at the outlet over the following 3-4 days. Additionally, discharge and meteorological conditions were recorded. The experiments were repeated each summer from 2007 to 2019. In summer 2019, electrical resistivity tomography was done during the experiment. We measured a profile perpendicular to the flow direction covering the whole study site. This data shows how subsurface patterns could influence water flow on the soil-bedrock interface. We investigated tracer recovery rates against cumulated runoff since tracer application. Substantially different transit times and qualitatively different behaviour of the breakthrough curves were observed, even under steady state conditions. We present first results how these differences could be linked to the structure of the subsurface.</p>

Subsurface flow is a prominent runoff process in sloping lands of purple soil in the upper Yangtze River basin. However, it remains difficult to identify and quantify. In this study, in situ runoff experimental plots were used to measure soil moisture dynamics using an array of time domain reflectometry (TDR) together with overland flow and subsurface flow using isolated collecting troughs. Frequency of preferential flow during rainfall events and the controls of subsurface flow processes were investigated through combined analysis of soil properties, topography, rainfall intensity, initial wetness, and tillage. Results showed that subsurface flow was ubiquitous in purple soil profiles due to well-developed macropores, especially in surface soils while frequency of preferential flow occurrence was very low (only 2 cases in plot C) during all 22 rainfall events. Dry antecedent moisture conditions promoted the occurrence of preferential flow. However, consecutive real-time monitoring of soil moisture at different depths and various slope positions implied the possible occurrence of multiple subsurface lateral flows during intensive storms. Rainfall intensity, tillage operation, and soil properties were recognized as main controls of subsurface flow in the study area, which allows the optimization of management practices for alleviating adverse environmental effects of subsurface flow in the region.

Hillslope threshold response to rainfall: (1) A field based forensic approach

Despite all efforts, agricultural contaminants remain at alarming concentrations in Quebec surface waters. Preferential flow (PF) of soluble contaminants has been suggested as a contributing factor but has not been specifically studied in the humid climate of eastern Canadian soils. Three tracers were surface applied on plots along a catena on a loamy sand under intensive agricultural production in Beauce, Quebec. Tracer distribution in soil profiles was monitored three times over a 12-month period. At the summit, finger flow rapidly transported tracers into the subsurface. The subsurface preferential lateral flow rapidly brought the tracers downslope. Narrow points of preferential seepage and discharge and underground lateral PF were observed at the footslope. The summit and the backslope of the catena showed strong vertical and lateral subsurface PF, which made their contribution to subsurface tracer movement toward surface water equal to or greater than that of the footslope, in part because of the hydrological connectivity between summit, backslope, and surface water. PF and matrix flow were both significant in all parts of the catena. Therefore, all parts of a catena, even those far from surface water, should be considered when evaluating potential belowground contaminant transport toward surface water.

Generalizable process knowledge on hillslope hydrological dynamics is still very poor, yet indispensable for numerous theoretical and practical applications. To gain insight into the organization of hillslope hydrological dynamics we intercompared 90 observations of shallow water table dynamics at three neighboring large‐scale (33 × 75 m) hillslopes with similar slope, aspect, curvature, geologic, and pedologic properties but differences in vegetation cover (grassland, coniferous forest, and mixed forest) over a time period of 9 months. High‐resolution measurements of water table fluctuations, rainfall, and discharge in the creek at the foot of all hillslopes allowed a good system characterization. The aim of this study was to explore the spatio‐temporal variability of water table fluctuations within and between hillslopes, the effect of event and antecedent characteristics on the observed dynamics, and how the hillslope subsurface flow (SSF) response is reflected in the runoff response. To intercompare the SSF behavior we conducted an event‐based analysis of the percentage of well activation, several metrics characterizing the shape and timing of the water table response curves, rainfall characteristics, antecedent wetness conditions, and several runoff response metrics. The analysis reveals that there are distinct differences in SSF response between the grassland hillslope and the forested hillslopes, with a lower frequency of well activation and absolute water table rise at the grassland hillslope. Second, spatial patterns of water table dynamics differ between wet fall/winter/spring (predominantly saturation of the lower part of the hillslope, weaker water table response, and slower response times) and dry summer conditions (whole‐hillslope activation but higher spatial variability, generally stronger water table dynamics, and quicker response times). The observed seasonally changing water table dynamics suggest the development of a preferential flow network during high‐intensity rainstorms under dry summer conditions. Third, catchment runoff is strongly driven by hillslope dynamics, yet contrasting hydrographs during events with similar hillslope dynamics indicate the influence of additional processes. Overall, the observed high spatio‐temporal variability of seemingly homogeneous hillslopes calls for rethinking of current monitoring strategies and developing and testing new conceptual models of hillslope hydrologic processes.

Subsurface flow in a soil-mantled subtropical dolomite karst slope: A field rainfall simulation study

Abstract. Subsurface flow in preferential pathways in soils may transport water more rapidly than the soil matrix, may be quickly activated during precipitation events and enhance infiltration or interflow. Vertical pathways are particularly important for runoff generation. However, identifying these pathways is challenging because traditional methods such as piezometers, soil moisture sensors, or hillslope trenches do not adequately capture the spatial scale and frequency of preferential flow features, while other experimental techniques like dye tracing are labor-intensive and invasive. In this study, we introduce a novel method to identify the locations of preferential flow by analysing vertical soil profiles of stable water isotopes. Across four catchments, we drilled 100 soil cores (1–3 m deep) per catchment and analyzed the stable isotope composition of the soil water in 10–20 cm depth intervals to construct depth profiles. We employed clustering techniques to group soil-water isotope profiles and selected those that matched to a seasonal sampling date to establish a reference profile for each catchment using LOESS regression, representing profiles influenced solely by matrix infiltration. Deviations from these reference profiles were then used as indicators of being influenced by vertical or lateral preferential flow. Our results revealed evidence of preferential flow in all studied catchments. Especially in the alpine catchment with highly heterogeneous soils, many profiles showed distinct preferential flow features, including multiple, vertically independent pathways occurring at variable depths, even among adjacent profiles. These findings demonstrate the feasibility of using soil water isotope profiles to assess preferential flow pathways and highlight the substantial spatial and vertical variability of preferential flowpaths at hillslope and catchment scales.

<p>Subsurface structural heterogeneity can route soil water through the vadose zone in complex ways that are difficult to observe or model in detail. The fill-and-spill hypothesis, which was developed to explain observations from field experiments at the hillslope scale, suggests that lateral flow at the soil-bedrock interface (or other permeability contrasts) is only activated once depressions in the microtopography of that interface become saturated and connect laterally. To reproduce this fill-and-spill phenomenon, a Richards equation-based numerical model would require a discretization of this morphology at a fine scale. A coarse representation smooths the interface eliminating the essential local storage-excess threshold behavior. However, the accuracy and abundance of field measurements rarely provide the data that would be needed to do this, and the fine discretization creates a high computational burden.</p><p>Here we explore the potential of an upscaled representation of the fill-and-spill mechanism based on parameterization of a permeability tensor with saturation-dependent anisotropy. By increasing empirical lateral hydraulic conductivity as they approach saturation, this anisotropy could reproduce how the fill-and-spill causes saturated patches at the permeability contrast to connect along preferential flow paths. Such a process representation could be incorporated into a Richards equation-based model enabling it to reproduce the fill-and-spill phenomenon even with a coarse spatial resolution.</p><p>We tested this idea using ParFlow to model virtual hillslopes with synthetic rugged internal topography. We then investigated the effective permeability tensor over an averaging volume that encloses the interface. By parameterizing the anisotropy of an interface block under changing moisture variables, we may be able to generate predictions of how the fine-scale fill-and-spill processes integrate to the hillslope and catchment scale.</p>

The distribution of frost table depths on a peat‐covered permafrost slope was examined in a discontinuous permafrost region in northern Canada over 4 consecutive years at a variety of spatial scales, to elucidate the role of active layer development on runoff generation. Frost table depths were highly variable over relatively short distances (0.25–1 m), and the spatial variability was strongly correlated to soil moisture distribution, which was partly influenced by lateral flow converging to frost table depressions. On an interannual basis, thaw rates were temporally correlated to air temperature and the amount of precipitation input. Simple simulations show that lateral subsurface flow is governed by the frost table topography having spatially variable storage that has to be filled before water can spill over to generate flow downslope, in a similar manner that bedrock topography controls subsurface flow. However, unlike the bedrock surface, the frost table is variable with time and strongly influenced by the heat transfer involving water. Therefore, it is important to understand the feedback between thawing and subsurface water flow and to properly represent the feedback in hydrological models of permafrost regions.

HighlightsPreferential flow (PF) can critically reduce riparian buffer contaminant removal efficiency.This collection presents research on PF measurement, visualization, modeling, and contaminant transport impacts.Future needs include tools to identify landscape-scale PF areas and conservation practices.Future models for research and practice should account for PF in riparian buffers.Abstract. Preferential flow in riparian buffers can substantially compromise their effectiveness in reducing contaminants from overland runoff. The objective of this article is to introduce a collection of five articles on current research into subsurface preferential flow measurement, visualization, modeling, and impacts on contaminant fate and transport at scales ranging from the subsurface pore scale to the plot scale to the watershed scale. This collection presents selected works from a broader invited session on “Preferential flow and piping in riparian buffers” at the 2020 ASABE Annual International Meeting. Major findings include: new methodologies, such as light transmission and geophysics, to characterize subsurface preferential flow; an infiltration partitioning approach to quantify preferential flow from field experiments; a kinematic dispersive wave model to effectively simulate subsurface preferential flow; and the significant impact of surface concentrated flow pathways on pesticide fate and transport both upstream and within a riparian buffer. Future work is needed to develop methods and tools to identify PF areas and management solutions within a landscape, and to update both research and design models to better quantify and account for PF processes. Keywords: Best management practice, Buffer strip, Agricultural conservation practice, Filter strip, Macropore, Nonpoint-source pollution.

Abstract. Subsurface stormflow (SSF) can play a key role for the runoff generation at hillslopes. Quantifications of SSF suffer from the limited ability to predict how SSF is formed at a particular hillslope and how it varies in time and space. This study concentrates on the temporal variability of SSF formation. Controlled sprinkling experiments at three experimental slopes were replicated with varying precipitation intensity and varying antecedent precipitation. SSF characteristics were observed with hydrometric measurements and tracer experiments. SSF response was affected in different ways and to varying degree by changes of precipitation intensity and antecedent precipitation. The study showed that the influence of antecedent precipitation on SSF response depends on how SSF is formed at a particular hillslope. As formation of SSF was hardly influenced by the increase of precipitation intensity subsurface flow rates were not increased by higher intensity. However, timing and relevance of subsurface flow response changed substantially at different precipitation intensities, because saturation and flow formation occurred above the soil-bedrock interface, but also within the topsoil depending on precipitation intensity.

The spatial heterogeneity and temporal variability of hillslope subsurface flow (SSF) generation are poorly understood in the alpine critical zone (CZ). Owing to the limitations of observation technology bottlenecks and harsh environments, existing studies lack a holistic and process‐based understanding of SSF generation, particularly for the deep CZ and annual freeze‐thaw (FT) cycle. This study improves the conventional trenching observation scheme and is the first to have successfully monitored SSF processes from the soil to the bedrock layer with high‐resolution from 2017 to 2020 in a headwater catchment of the Qinghai‐Tibetan Plateau. The results show that hillslope runoff generation exhibited similar trends over the years while exhibiting distinct patterns across seasons. Spatially, slope aspects significantly affect the runoff amount and ratio. The south‐facing slope is dominated by overland flow (OF, 84.2%–97.0%), while the north‐facing slope is dominated by SSF (88.9%–97.3%). The runoff depth of the north‐facing slope is 11.0 times that of the south‐facing slope. The SSF in the deep CZ accounts for 20.8%–37.8% of the total runoff along the north‐facing slope. Temporally, runoff generation exhibits distinct patterns across seasons. OF exhibits two different patterns, while SSF exhibits five across seasons. Our results indicate that runoff generation in different FT stages should be studied by the time phase. Therefore, we propose seasonal or FT period‐based classification criteria and divide OF and SSF into two and three consecutive periods throughout the calendar year. We suggest that these inferences and observations should be considered in the framework of hydrological models in cold regions.

Preferential flow (PF)‐dominated soil structure is often considered a unique system consisting of micropores and macropores and thus supposed to provide dual‐pore filtering effects on hydrological signals, through which smoothing effects are likely to be stronger for matrix flow and weaker for PF via macropores. By using time series of hydrological signals (precipitation, canopy interception, throughfall, soil moisture, evapotranspiration, water storage in soil and groundwater, and catchment discharge) propagating through the Shale Hills Catchments and representative soil series, the filtering effects of the catchment and soil profiles were tested through the wavelet analysis. Typical filtering effects, that is, the characteristic of the role of soil and the groundwater table as a buffer, were observed from the wavelet spectrum, gradually smoothing off the precipitation signal. The hypothesized dual‐pore style filtering effects of the soil profile were also confirmed through the coherence spectra and phase differences, rendering them applicable for possible use as “fingerprints” of PF to infer subsurface flow features. We found that PF dominates the catchment's discharge response at the scales from 3 to 12 days, which contributes to the catchment discharge mainly as subsurface lateral flow at upper or middle soil horizons. Through subsurface PF pathways, even the hilltop is likely hydrologically connected to the valley floor, building connections with or making contributions to the catchment discharge. This study highlights the potential of wavelet analysis for retrieving and characterizing subsurface flow processes based on the revealed dual‐pore filtering effects of the soil system. Although some limitations and uncertainties still exist, we believe that wavelet methods provide a highly potential but underexplored approach to studying subsurface hydrology.