Lateral Subsurface Flow Research Articles

Assessment of the spatial variability of apparent electrical conductivity in a tile drained catchment in Fensholt subcatchment, Jutland, Denmark for improved small-scale prediction of highly reducing areas

The implementation of a targeted, site-specific approach on reducing the nitrogen load from agriculture requires fine-scale assessment of soil heterogeneity. In this study, the soil heterogeneity was described through the spatial heterogeneity of the apparent electrical conductivity values (ECa) and its implications to the patterns of redox potential values (Eh) in an intensely tile drained agricultural area. Piezometers and platinum-tipped redox probes for water level and Eh measurements were installed in transects along the assumed subsurface lateral water flow direction at different depths overlying the tile drains. Water level and Eh were determined from February to August 2017. A statistical analysis of the average ECa and slope measurements of each transect showed that they could be classified into four operational soil combinations (SC): high ECa, low slope (SC-1), low ECa, low slope (SC-2), high ECa, high slope (SC-3), low ECa, high slope (SC-4). A significant correlation between the inverted electrical conductivity (EC) values, clay and soil organic carbon (SOC) content suggested that the EC correlates with clay content. The high clay percentage associated with high ECa is assumed to facilitate development of anoxic microsites which led to more reducing conditions. Due to the generally reducing conditions and higher occurrence of saturated piezometers, transects under SC-1 were considered to have the most potential for intense nitrate reduction. In a tile drained setting, classifications based on average ECa and slope values may be utilized as parameters for a simple mapping tool for improved fine scale prediction of potential hotspots for nitrate reduction.

Dynamic domain kinematic modelling for predicting interflow over leaky impeding layers

AbstractTraditional Boussinesq or kinematic simulations of interflow (i.e., lateral subsurface flow) assume no leakage through the impeding layer and require a no‐flow boundary condition at the ridge top. However, recent analyses of many interflow‐producing landscapes indicate that leaky impeding layers are common, that most interflow percolates well before reaching the toe slope, and therefore, the downslope contributing length is shorter than the hillslope length. In watersheds characterised by perched interflow over a low conductivity layer through permeable topsoil, interflow with percolation may be modelled with a kinematic wave model using a mobile upslope boundary condition defining the hillslope portion contributing interflow to valleys. Here, we developed and applied a dynamic interflow model to simulate interflow using a downslope travel distance concept such that only the active contributing length is modelled at any time. The model defines a variable active area based on the depth of the perched layer, the topographic slope and the ratio of the hydraulic conductivity of topsoil to that of the impeding layer. It incorporates a two‐layer soil moisture accounting water balance analysis, a pedo‐transfer function, and percolation and evaporation routines to predict interflow rates in continuous and event‐based scenarios. We tested the modelling concept on two sets of data (2‐year dataset of rainfall observations for the continuous simulation and a multi‐day irrigation experiment for the event simulation) from a 121‐m‐long open interflow collection trench on an experimental hillslope at the Savannah River Site, South Carolina. The continuous model simulation partially represented the observed interflow hydrograph and perched water depth in the experimental hillslope with correlation coefficients of 0.85 and 0.35, respectively. Model performance improved significantly at event‐scale analysis. The modelling approach realistically represents interflow dynamics in hillslopes with leaky impeding layers and can be integrated into catchment‐scale hydrology models for more detailed hillslope process modelling.

Changes in plot-scale runoff generation processes from the spring–summer transition period to the summer months in a permafrost-dominated catchment

The hydrological regimes in permafrost-dominated catchments have unique characteristics. However, studies based on plot-scale experiments of runoff generation processes and the factors influencing these processes are limited on the Tibetan Plateau, which is experiencing rapid warming and permafrost degradation. Runoff generation processes were studied on a standard runoff plot (5 × 20 m) in a permafrost-dominated catchment on the Tibetan Plateau with an alpine meadow cover to analyze these processes during the spring–summer transition period (from mid-May to early June) and in the summer months of June and July. The discharge, including surface and subsurface runoff, the soil hydrothermal and moisture conditions, and the meteorological conditions were monitored from May 2018 to May 2019. Partial Least-Squares Path Modeling was used to identify the influencing factors in the two time periods. The lateral subsurface flow at 30–60 cm depth accounted for the majority of the discharge from the runoff plot (>99%) and showed a sharp increase when the soil water contents of the thawed soil layers exceeded a threshold value that varied from 0.41 to 0.52. During the spring–summer transition period, the lateral subsurface flow at 30–60 cm depth was promoted by both event precipitation and the antecedent precipitation with an average runoff ratio of 1.7, indicating that the antecedent moisture condition maintained by the underlying permafrost drives runoff generation via meltwater from snow and frozen soil. In the summer months, the lateral subsurface flow at 30–60 cm depth was directly promoted by event precipitation, especially the moderate precipitation that occurred in this period, with a reduced runoff ratio of 0.5 as a result of deepening of the thawed soil layer from 30 to >60 cm, enhanced evapotranspiration and an increased soil water storage capacity caused by an increase of air temperature and the soil temperature. Prediction models obtained via Multiple Linear Regressions of the identified influencing factors were able to accurately estimate the discharge. This study shows the important role of lateral subsurface flow in runoff generation processes in this permafrost area and the most important influencing factors in different seasons.

Dynamic spreading of re-melted volcanic ash bead on thermal barrier coatings

Dynamic spreading of 1 mm diameter volcanic ash bead on thermal barrier coatings (TBCs) surfaces and the infiltration and subsurface lateral flow of these melts within TBCs were quantitatively investigated at 1200−1600 °C for 1−10000 min. The spreading areas, infiltration depths and lateral flow distances are controlled by melt viscosity, coating surface roughness and microstructures. The lateral flow along the inter-column of the coating was dramatically accelerated at higher temperature with a diffusion distance of more than 5.3 mm within 1 min at 1600 °C. These findings are critical for understanding the corrosion mechanism of TBC in contaminated environments.

Establishing irrigation potential of a hillside aquifer in the African highlands

AbstractFeeding 9 billion people in 2050 will require sustainable development of all water resources, both surface and subsurface. Yet, little is known about the irrigation potential of hillside shallow aquifers in many highland settings in sub‐Saharan Africa that are being considered for providing irrigation water during the dry monsoon phase for smallholder farmers. Information on the shallow groundwater being available in space and time on sloping lands might aid in increasing food production in the dry monsoon phase. Therefore, the research objective of this work is to estimate potential groundwater storage as a potential source of irrigation water for hillside aquifers where lateral subsurface flow is dominant. The research was carried out in the Robit Bata experimental watershed in the Lake Tana basin which is typical of many undulating watersheds in the Ethiopian highlands. Farmers have excavated more than 300 hand dug wells for irrigation. We used 42 of these wells to monitor water table fluctuation from April 16, 2014 to December 2015. Precipitation and runoff data were recorded for the same period. The temporal groundwater storage was estimated using two methods: one based on the water balance with rainfall as input and baseflow and evaporative losses leaving the watershed as outputs; the second based on the observed rise and fall of water levels in wells. We found that maximum groundwater storage was at the end of the rain phase in September after which it decreased linearly until the middle of December due to short groundwater retention times. In the remaining part of the dry season period, only wells located close to faults contained water. Thus, without additional water sources, sloping lands can only be used for significant irrigation inputs during the first 3 months out of the 8 months long dry season.

Parameterization of a hydrologic model with geophysical data to simulate observed subsurface return flow paths

AbstractA portion of water not consumed by crops during flood irrigation can flow back across the surface or through the subsurface to adjacent surface water bodies and streams as return flow. Few studies have directly addressed subsurface processes governing return flow and the importance of structural complexity on hydrologic process representation. It is challenging to measure and model these subsurface flow paths using traditional hydrologic observations. In this study, we assess the impact of subsurface structural complexity on vadose zone flow representation in a two‐dimensional transport model by varying structural complexity derived from background geophysical data. We assessed four model structures each with three soil types of homogeneous hydrologic properties, two of which were evaluated with and without an anisotropy factor. Wetting front arrival times, derived from time‐lapse electrical resistivity measurements during flood irrigation field experiments, were used to evaluate the different representations of soil profile structures. These data indicated both vertical and lateral preferential flow in the subsurface during flood irrigation. Inclusion of anisotropy in the saturated hydraulic conductivity field improved the ability to model subsurface hydrologic behavior when flow processes shifted from uniform to heterogeneous flow, as occurs with lateral subsurface return flow under flood irrigation driven by a large pressure gradient. This reduced the need for detailed spatial discretization to represent these observed subsurface flow processes. The resulting simple three‐layer model structure was better able to model both the vertical and lateral flow processes than a more complex geospatial structure, suggesting that overinterpretation of smoothed inverted profiles could lead to misrepresentation of the subsurface structure.

Hydrological functioning of thawing soil water in a permafrost‐influenced alpine meadow hillslope

AbstractThe complex variation of the hydrological functioning of soil water has been widely studied using measurements of the stable isotopic composition of either the bulk soil water (BSW) collected by the cryogenic vacuum extraction method or the mobile soil water (MSW) collected by the zero‐tension soil lysimeter method. We collected samples of precipitation, BSW, MSW, and hillslope runoff in an alpine meadow hillslope influenced by permafrost to investigate the hydrological functioning of thawing soil water. The results showed that the δ2H values in BSW were generally more negative than those of the MSW, which may be a result of the depleted spring snowmelt that formed the tightly bound soil water. The relatively stable characteristics of δ2H in the MSW were related to the dynamic mixing of newly infiltrated precipitation with slightly enriched mobile water held by the low soil matrix potential. The δ2H values in the BSW showed higher temporal variations than the values in the MSW, which were mainly related to the mixing of depleted tightly bound soil water and enriched MSW in varying fractions. The MSW better represented the lateral subsurface flow (SSF, which further generated streamflow) than the BSW samples considering the relatively close stable isotopic values between the lateral SSF and the MSW in the same soil layer. The results of this study show the influences of the varying water level maintained by the underlying permafrost on the water and isotopic dynamics of both the soil water pools and streamflow.

Land cover effects on soil infiltration capacity measured using plot scale rainfall simulation in steep tropical lowlands of Central Panama

AbstractUnderstanding land use/land cover (LULC) effects on tropical soil infiltration is crucial for maximizing watershed scale hydro‐ecosystem services and informing land managers. This paper reports results from a multiyear investigation of LULC effects on soil bulk infiltration in steep, humid tropical, and lowland catchments. A rainfall simulator applied water at measured rates on 2 × 6 m plots producing infiltration through structured, granulated, and macroporous Ferralsols in Panama's central lowlands. Time‐lapse electrical resistivity tomography (ERT) helped to visualize infiltration depth and bulk velocity. A space‐for‐time substitution methodology allowed a land‐use history investigation by considering the following: (a) a continuously heavy‐grazed cattle pasture, (b) a rotationally grazed traditional cattle pasture, (c) a 4‐year‐old (y.o.) silvopastoral system with nonnative improved pasture grasses and managed intensive rotational grazing, (d) a 7 y.o. teak (Tectona grandis) plantation, (e) an approximately 10 y.o. secondary succession forest, (f) a 12 y.o. coffee plantation (Coffea canephora), (g) an approximately 30 y.o. secondary succession forest, and (h) a >100 y.o. secondary succession forest. Within a land cover, unique plot sites totalled two at (a), (c), (d), (e), and (g); three at (b); and one at (f) and (h). Our observations confirmed measured infiltration scale dependency by comparing our 12 m2 plot‐scale measurements against 8.9 cm diameter core‐scale measurements collected by others from nearby sites. Preferential flow pathways (PFPs) significantly increased soil infiltration capacity, particularly in forests greater than or equal to 10 y.o. Time‐lapse ERT observations revealed shallower rapid bulk infiltration and increased rapid lateral subsurface flow in pasture land covers when compared with forest land covers and highlighted how much subsurface flow pathways can vary within the Ferralsol soil class. Results suggest that LULC effects on PFPs are the dominant mechanism by which LULC affects throughfall partitioning, runoff generation, and flow pathways.

Groundwater controls on colloidal transport in forest stream waters.

Biogeochemical changes of whole catchments may, at least in part, be deduced from changes in stream water composition. We hypothesized that there are seasonal variations of natural nanoparticles (NNP; 1-100nm) and fine colloids (<300nm) in stream water, which differ in origin depending on catchment inflow parameters. To test this hypothesis, we assessed the annual dynamics of the elemental composition of NNP and fine colloids in multiple water compartments, namely in stream water, above and below canopy precipitation, groundwater and lateral subsurface flow from the Conventwald catchment, Germany. In doing so, we monitored meteorological and hydrological parameters, total element loads, and analyzed element concentrations of org C, Al, Si, P, Ca, Mn and Fe by Asymmetric Flow Field Flow Fractionation (AF4). The results showed that colloid element concentrations were<5µmol/L. Up to an average of 55% (Fe) of total element concentrations were not truly dissolved but bound to NNP and fine colloids. The colloid patterns showed seasonal variability with highest loads in winter. The presence of groundwater-derived colloidal Ca in stream water showed that groundwater mainly fed the streams throughout the whole year. Overall, the results showed that different water compartments vary in the NNP and fine colloidal composition making them a suitable tool to identify the streams NNP and fine colloid sources. Given the completeness of the dataset with respect to NNP and fine colloids in multiple water compartments of a single forest watershed this study adds to the hitherto underexplored role of NNP and fine colloids in natural forest watersheds.

Representing Intrahillslope Lateral Subsurface Flow in the Community Land Model

AbstractThe concept of using representative hillslopes to simulate hydrologically similar areas of a catchment has been incorporated in many hydrologic models but few Earth system models. Here we describe a configuration of the Community Land Model version 5 in which each grid cell is decomposed into one or more multicolumn hillslopes. Within each hillslope, the intercolumn connectivity is specified, and the lateral saturated subsurface flow from each column is passed to its downslope neighbor. We first apply the model to simulate a headwater catchment and assess the results against runoff and evapotranspiration flux measurements. By redistributing soil water within the catchment, the model is able to reproduce the observed difference between evapotranspiration in the upland and lowland portions of the catchment. Next, global simulations based on hypothetical hillslope geomorphic parameters are used to show the model's sensitivity to differences in hillslope shape and discretization. Differences in evapotranspiration between upland and lowland hillslope columns are found to be largest in arid and semiarid regions, while humid tropical and high‐latitude regions show limited evapotranspiration increases in lowlands relative to uplands.

Distinct stores and the routing of water in the deep critical zone of a snow-dominated volcanic catchment

Abstract. This study combines major ion and isotope chemistry, age tracers, fracture density characterizations, and physical hydrology measurements to understand how the structure of the critical zone (CZ) influences its function, including water routing, storage, mean water residence times, and hydrologic response. In a high elevation rhyolitic tuff catchment in the Jemez River Basin Critical Zone Observatory (JRB-CZO) within the Valles Caldera National Preserve (VCNP) of northern New Mexico, a periodic precipitation pattern creates different hydrologic flow regimes during spring snowmelt, summer monsoon rain, and fall storms. Hydrometric, geochemical, and isotopic analyses of surface water and groundwater from distinct stores, most notably shallow groundwater that is likely a perched aquifer in consolidated collapse breccia and deeper groundwater in a fractured tuff aquifer system, enabled us to untangle the interactions of these groundwater stores and their contribution to streamflow across 1 complete water year (WY). Despite seasonal differences in groundwater response due to water partitioning, major ion chemistry indicates that deep groundwater from the highly fractured site is more representative of groundwater contributing to streamflow across the entire water year. Additionally, the comparison of streamflow and groundwater hydrographs indicates a hydraulic connection between the fractured welded tuff aquifer system and streamflow, while the shallow aquifer within the collapse breccia deposit does not show this same connection. Furthermore, analysis of age tracers and oxygen (δ18O) and stable hydrogen (δ2H) isotopes of water indicates that groundwater is a mix of modern and older waters recharged from snowmelt, and downhole neutron probe surveys suggest that water moves through the vadose zone both by vertical infiltration and subsurface lateral flow, depending on the lithology. We find that in complex geologic terrain like that of the JRB-CZO, differences in the CZ architecture of two hillslopes within a headwater catchment control water stores and routing through the subsurface and suggest that shallow groundwater does not contribute significantly to streams, while deep fractured aquifer systems contribute most to streamflow.

Pairing geophysical techniques improves understanding of the near-surface Critical Zone: Visualization of preferential routing of stemflow along coarse roots

There is compelling evidence from aboveground observations that trees considerably affect precipitation partitioning through the Critical Zone. However, due to the lack of appropriate methods, the role of root systems (the hidden half of trees) on redistributing precipitation and infiltration into and routing through the soil remains inadequately visualized and understood. Here, we designed a novel experiment to pair two non-invasive geophysical techniques, ground-penetrating radar (GPR) and electrical resistivity tomography (ERT), to trace stemflow through the soil in a forested hillslope after water was released on the trunk of an American beech (Fagus grandifolia Ehrh.) to induce stemflow. We used GPR to locate lateral coarse roots and GPR and ERT together to confirm the wetting areas in response to the non-uniform transport of stemflow. Signal changes between time-lapse geophysical images were used to reveal both the response time and location of stemflow infiltration into and redistribution through the soil. This first known study to investigate the subsurface routing of stemflow by combining GPR and ERT revealed that the belowground funneling of stemflow along laterally oriented coarse roots transported water 2.8 m downslope from the study tree in 30 min after stemflow was initiated. In situ excavation validated the distribution of lateral roots and lateral root-derived preferential flow paths identified in geophysical images, confirming the utility of pairing GPR and ERT to gain insights into the temporal dynamics and spatial distribution of subsurface routing of stemflow. The proposed method visualized and confirmed the funneling effect of roots on belowground water redistribution that contributed to subsurface lateral flow. Pairing GPR and ERT provides a useful combination of geophysical methods to advance our understanding of the complex interactions between plant and soil, such as the role of tree roots in soil hydrological process by revealing areas of funneling in the hidden part of the Critical Zone.

Heat transfer within frozen slopes in subarctic Yukon, Canada

The dominant mechanism of heat transfer during ground thaw is typically assumed to be vertical conduction. However, the addition of lateral subsurface water flow introduces the potential for the forced convection of energy, having an influence on ground temperatures and thaw rates. Field observations of snowmelt run-off and rates of ground thaw for two slopes within the Wolf Creek basin, Yukon, Canada, highlighted different rates of ground thaw with slope position. Ground temperatures were numerically simulated to evaluate the relative influence of conduction and convection on the thawing of these slopes; each slope comprised different soils and had a different slope aspect. Both slopes were composed of an organic layer overlying a mineral soil. Lateral water flow above the frozen layer occurred within both slopes as a result of a perched saturated zone above the organic–mineral interface. The numerical models reveal that lateral diversion within a surficial, high-hydraulic conductivity layer, such as an organic layer, can initiate convective heat transfer. However, the observed differential thaw was determined to be a result of conduction and variations in the initial ice content and snow cover rather than lateral convection of heat.

Nitrate subsurface transport and losses in response to its initial distributions in sloped soils: An experimental and modelling study

AbstractTransport and losses of nitrate from sloped soils are closely linked to nitrogen fertilizer management. Previous studies have always focused on different types of fertilizer applications and rarely analysed various initial nitrate distributions as a result of nitrogen fertilizer applications. Under certain conditions, both subsurface lateral saturated flow and vertical leaching dominate nitrate losses. Soil tank experiments and HYDRUS‐2D modelling were used to better understand the subsurface nitrate transport and losses through lateral saturated flow and vertical leaching under various initial nitrate distributions. Low (L: 180 mg L−1), normal (N: 350 mg L−1), and high (H: 500 mg L−1) nitrate concentrations were used in five different distributions (NNNN, NLLN, LHHL, LNLN, and HNHN) along the slope of the tank. The first two treatments (NNNN and NLLN) were analysed both experimentally and numerically. Experiments were conducted under 12 rainfall events at intervals of 3 days. The HYDRUS‐2D model was calibrated and validated against the experimental data and demonstrated good model performance. The other three treatments (LHHL, LNLN, and HNHN) were investigated using the calibrated model. Nitrate concentrations in purple sloped soils declined exponentially with time under intermittent rainfalls, predominantly in the upper soil layers. Non‐uniform initial nitrate distributions contributed to larger differences between four locations along the slope in deeper soil layers. The non‐uniform nitrate distribution either enhanced or reduced decreases in nitrate concentrations in areas with higher or lower initial nitrate concentrations, respectively. Higher nitrate concentrations at the slope foot and along the slope were reduced mainly by lateral flow and vertical leaching, respectively. Increasing nitrogen application rates increased subsurface nitrate losses. Mean subsurface lateral nitrate fluxes were twice as large as mean vertical leaching nitrate fluxes. However, due to longer leaching durations, total nitrate losses due to vertical leaching were comparable with those due to lateral flow, which indicated comparable environmental risks to surface waters and groundwater.

Preliminary analysis on the impacts of the rhizosphere on occurrence of rainfall-induced shallow landslides

Rainfall-induced diffuse shallow landslides are one of the most critical natural hazards as they often evolve into highly destructive flow slides and debris flows. Vegetation is recognised to play a key role in landslide occurrence and is frequently invoked as a potential remedial measure for slope stabilisation at the catchment scale. The beneficial action of vegetation is generally associated with mechanical (root anchoring) and hydrological (suction generated by root water uptake) effects. There is indeed a third effect that has thus far been little explored. The rhizosphere, the portion of soil directly affected by plant roots, is characterised by hydraulic conductivity higher than the underlying soil horizons. This significantly affects hillslope hydrology by promoting lateral diversion of rainwater. This paper presents a case study in Scotland where the rhizosphere is demonstrated to play a major role in controlling shallow landslides. Field investigation and laboratory testing were carried out to characterise the hydraulic conductivity of the rhizosphere and deeper horizons. In turn, this formed the basis for the development of a physically based model for the slope. The model was first validated against its capability to simulate failure of two historical landslides and then exploited to demonstrate the beneficial effect of the rhizosphere. The lesson learned from this study is that shallow landslide hazard can be mitigated by enhancing the capacity of the rhizosphere to act as a natural lateral drainage. This implies that plants with root-system architecture that enhances lateral subsurface flow should be privileged when designing vegetation-based remedial measures.

Comparison of nutrient loss pathways: Run‐off and seepage flow in Vertisols

AbstractRecent studies have highlighted the agronomic and environmental importance of phosphorus (P) movement through the soil profile. Thus, faced with challenges such as high‐profile cases of P enrichment of surface water, better understanding of nutrient movement through soil is needed to better manage agricultural fertilizers and manures and their contribution to water quality degradation. In particular, field‐scale research is especially needed in soils with preferential flow transport pathways. Thus, we collected nitrogen (N) and P transport data in run‐off and seepage (lateral subsurface return flow) from 13 field‐ and farm‐scale watersheds on Vertisols in Central Texas for a 14‐year period. For 2004–2017, seepage accounted for ~20% of the total surface flow, and nutrient concentrations were generally similar in run‐off and seepage. As surface run‐off contributed ~80% of the flow, it follows that median annual N and P loads in run‐off were significantly greater than in seepage for every watershed. N loads in both run‐off and seepage flow from cultivated land were an order of magnitude greater than in native prairie and improved pasture, and the highest run‐off and seepage P loads both occurred on cultivated land with organic fertilizer sources. Increasing watershed scale (size) did not to produce consistent patterns in N or P loss in run‐off or seepage. Land use and watershed scale produced significant differences in seepage volume but did not affect run‐off volumes or total surface flow/rainfall. Although less significant in terms of total offsite flux, nutrient movement in vadose zones has important agronomic and environmental implications as considerable N and P are transported through and within the root zone and eventually offsite. And in terms of P, this contradicts the traditionally held scientific viewpoint that P movement through the vadose zone is unimportant agronomically and environmentally.

The behavior of wetting front on slopes with different slope morphologies during rainfall

The aim of this research is to investigate the influence of slope morphology on the wetting front arrival during rainfall. Temporal variations in soil water content at different times and depths are measured in a slope gully and in a non-gullied slope. The arrival time of wetting front (Ts) and the time when soil shear strength reaches its lowest level during rainfall (Te) at different depths and at different elevations in the slopes and in rainfall events with different intensities are collected in the study and are used to represent the hydrological behavior in a slope. Ts and Te values noticeably decrease with depth in the slope gully lower elevation during moderate intensity rainfall, whereas these values increase with depth in a non-gullied slope. Rainfall characteristics considerably affect the behavior of Ts and Te values in the slope gully. Ts values vary slightly along depths during extremely high intensity rainfall compared with that of moderate rainfall intensity. Slope morphology and rainfall characteristics considerably affect the hydrological behavior in a slope. Subsurface lateral flow in a slope gully during rainfall is more conspicuous than in a non-gullied slope due to its topographic features. In a landslide prone area, soil instability in a slope gully may occur faster than in a non-gullied slope during severe rainfall activity.

Uniform and lateral preferential flows under flood irrigation at field scale

AbstractFlood irrigation is globally one of the most used irrigation methods. Typically, not all water that is applied during flood irrigation is consumed by plants or lost to evaporation. Return flow, the portion of applied water from flood irrigation that returns back to streams either via surface or subsurface flow, can constitute a large part of the water balance. Few studies have addressed the connection between vertical and lateral subsurface flows and its potential role in determining return flow pathways due to the difficulty in observing and quantifying these processes at plot or field scale. We employed a novel approach, combining induced polarization, time‐lapse electrical resistivity tomography, and time‐lapse borehole nuclear magnetic resonance, to identify flow paths and quantify changes in soil hydrological conditions under nonuniform application of flood irrigation water. We developed and tested a new method to track the wetting front in the subsurface using the full range of inverted resistivity values. Antecedent soil moisture conditions did not play an important role in preferential flow path activation. More importantly, boundaries between lithological zones in the soil profile were observed to control preferential flow pathways with subsurface run‐off occurring at these boundaries when saturation occurred. Using the new method to analyse time‐lapse resistivity measurements, we were able to track the wetting front and identify subsurface flow paths. Both uniform infiltration and preferential lateral flows were observed. Combining three geophysical methods, we documented the influence of lithology on subsurface flow processes. This study highlights the importance of characterizing the subsurface when the objective is to identify and quantify subsurface return flow pathways under flood irrigation.

Experimental Investigation of Lateral Subsurface Flow Depending on Land Use and Soil Cultivation

The magnitude and timing of flood events are influenced by surface and subsurface flow generation as well as by present land use distribution. An integrated understanding of the interactions of soil properties, land use and flow generation is still missing. Therefore, field experiments are required to gain further knowledge about land use dependencies of discharge generation and concentration processes. In our research, we built an experimental setup consisting of three sites with similar soil and topographic conditions and different land use types (cropland, grassland, forest). The applied multimethod approach includes meteorological parameters, soil moisture, soil moisture tension, surface runoff, lateral subsurface flow, and stream discharge observations. The results show that low subsurface flow discharges more often occur at the cropland site, while large flow volumes were mainly observed at the grassland site. A correlation of the horizontal distribution of subsurface flow volumes and the accumulation areas of the surface topography has been found (r² = 0.76). The observed average response times for advective events increase from the forest site (6.0 h) to the grassland site (12.4 h) to the cropland site (20.9 h). Response times of convective events were shorter than 1 h at all sites.

Hydrogeophysical monitoring reveals primarily vertical movement of an applied tracer across a shallow, sloping low-permeability till interface: Implications for agricultural nitrate transport

Intensive potato production is an important contributor to groundwater and surface water nitrate contamination in the province of Prince Edward Island (PEI), Canada. In PEI, the presence of a permeability contrast between soil and the underlying compact basal till at approximately 1 m depth is known to cause temporary perched water table conditions, but the implications for agricultural nitrate transport are not well understood. This study is the first detailed field investigation to examine the impact of the soil – compact basal till interface on vertical and lateral subsurface flow of water and solutes on a moderate hillslope in PEI. A surface-applied tracer test was conducted and a shallow drain-tile pipe system, with a tipping bucket gauge, was installed down-gradient of the tracer application to monitor lateral subsurface flow. In addition, three-dimensional electrical resistivity imaging (ERI) was employed to monitor the evolution of the applied tracer plume over a period of 1 year. A small scale, transient flow and transport model was used to simulate the tracer test and to investigate the sensitivity of tracer transport to selected parameters (primarily, the hydraulic conductivities of the soils and underlying compact basal till). Monitoring, conducted from October 2013 to June 2015, and numerical modeling showed a strong tendency for vertical flow of water and tracer through the soil – compact basal till interface as opposed to lateral down-slope subsurface flow. Water balance calculations estimated a total vertical flow of 94.5% of the infiltration for the October 2014 to January 2015 period. The preference for vertical over lateral flow and transport was consistent with electrical resistivity images for the same time period. These findings indicate that the fate of infiltrating water and dissolved nitrate is the deeper bedrock aquifer water table, and that intermittent perched water table conditions do not result in significant lateral flow toward nearby surface waters.