ABSTRACT:

This file includes the data published in: Mares, R., Barnard, H.R., Mao, D., Revil, A. and Singha, K. (2016). Examining diel patterns of soil and xylem moisture using electrical resistivity imaging. Journal of Hydrology, https://doi.org/10.1016/j.jhydrol.2016.03.003, 12 p.

The feedbacks among forest transpiration, soil moisture, and subsurface flowpaths are poorly understood. We investigate how soil moisture is affected by daily transpiration using time-lapse electrical resistivity imaging (ERI) on a highly instrumented ponderosa pine and the surrounding soil throughout the growing season. By comparing sap flow measurements to the ERI data, we find that periods of high sap flow within the diel cycle are aligned with decreases in ground electrical conductivity and soil moisture due to drying of the soil during moisture uptake. As sap flow decreases during the night, the ground conductivity increases as the soil moisture is replenished. The mean and variance of the ground conductivity decreases into the summer dry season, indicating drier soil and smaller diel fluctuations in soil moisture as the summer progresses. Sap flow did not significantly decrease through the summer suggesting use of a water source deeper than 60 cm to maintain transpiration during times of shallow soil moisture depletion. ERI captured spatiotemporal variability of soil moisture on daily and seasonal timescales. ERI data on the tree showed a diel cycle of conductivity, interpreted as changes in water content due to transpiration, but changes in sap flow throughout the season could not be interpreted from ERI inversions alone due to daily temperature changes.

ABSTRACT:

Data from Wieting, C., Ebel, B., and Singha, K. (2017). Quantifying the effects of wildfire on changes in soil properties by surface burning of soils from the Boulder Creek Critical Zone Observatory. Journal of Hydrology-Regional Studies, http://dx.doi.org/10.1016/j.ejrh.2017.07.006, 43-57.

Infiltration processes are not well understood in fire-affected soils because soil hydraulic properties and soil-water content are altered by the heat. This study uses intact soil cores, which should maintain preferential flow paths, that were collected in the field to explore the impacts of fire on soil properties and infiltration processes during rainfall. Three soil scenarios are presented here: unburned control soils, and low- and high-severity burned soils. Fire severity was simulated in the laboratory using a heating gun, and established based on temperature and duration of heating. Soil properties pre- and post-burn were measured using laboratory techniques including: Mini Disk Infiltrometer tests, Water Drop Penetration Time (WDPT) Tests, and measurements of dry bulk density and total organic carbon (TOC). Soil moisture and temperature were recorded at approximately 2.5 cm and 7.5 cm in soil cores as was the cumulative volume of water exiting the core during rainfall simulations. Mini Disk infiltration experiments suggest a decrease in both cumulative infiltration and infiltration rates from unburned to low-severity burned soils. High-severity burned soils saw an increase in cumulative infiltration. We interpret these changes as a result of the burning off of organic materials, enabling water to infiltrate more instead of being stored in the organics. The field saturated hydraulic conductivity did not vary from unburned to low-severity burned soils, but increased in high-severity burned soils due to the lack of organics that help inhibit water movement. During rainfall simulations, soil-water storage decreased from when soils were burned, likely because of the inability to store water within organic materials since they were burned. Vulnerability to raindrop impact also increased with fire severity. Together, these results indicate that fire-induced changes from low-severity wildfires were not as drastic as high-severity wildfires, and that high-severity burned soils can infiltrate more water, but not necessarily store it. Quantifying soil properties affected by wildfire, which can be gained through controlled laboratory simulations like this study, will aid in predicting post-wildfire behavior on the watershed scale.

ABSTRACT:

Data from Singha, K. and Gorelick, S.M. (2005). Saline tracer visualized with electrical resistivity tomography: field scale spatial moment analysis. Water Resources Research, 41, W05023, https://doi.org/10.1029/2004WR003460, 17 p.

Cross-well electrical resistivity tomography (ERT) was used to monitor the migration of a saline tracer in a two-well pumping-injection experiment conducted at the Massachusetts Military Reservation in Cape Cod, Massachusetts. After injecting 2200 mg/Lof sodium chloride for 9 hours, ERT data sets were collected from four wells every 6 hours for 20 days. More than 180,000 resistance measurements were collected during the tracer test. Each ERT data set was inverted to produce a sequence of 3-D snapshot maps that track the plume. In addition to the ERT experiment a pumping test and an infiltration test were conducted to estimate horizontal and vertical hydraulic conductivity values. Using modified moment analysis of the electrical conductivity tomograms, the mass, center of mass, and spatial variance of the imaged tracer plume were estimated.Although the tomograms provide valuable insights into field-scale tracer migration behavior and aquifer heterogeneity, standard tomographic inversion and application of Archie’s law to convert electrical conductivities to solute concentration results in underestimation of tracer mass. Such underestimation is attributed to (1) reduced measurement sensitivity to electrical conductivity values with distance from the electrodes and (2) spatial smoothing (regularization) from tomographic inversion. The center of mass estimated from the ERT inversions coincided with that given by migration of the tracer plume using 3-D advective-dispersion simulation. The 3-D plumes seen using ERT exhibit greater apparent dispersion than the simulated plumes and greater temporal spreading than observed in field data of concentration breakthrough at the pumping well.

ABSTRACT:

The data presented here are published in Foster, A., Trautz, A.C., Bolster, D., Illangasekare, I., and Singha, K. (2021). Effects of large-scale heterogeneity and temporally varying hydrologic processes on estimating immobile pore space: A mesoscale-laboratory experimental and numerical modeling investigation. Journal of Contaminant Hydrology, https://doi.org/10.1016/j.jconhyd.2021.103811.

The advection-dispersion equation (ADE) often fails to predict solute transport, in part due to incomplete mixing in the subsurface, which the development of non-local models has attempted to deal with. One such model is dual-domain mass transfer (DDMT); one parameter that exists within this model type is called immobile porosity. Here, we explore the complexity of estimating immobile porosity under varying flow rates and density dependencies in a large-scale heterogeneous system. Immobile porosity is estimated experimentally and using numerical models in 3-D flow systems, and is defined by domains of comparatively low advective velocity instead of truly immobile regions at the pore scale. Tracer experiments were conducted in a mesoscale 3-D tank system with embedded large impermeable zones and the generated data were analyzed using a numerical model. The impermeable zones were used to explore how large-scale structure and heterogeneity affect parameter estimation of immobile porosity, assuming a dual-porosity model, and resultant characterization of the aquifer system. Spatially and temporally co-located fluid electrical conductivity (σ_f) and bulk apparent electrical conductivity (σ_b)—using geophysical methods—were measured to estimate immobile porosity, and numerical modeling (i.e., SEAWAT and R3t) was conducted to explore controls of the immobile zones on the experimentally observed flow and transport. Results showed that density-dependent flow increased the hysteresis between measured fluid and bulk electrical conductivity, resulting in larger interpreted immobile pore-space estimates. Increasing the dispersivity in the model simulations decreased the estimated immobile porosity; flow rate had no impact. Overall, the results of this study highlight the difficulty faced in determining immobile porosity values in field settings, where hydrogeologic processes may vary temporally. Our results also highlight that immobile porosity is an effective parameter in an upscaled model whose physical meaning is not necessarily clear and that may not align with intuitive interpretations of a porosity.

Column experiments from Allan Foster's thesis contributing to this work are also included below.

ABSTRACT:

This file includes the data published in: Malenda, H.F., Sutfin, N.A., Stauffer, S., Guryan. G., Rowland, J.C., Williams, K.H., and Singha, K. (2019). From Grain to Floodplain: Evaluating heterogeneity of floodplain hydrostatigraphy using sedimentology, geophysics, and remote sensing. Earth Surface and Planetary Landforms, doi:10.1002/esp.4613.

Floodplain stratigraphy, a major structural element of alluvial aquifers, is a fundamental component of floodplain heterogeneity, hydraulic conductivity, and connectivity. Watershed-scale hydrological models often simplify floodplains by modeling them as largely homogeneous, which inherently overlooks natural floodplain heterogeneity and anisotropy and their effects on hydrologic processes such as groundwater flow and transport and hyporheic exchange. This study, conducted in the East River Basin, Colorado, USA, combines point-, meander-, and floodplain-scale data to explore the importance of detailed field studies and physical representation of alluvial aquifers. We combine sediment core descriptions, hydraulic conductivity estimates from slug tests, ground-penetrating radar (GPR), historical maps of former channels, LiDAR-based elevation and Normalized Difference Vegetation Index data to infer 3-D fluvial stratigraphy. We compare and contrast stratigraphy of two meanders with disparate geometries to explore floodplain heterogeneity and connectivity controls on flow and transport. We identify buried point bars, former channels, and overbank deposits using GPR, corroborated by point sediment descriptions collected during piezometer installment and remotely sensed products. We map heterogeneous structural features that should control resultant flow and transport; orientation and connectivity of these features would control residence times important in hydrologic models.

ABSTRACT:

Data from Doughty, M., Sawyer, A., Wohl, E., and Singha, K. (2020). Mapping increases in hyporheic exchange from channel-spanning logjams, Journal of Hydrology, https://doi.org/10.​1016/​j.​jhydrol.​2020.​124931.

Human impacts such as timber harvesting, channel engineering, beaver removal, and urbanization alter the physical and chemical characteristics of streams. These anthropogenic changes have reduced fallen trees and loose wood that form blockages in streams. Logjams increase hydraulic resistance and create hydraulic head gradients along the streambed that drive groundwater-surface water exchange. Here, we quantify changes in hyporheic exchange flow (HEF) due to a channel-spanning logjam using field measurements and numerical modeling in MODFLOW and MT3DMS. Electrical resistivity (ER) imaging was used to monitor the transport of solutes into the hyporheic zone during a series of in-stream tracer tests supplemented by in-stream monitoring. We conducted experiments in two reaches in Little Beaver Creek, Colorado (USA): one with a single, channel-spanning logjam and the second at a control reach with no logjams. Our results show that 1) higher HEF occurred at the reach with a logjam, 2) logjams create complex HEF pathways that can cause bimodal solute breakthrough behavior downstream, and 3) higher discharge rates associated with spring snowmelt increase the extent and magnitude of HEF. The numerical modeling supports all three field findings, and also suggest that lower flows increase solute retention in streams, although this last conclusion is not supported by field results. This study represents the first use of ER to explore HEF around a naturally occurring logjam over different stream discharges and has implications for understanding how logjams influence the transport of solutes, the health of stream ecosystems, and stream restoration and conservation efforts.

ABSTRACT:

Data from Harmon, R., Barnard, H., and Singha, K. (2020). Water-table depth and bedrock permeability control magnitude and timing of transpiration-induced diel fluctuations in groundwater. Water Resources Research, 56, e2019WR025967. https://doi.org/10.1029/2019WR025967.

The subsurface processes that mediate the connection between evapotranspiration and groundwater within forested hillslopes are poorly defined. Here, we investigate the origin of diel signals in unsaturated soil water, groundwater, and stream stage on three forested hillslopes in the H.J. Andrews Experimental Forest in western Oregon, USA, during the summer of 2017, and assess how the diurnal signal in evapotranspiration (ET) is transferred through the hillslope and into these stores. There was no evidence of diel fluctuations in upslope groundwater wells, suggesting that tree water uptake in upslope areas does not directly contribute to the diel signal observed in near-stream groundwater and streamflow. The water table in upslope areas resided within largely consolidated bedrock, which was overlain by highly fractured unsaturated bedrock. These subsurface characteristics inhibit formation of diel signals in groundwater and impeded the transfer of diel signals in soil moisture to groundwater because (1) the bedrock where the water table resides limited root penetration and (2) the low unsaturated hydraulic conductivity of the highly fractured rock weakened the hydraulic connection between groundwater and soil/rock moisture. Transpiration-driven diel fluctuations in groundwater were limited to near-stream areas but were not ubiquitous in space and time. The depth to the groundwater table and the geologic structure at that depth likely dictated rooting depth and thus controlled where and when the transpiration-driven diel fluctuations were apparent in riparian groundwater. This study outlines the role of hillslope hydrogeology and its influence on the translation of evapotranspiration and soil moisture fluctuations to groundwater and stream fluctuations.

ABSTRACT:

This file includes the data published in: Johnston, A.J., Runkel, R.L., Navarre-Sitchler, A. and Singha, K. (2017). Exploration of diffuse and discrete sources of acid mine drainage to a headwater mountain stream in Colorado, USA. Mine Water and the Environment, doi:10.1007/s10230-017-0452-6, 16 p.

We investigated the impact of acid mine drainage (AMD) contamination from the Minnesota Mine, an inactive gold and silver mine, on Lion Creek, a headwater mountain stream near Empire, Colorado. The objective was to map the sources of AMD contamination, including discrete sources visible at the surface and diffuse inputs that were not readily apparent. This was achieved using geochemical sampling, in-stream and in-seep fluid electrical conductivity (EC) logging, and electrical resistivity imaging (ERI) of the subsurface. The low pH of the AMD-impacted water correlated to high fluid EC values that served as a target for the ERI. From ERI, we identified two likely sources of diffuse contamination entering the stream: (1) the subsurface extent of two seepage faces visible on the surface, and (2) rainfall runoff washing salts deposited on the streambank and in a tailings pile on the east bank of Lion Creek. Additionally, rainfall leaching through the tailings pile is a potential diffuse source of contamination if the subsurface beneath the tailings pile is hydraulically connected with the stream. In-stream fluid EC was lowest when stream discharge was highest in early summer and then increased throughout the summer as stream discharge decreased, indicating that the concentration of dissolved solids in the stream is largely controlled by mixing of groundwater and snowmelt. Total dissolved solids (TDS) load is greatest in early summer and displays a large diel signal. Identification of diffuse sources and variability in TDS load through time should allow for more targeted remediation options.

ABSTRACT:

A series of hyporheic exchange studies were conducted in watersheds 01 and 03 during the summer of 2010 using saline tracers coupled with electrical resistivity to image the temporal and spatial extent of the hyporheic zone during baseflow recession. A series of four 48-hr tracer tests were conducted in each watershed on a rotational schedule with each tracer test starting approximately 2 weeks following the start of the previous test in each watershed. Each tracer injection was targeted to enrich the stream electrical conductivity by ~100 uS/cm. Electrical resistivity surveys were conducted on up to 6 transects of electrodes (12 electrodes per transect) in each watershed for each test. Resistivity surveys were collected, on a high temporal frequency ranging from continuous to every 4 hrs, for pre-injection, injection, and post-injection until conductivity measurements in the shallow groundwater well network returned to pre-injection magnitudes. During each injection conductivity magnitudes were measured in the stream and each accessible groundwater well in the watershed using a handheld conductivity meter on a frequency ranging from near continuous (~15-30 min), during tracer start-up and shutoff, to every 2-6 hrs depending on position within the tracer test. Hydraulic head data was collected approximately every 15 minutes by downwell pressure transducers from a select set of groundwater wells in each watershed for nearly the full summer 2010.

These data were published in a series of papers outlined below.

ABSTRACT:

These data are described in Beetle-Moorcroft, F., Shanafield, M., and and Singha, K. (2021). Exploring conceptual models of infiltration and groundwater recharge on an intermittent river: the role of geologic controls. Journal of Hydrology-Regional Studies, https://doi.org/10.1016/j.ejrh.2021.100814.

Non-perennial rivers and streams are the main surface water resource in arid climates, and streambed infiltration in these systems is a vital component of groundwater recharge. Subsurface geology controls the extent and location of streambed infiltration and therefore impacts both streamflow and groundwater levels. This study explores geological controls on groundwater recharge through an intermittent river streambed using scenario evaluation with numerical models constrained by field observations. Our conceptual model included five fundamental variations in the system that could impact where and how much recharge is possible: 1) the presence of a fault; 2) variation in the alluvial aquifer hydraulic conductivity; 3) variation in the thickness of the streambed; 4) presence or absence of a confining unit; and 5) groundwater withdrawals via pumping. To achieve a realistic outcome, we parameterized the model using field observations from the Alamosa River in Colorado as an example. Scenarios that changed hydraulic conductivity values resulted in the most notable changes to infiltration, streamflow, and deep aquifer recharge; conversely, variations in streambed thickness had the least impact. The extent to which streambed infiltration occurs is dependent on streambed properties as well as the hydraulic properties of the underlying alluvial aquifer, and this in turn controls the impacts on streamflow. This research shows that subsurface heterogeneities are a fundamental control on non-perennial rivers’ hydrogeologic systems and are key to their resilience.

ABSTRACT:

These data are described in Rickel, A., Hoagland, B., Navarre-Sitchler, A. and Singha, K. (2021). Seasonal shifts in surface water-groundwater connections from electrical resistivity in a ferricrete-impacted stream. Geophysics, v. 86, no. 5, 13 p. 10.1190/GEO-2020-0599.1.

The efficacy of the hyporheic zone (HZ) — where surface water and groundwater mix — for processing nutrients or uptake of metals is dependent on streambed hydraulic conductivity and stream discharge, among other characteristics. Here, we explore electrical resistivity tomography (ERT) of hyporheic exchange in Cement Creek near Silverton, Colorado, which is affected by ferricrete precipitation. To quantify flows through the HZ, we conducted four-hour salt injection tracer tests and collected time-lapse ERT of the streambed and banks of Cement Creek at high and low flow. We installed piezometers to conduct slug tests, which suggested a low permeability zone at 44-cm depth likely comprised of ferricrete that cemented cobbles together. Based on the ERT, the tracer released into the stream was constrained within the shallow streambed with little subsurface flow through the banks. Tracer was detected in the HZ for a longer time at high flow compared to low flow, suggesting that more flow paths were available to connect the stream to the HZ. Tracer was confined above the ferricrete layer during both the high- and low-flow tests. Mass transfer and storage area parameters were calculated from combined analysis of apparent bulk conductivity derived from ERT and numerical modeling of the tracer breakthrough curves. The hyporheic storage area estimated at low discharge (0.1 m2) was smaller than at high discharge (0.4 m2) and residence times were 2.7 h at low discharge and 4.1 h at high discharge. During high discharge, in-stream breakthrough curves displayed slower breakthrough and longer tails, which was consistent with the time-lapse electrical inversions and One-dimensional Transport with Inflow and Storage (OTIS) modeling. Our findings indicate that ferricrete reduces the hydraulic conductivity of the streambed and limits the areal extent of the HZ, which may lower the potential for pollutant attenuation from the metal-rich waters of Cement Creek.

ABSTRACT:

These data are published in Harmon, R., Barnard, H., Day-Lewis, F.D., Mao, D., and Singha, K. (2021). Exploring environmental factors that drive diel variations in tree water storage using wavelet analysis. Frontiers in Water, doi: 10.3389/frwa.2021.682285.

Internal water storage within trees can be a critical reservoir that helps trees overcome both short- and long-duration environmental stresses. We monitored changes in internal tree water storage in a ponderosa pine using moisture probes, a dendrometer, and time-lapse electrical resistivity imaging (ERI) to investigate how patterns of in-tree water storage are affected by changes in sapflow rates, soil moisture, and meteorologic factors such as vapor pressure deficit. ERI measurements are influenced by changes in moisture, temperature, solute concentration, and material properties; thus, to evaluate changes in moisture based on ERI, the first three factors must be considered. Measurements of xylem fluid electrical conductivity were constant in the early growing season, while inverted sapwood electrical conductivity steadily increased, suggesting that increases in electrical conductivity of the sapwood did not result from an increase xylem fluid electrical conductivity. Seasonal increases in stem electrical conductivity corresponded with seasonal increases in trunk diameter, suggesting that increased electrical conductivity may result from new growth. Changes in diel amplitudes of inverted sapwood electrical conductivity, which correspond to diel changes in sapwood moisture, indicated that tree water storage use was greatest ~4-5 days after storm events, when sapwood inverted electrical conductivity measurements suggest internal stores were high. A decrease in diel amplitudes of inverted sapwood electrical conductivity during dry periods, suggest that the ponderosa pine relied on internal water storage to supplement transpiration demands, but as drought conditions progressed, tree water storage contributions to transpiration decreased. Wavelet analyses indicated that lag times between inverted sapwood electrical conductivity and sapflow increased after storm events, suggesting that as soils dried reliance on internal water storage increased and the time required to refill daily deficits in internal water storage increased. Lag times peaked when soil moisture returned to pre-storm event levels and then decreased as drought progressed. Short time lags between sapflow and inverted sapwood electrical conductivity corresponded with dry conditions, when ponderosa pine are known to reduce stomatal conductance to avoid xylem cavitation. Time-lapse ERI- and wavelet-analysis results highlighted the important role internal tree water storage plays in supporting transpiration throughout the course of a day, and during periods of declining subsurface moisture.

ABSTRACT:

Dual-porosity models are often used to describe solute transport in heterogeneous media, but the parameters within these models (e.g., immobile porosity and mobile/immobile exchange rate coefficients) are difficult to identify experimentally or relate to measurable quantities. Here, we performed synthetic, pore-scale millifluidics simulations that coupled fluid flow, solute transport, and electrical resistivity (ER). A conductive-tracer test and the associated geoelectrical signatures were simulated for four flow rates in two distinct pore-scale model scenarios: one with intergranular porosity, and a second with an intragranular porosity also defined. With these models, we explore how the effective characteristic-length scale estimated from a best-fit dual domain mass transfer (DDMT) model compares to geometric aspects of the flow field. In both model scenarios we find that: (1) mobile domains and immobile domains develop even in a system that is explicitly defined with one domain; (2) the ratio of immobile to mobile porosity is larger at faster flow rates as is the mass-transfer rate; and (3) a comparison of length scales associated with the mass-transfer rate (Lα) and those associated with calculation of the Peclet number (LPe) show LPe is commonly larger than Lα. These results suggest that estimated immobile porosities from a DDMT model are not only a function of physically mobile or immobile pore space, but also are a function of the average linear pore-water velocity and physical obstructions to flow, which can drive the development of immobile porosity even in single-porosity domains.

ABSTRACT:

Shallow subsurface flow is a dominant process controlling hillslope runoff generation, soil development, and solute reaction and transport. Despite their importance, the location and geometry of these flowpaths are difficult to determine. In arctic environments, shallow subsurface flowpaths are limited to a thin zone of seasonal thaw above permafrost, which is traditionally assumed to mimic surface topography. Here we use a combined approach of electrical resistivity tomography (ERT) and self-potential measurements (SP) to map shallow subsurface flowpaths in and around water tracks, drainage features common to arctic hillslopes. ERT measurements delineate thawed zones in the subsurface that control flowpaths, while SP is sensitive to groundwater flow. We find that areas of low electrical resistivity in the water tracks are deeper than manual thaw depth estimates and vary from surface topography. This finding suggests that traditional techniques may underestimate active layer thaw and the extent of the flowpath network on arctic hillslopes. SP measurements identify complex 3-D flowpaths in the thawed zone. Our results lay the groundwork for investigations into the seasonal dynamics, hydrologic connectivity, and climate sensitivity of spatially distributed flowpath networks on arctic hillslopes.

ABSTRACT:

Movement of soil moisture associated with tree root-water uptake is ecologically important but technically challenging to measure. Here, the self-potential (SP) method, a passive electrical geophysical method, is used to characterize water flow in situ. Unlike tensiometers, which use a measurement of state (i.e., matric pressure) at two locations to infer fluid flow, the SP method directly measures signals generated by water movement. We collected SP measurements in a two-dimensional array at the base of a Douglas-fir tree (Pseudotsuga menziesii) in the H.J. Andrews Experimental Forest in western Oregon over 5 months to provide insight on the propagation of transpiration signals into the subsurface under variable soil moisture. During dry conditions, SP data appear to show downward unsaturated flow, whereas nearby tensiometer data appear to suggest upward flow during this period. After the trees enter dormancy in the fall, precipitation-induced vertical flow dominates in the SP and tensiometer data. Diel variations in SP data correspond to periods of tree transpiration. Changes in volumetric water content occurring from soil moisture movement during transpiration are not large enough to appear in volumetric water content data. Fluid flow and electrokinetic coupling (i.e., electrical potential distribution) were simulated using COMSOL Multiphysics to explore the system controls on field data. The coupled model, which included a root-water uptake term, reproduced components of both the long-term and diel variations in SP measurements, thus indicating that SP has potential to provide spatially and temporally dense measurements of transpiration-induced changes in water flow. This manuscript presents the first SP measurements focusing on the movement of soil moisture in response to tree transpiration.