Groundwater provides a fundamental resource for modern life. Throughout the world, groundwater is managed by storing (recharging) it underground in natural aquifers for future withdrawal and consumptive use. In Arizona, over 4 million people benefit from managed aquifer storage, but little effort is made to track the movement of recharged water through the subsurface. Motivated by current limitations in our ability to monitor percolation and groundwater movement at the scale of a recharge facility, an effort to collect time-lapse gravity data was carried out at the Southern Avra Valley Storage and Recovery Project (SAVSARP) operated by the City of Tucson, Arizona. In addition to collecting water-level data 12 wells, there were three primary gravity experiments: (1) five continuously-recording gravity meters (2 iGrav superconducting gravity meters and 3 gPhone gravity meters) were installed semi-permanently in control buildings adjacent to the recharge basins, (2) absolute gravity measurements were made at nine locations over a 17 month period, and (3) three relative-gravity campaigns were carried out on a network of 70 stations. This large-scale controlled experiment, with known infiltration and pumping rates, resulted in one of the most comprehensive datasets of its kind. Gravity data led to several hydrologic insights, both through direct measurement and modeling. First, the infiltration rate could be estimated accurately based on the initial rate of gravity change during infiltration, regardless of the specific yield. Using two gravity meters, the depth, and therefore speed, of the wetting front beneath a recharge basin was observed, including the time at which the water table was reached. Spatial maps of gravity change from relative gravity surveys show areas where infiltration efficiency is highest, and where groundwater accumulates; storage accumulated preferentially to the west of the recharge basins, away from pumping wells. Such information would be valuable for planning the location of pumping wells at a new facility. Gravity data were useful for calibration of a Modflow-NWT groundwater-flow model using the Unsaturated Zone Flow package to simulate recharge; the reduction in the posterior parameter distribution compared to the a priori estimate was substantial and similar to head data. In contrast to model-simulated head data, model-simulated gravity data were less sensitive to more distant model elements and more effective for calibration of a superposition-type model. Observed head data had a strong regional signal reflecting basin-scale conditions with only minor variation associated with individual recharge basins, and were therefore of limited usefulness for model calibration. Together, the methods developed by the study and interpretations they made possible suggest that gravity data are an effective way to better understand large-scale infiltration and groundwater movement.

>Magister Scientiae - MSc / The need to improve groundwater security remains critical especially in urban areas where demand for groundwater as an alternative source of water supply increases. Declining trends in availability of surface water because of climate change effects further exacerbates problems of water supply shortage to meet the increasing demand for water, hence the need for groundwater sources. The use of hydrogeophysics data and derivative analysis in understanding aquifer dynamics remains limited and poorly understood therefore, the study argues that when hydrogeophysics data and derivative analysis are not used in aquifer characterization, it results in models that are not solution-based and cannot guide groundwater management. The study was aimed at providing improved understanding on characterization of aquifer dynamics for solution-based modelling while addressing the importance of integrating hydrogeophysics data and derivative analysis in amplifying the heterogeneities that exist in aquifer system.

The Geropotamos aquifer on the north‐central coast of Crete, Greece, is invaded in some places by salt water from the Aegean Sea, with impact on freshwater supplies for domestic and business uses, including agriculture. The geological setting of the study area is considered complex, as Miocene biogenic limestones, marls, clays and conglomerates crop out in the central and the western part and clastic limestones and dolomites of the Tripolis and Plattenkalk nappe (the bedrock) in the eastern part of the study area. The phyllitequartzite nappe (which forms the oldest rock of the study area) lays on the northern part of Geropotamos basin. The local tectonic regime of the study area is characterized by faults of NW‐SE and NE‐SW directions. Investigation of the aquifer using Transient ElectroMagnetic method (TEM) and Vertical Electrical Resistivity (VES) measurement technique has resulted in 1D models and 2D/3D imaging of geoelectric structures, depicting the zones of salination of groundwater in the aquifer. 1179 TEM soundings in 372 sites have been carried out in a detailed survey grid (about 200m in X and Y dimension) and 3 VES soundings were acquired in three different sites (different geological conditions). For the 2 of them, multidirectional measurements were also acquired since the structure is more complex than a 1D model that VES technique is able to model. Moreover, 3 water samplings carried out. At each sampling, samples from 22 boreholes and 2 springs were analysed and 16 chemical parameters were determined. Detailed geochemical analysis, including Piper, Durov, Ternary, Stiff, Wilcox, Dispersion diagrams and Factors controlling the groundwater quality, was accomplished showing very good results and the relationship with the geophysical methods. All data were inserted in GIS environment and Groundwater Quality Maps were produced. Furthermore, Remote Sensing application, geological mapping and hydro‐lithological data showed that the physical characteristics of geomorphology and geology are in great relationship with the chemical and geophysical properties as well. Suggestions that Miocene evaporites led to groundwater salination are unconfirmed, and seawater intrusion is the most probable cause, supported by the results of this research. It is indicated that saline intrusion is likely to occur along fractures in a fault zone through otherwise low‐permeability phyllite‐quartzite bedrock, and it is emphasized the critical role of fracture pathways in salination problems of coastal aquifers.

Groundwater-level measurements in monitoring wells or piezometers are the most common, and often the only, hydrologic measurements made at artificial recharge facilities. Measurements of gravity change over time provide an additional source of information about changes in groundwater storage, infiltration, and for model calibration. We demonstrate that for an artificial recharge facility with a deep groundwater table, gravity data are more sensitive to movement of water through the unsaturated zone than are groundwater levels. Groundwater levels have a delayed response to infiltration, change in a similar manner at many potential monitoring locations, and are heavily influenced by high-frequency noise induced by pumping; in contrast, gravity changes start immediately at the onset of infiltration and are sensitive to water in the unsaturated zone. Continuous gravity data can determine infiltration rate, and the estimate is only minimally affected by uncertainty in water-content change. Gravity data are also useful for constraining parameters in a coupled groundwater-unsaturated zone model (Modflow-NWT model with the Unsaturated Zone Flow (UZF) package).

While restoring hyporheic flowpaths has been cited as a benefit to stream restoration structures, little documentation exists confirming that constructed restoration structures induce hyporheic exchange comparable to natural stream features. This study compares a stream restoration structure (cross-vane) to a natural feature (riffle) concurrently in the same stream reach using time-lapsed electrical resistivity (ER) tomography. Using this hydrogeophysical approach, I am able to quantify hyporheic extent and transport beneath the cross-vane structure and riffle. I interpret from the geophysical data that the cross-vane and natural riffle induced spatially and temporally unique hyporheic extent and transport, and the cross-vane created both spatially larger and temporally longer hyporheic flowpaths than the natural riffle. Tracer from the 4.67-hr injection was detected along flowpaths for 4.6-hrs at the cross-vane and 4.2-hrs at the riffle. The spatial extent of the hyporheic zone at the cross-vane was 12% larger than at the riffle. I compare ER results of this study to vertical fluxes calculated from temperature profiles and conclude significant differences in the interpretation of hyporheic transport from these different field techniques. Results of this study demonstrate a high degree of heterogeneity in transport metrics at both the cross-vane and riffle and significant differences between the hyporheic flowpath networks at the two different features. Our results suggest that restoration structures may be capable of creating sufficient exchange flux and residence times to achieve the same ecological functions as natural features, but engineering of the physical and biogeochemical environment may be necessary to realize those benefits.

Declining groundwater tables threatens several municipalities in Sweden which drinking water is collected from. To ensure a sound drinking water supply, the Geological Survey of Sweden has initiated a groundwater exploration plan. Airborne electromagnetic measurements have seen an uprise in hydrogeophysical projects and have a great potential to achieve high-quality models, especially when combined with drilling data. In 2018, the Geological Survey of Sweden conducted an airborne electromagnetic survey, using the SkyTEM system, in the outskirts of Örebro, Sweden. SkyTEM is a time-domain system and is the most favoured system in hydrogeophysical investigations and was developed especially with hydrogeophysical applications in mind. It is unique by being able to measure interleaved low and high moment current pulses which enables for both high resolution close to surface and increased depth of investigation. During 2019, further drilling in the area including both lithological, and geophysical logging including natural gamma and normal resistivity were carried out. High natural gamma radiation typically indicates content of clay in the rocks. The geology in the area is well explored since the 1940’s when oil was extracted from alum shale in Kvarntorp, located in the survey area. Rocks of sedimentary origin reaches approximately 80 m down until contact with bedrock. Well preserved layers of limestone, shale, alum shale and sandstone are common throughout the area. Combining SkyTEM data with borehole data increases the confidence and generates a model better reflecting the geology in the area. The AarhusInv inversion code was used to perform the modelling, developed by the HydroGeophysical Group (HGG) at Aarhus University, Denmark. Four different models along one single line were generated by using 3, 4, 6 and 30 layers for the reference model in the inversion. Horizontal constraints were applied to all models. Vertical constraints were only applied to the 30 layer model. The survey flight altitude is considered high and in combination with removal of data points being affected by noise, the maximum number of layers in the final model is limited to three. This suggests that the 3 layer model is the most representative model for this survey. The conductive shale seen in the geophysical log is visible in all models at a depth of roughly 40-60 m which is consistent with the geophysical log. No information is retrieved below the shale which concludes that the contact between the sandstone and crystalline rock is not resolved. The lack of information below a highly conductive structure is expected due to shielding effects. This study recommend to carefully assess the flight altitude at quality-control analysis during survey design.

An acoustic frequency-swept wave was investigated as a means for determining Snow Water Equivalent (SWE) in cold wind-swept prairie and sub-alpine environments. Building on previous research conducted by investigators who have examined the propagation of sound in snow, digital signal processing was used to determine acoustic pressure wave reflection coefficients at the interfaces between 'layers' indicative of changes in acoustic impedance. Using an iterative approach involving boundary conditions at the interfaces, the depth-integrated SWE was determined using the Berryman equation from porous media physics. Apparatuses used to send and receive sound waves were designed and deployed during the winter season at field sites situated near the city of Saskatoon, Saskatchewan, and in Yoho National Park, British Columbia. Data collected by gravimetric sampling was used as comparison for the SWE values determined by acoustic sounding. The results are encouraging and suggest that this procedure is similar in accuracy to SWE data collected using gravimetric sampling. Further research is required to determine the applicability of this technique for snow situated at other geographic locations.

Near-surface sediments in the vadose zone play a fundamental role in the hydrologic system. The shallow vadose zone can act as a buffer to delay or attenuate surface contaminants before they reach the water table. It also acts as a temporary soil moisture reservoir for plant and atmospheric uptake, and regulates the seasonal groundwater recharge process. Over the past few decades, geophysical methods have received unprecedented attention as an effective vadose zone characterization tool offering a range of non-invasive to minimally invasive techniques with the capacity to provide detailed soil moisture information at depths typically unattainable using conventional point-measurement sensors. Ground-penetrating radar (GPR) has received much of this attention due to its high sensitivity to the liquid water phase in geologic media. While much has been learned about GPR soil moisture monitoring and characterization techniques, it has not been evaluated across highly dynamic natural soil conditions. Consequently, GPR’s capacity to characterize a complete range of naturally occurring vadose zone conditions including wetting/drying and freeze/thaw cycles, is not yet fully understood. Further, the nature of GPR response during highly dynamic moisture periods has not been thoroughly investigated. The objective of this thesis is to examine the capacity of various surface GPR techniques and methodologies for the characterization of soil moisture dynamics in the upper few meters of vadose zone, and to develop measurement strategies capable of providing quantitative information about the current and future state of the shallow hydrologic system. To achieve this, an exhaustive soil moisture monitoring campaign employing a range of GPR antenna frequencies and survey acquisition geometries was initiated at three different agricultural field sites located in southern Ontario, Canada, between May 2006 and October 2008. This thesis represents the first attempt to evaluate multiple annual cycles of soil conditions and associated hydrological processes using high-frequency GPR measurements. Summaries of the seven major works embodied in this thesis are provided below. Direct ground wave (DGW) measurements obtained with GPR have been used in a number of previous studies to monitor volumetric water content changes in the root zone; however, these studies have involved controlled field experiments or measurements collected across limited ranges in soil moisture. To further investigate the capacity of the DGW method, multi-frequency (i.e., 225 MHz, 450 MHz and 900 MHz) common-midpoint (CMP) measurements were used to monitor a complete annual cycle of soil water content variations at three sites with different soil textures (i.e., sand, sandy loam and silt loam). CMP surveys permitted characterization of the nature and evolution of the near-surface electromagnetic wavefields, and their subsequent impact on DGW velocity measurements. GPR results showed significant temporal variations in both the near-surface wavefield and multi-frequency DGW velocities corresponding to both seasonal and shorter term variations in soil conditions. While all of the measurement sites displayed similar temporal responses, the rate and magnitude of these velocity variations corresponded to varying soil water contents which were primarily controlled by the soil textural properties. Overall, the DGW measurements obtained using higher frequency antennas were less impacted by near-surface wavefield interference due to their shorter signal pulse duration. The estimation of soil water content using GPR velocity requires an appropriate petrophysical relationship between the dielectric permittivity and volumetric water content of the soil. The ability of various empirical relationships, volumetric mixing formulae and effective medium approximations were evaluated to predict near-surface volumetric soil water content using high-frequency DGW velocity measurements obtained from CMP soundings. Measurements were collected using 225, 450 and 900 MHz antennas across sand, sandy loam and silt loam soil textures over a complete annual cycle of soil conditions. A lack of frequency dependence in the results indicated that frequency dispersion had minimal impact on the data set. However, the accuracy of soil water content predictions obtained from the various relationships ranged considerably. The best fitting relationships did exhibit some degree of textural bias that should be considered in the choice of petrophysical relationship for a given data set. Further improvements in water content estimates were obtained using a field calibrated third-order polynomial relationship and three-phase volumetric mixing formula. While DGW measurements provide valuable information within the root zone, the characterization of vertical moisture distribution and dynamics requires a different approach. A common approach utilizes normal-moveout (NMO) velocity analysis of CMP sounding data. To further examine this approach, an extensive field study using multi-frequency (i.e., 225 MHz, 450 MHz, 900 MHz) CMP soundings was conducted to monitor a complete annual cycle of vertical soil moisture conditions at the sand, sandy loam and silt loam sites. The use of NMO velocity analysis was examined for monitoring highly dynamic vertical soil moisture conditions consisting of wetting/drying and freeze/thaw cycles with varying degrees of magnitude and vertical velocity gradient. NMO velocity analysis was used to construct interval-velocity-depth models at a fixed location collected every 1 to 4 weeks. Time-lapse models were combined to construct temporal interval-velocity fields, which were converted into soil moisture content. These moisture fields were used to characterize the vertical distribution, and dynamics of soil moisture in the upper few meters of vadose zone. Although the use of multiple antenna frequencies provided varying investigation depths and vertical resolving capabilities, optimal characterization of soil moisture conditions was obtained with 900 MHz antennas. The integration of DGW and NMO velocity data from a single CMP sounding could be used to assess the nature of shallow soil moisture coupling with underlying vadose zone conditions; however, a more quantitative analyses of the surface moisture dynamics would require definitive knowledge of GPR sampling depth. Although surface techniques have been used by a number of previous researchers to characterize soil moisture content in the vadose zone, limited temporal sampling and low resolution near the surface in these studies impeded the quantitative analysis of vertical soil moisture distribution and its associated dynamics within the shallow subsurface. To further examine the capacity of surface GPR, an extensive 26 month field study was undertaken using concurrent high-frequency (i.e., 900 MHz) reflection profiling and CMP soundings to quantitatively monitor soil moisture distribution and dynamics within a sandy vadose zone environment. An analysis on the concurrent use of reflection and CMP measurements was conducted over two contrasting annual cycles of soil conditions. Reflection profiles provided high resolution traveltime data between four stratigraphic reflection events while cumulative results of the CMP sounding data set produced precise depth estimates for those reflecting interfaces, which were used to convert interval traveltime data into soil water content estimates. The downward propagation of episodic infiltration events associated with seasonal and transient conditions were well resolved by the GPR data. The GPR data also revealed variations in the nature of these infiltration events between contrasting annual cycles. The use of CMP soundings also permitted the determination of DGW velocities, which enabled better characterization of short-duration wetting/drying and freezing/thawing processes. This higher resolution information can be used to examine the nature of the coupling between shallow and deep moisture conditions. High-resolution surface GPR measurements were used to examine vertical soil moisture distribution and its associated dynamics within the shallow subsurface over a 26 month period. While the apparent ability of surface GPR methods to give high quality estimates of soil moisture distribution in the upper 3 meters of the vadose zone was demonstrated, the nature of these GPR-derived moisture data needed to be assessed in the context of other hydrological information. As a result, GPR soil moisture estimates were compared with predictions obtained from a well-accepted hydrological modeling package, HYDRUS-1D (Simunek et al., 2008). The nature of transient infiltration pulses, evapotranspiration episodes, and deep drainage patterns were examined by comparing them with vertical soil moisture flow simulations. Using laboratory derived soil hydraulic property information from soil samples and a number of simplifying assumptions about the system, very good agreement was achieved between measured and simulated soil moisture conditions without model calibration. The overall good agreement observed between forward simulations and field measurements over the vertical profile validated the capacity of surface GPR to provide detailed information about hydraulic state conditions in the upper few meters of vadose zone. A unique DGW propagation phenomenon was observed during early soil frost formation. High-frequency DGW measurements were used to monitor the seasonal development of a thin, high velocity frozen soil layer over a wet low velocity unfrozen substratum. During the freezing process, the progressive attenuation of a low velocity DGW and the subsequent development of a high velocity DGW were observed. Numerical simulations using GPRMAX2D (Giannopoulos, 2005) showed that low velocity DGW occurring after freezing commenced was due to energy leaking across the frozen layer from the spherical body wave in the unfrozen half space. This leaky phase progressively dissipated until the frozen layer reached a thickness equivalent to one quarter of the dominant wavelength in the frozen ground. The appearance of the high velocity DGW was governed by its destructive interference with the reflection events from the base of the frozen layer. This interference obscured the high velocity DGW until the frozen layer thickness reached one half of the dominant wavelength in the frozen ground. While GPR has been extensively used to study frozen soil conditions in alpine environments, its capacity to characterize highly dynamic shallow freeze-thaw processes typically observed in temperate environments is not well understood. High-frequency reflection profiles and CMP soundings were used to monitor the freezing and thawing process during the winter seasonal period at the sand and silt loam sites. Reflection profiles revealed the long-term development of a very shallow (<0.5 m) soil frost zone overlying unfrozen wet substratum. During the course of the winter season, long-term traveltime analysis yielded physical properties of the frozen and unfrozen layers as well as the spatial distribution of the base of the soil frost zone. Short-term shallow thawing events overlying frozen substratum formed a dispersive waveguide for both the CMP and reflection profile surveys. Inversion of the dispersive wavefields for the CMP data yielded physical property estimates for the thawed and frozen soils and thawed layer thickness. It was shown that GPR can be used to monitor very shallow freezing and thawing events by responding to changes in the relative dielectric permittivity of the soil water phase. The works embodied in this thesis demonstrate the effectiveness of high-frequency GPR as a non-invasive soil moisture monitoring tool under a full range of naturally occurring moisture conditions with the temporal and vertical resolution necessary to quantitatively examine shallow vadose zone moisture dynamics. Because this study encompassed an unprecedented range of naturally occurring soil conditions, including numerous short and long duration wetting/drying and freezing/thawing cycles, complex geophysical responses were observed during highly dynamic soil moisture processes. Analysis and interpretation of these geophysical responses yielded both qualitative and quantitative information about the state of the hydrologic system, and hence, provided a non-invasive means of characterizing soil moisture processes in shallow vadose zone environments. In the future, these GPR soil moisture monitoring strategies should be incorporated into advanced land-surface hydrological modeling studies to improve our understanding of shallow hydrologic systems and its impacts on groundwater resources.

An acoustic frequency-swept wave was investigated as a means for determining Snow Water Equivalent (SWE) in cold wind-swept prairie and sub-alpine environments. Building on previous research conducted by investigators who have examined the propagation of sound in snow, digital signal processing was used to determine acoustic pressure wave reflection coefficients at the interfaces between 'layers' indicative of changes in acoustic impedance. Using an iterative approach involving boundary conditions at the interfaces, the depth-integrated SWE was determined using the Berryman equation from porous media physics. Apparatuses used to send and receive sound waves were designed and deployed during the winter season at field sites situated near the city of Saskatoon, Saskatchewan, and in Yoho National Park, British Columbia. Data collected by gravimetric sampling was used as comparison for the SWE values determined by acoustic sounding. The results are encouraging and suggest that this procedure is similar in accuracy to SWE data collected using gravimetric sampling. Further research is required to determine the applicability of this technique for snow situated at other geographic locations.

Near-surface sediments in the vadose zone play a fundamental role in the hydrologic system. The shallow vadose zone can act as a buffer to delay or attenuate surface contaminants before they reach the water table. It also acts as a temporary soil moisture reservoir for plant and atmospheric uptake, and regulates the seasonal groundwater recharge process. Over the past few decades, geophysical methods have received unprecedented attention as an effective vadose zone characterization tool offering a range of non-invasive to minimally invasive techniques with the capacity to provide detailed soil moisture information at depths typically unattainable using conventional point-measurement sensors. Ground-penetrating radar (GPR) has received much of this attention due to its high sensitivity to the liquid water phase in geologic media. While much has been learned about GPR soil moisture monitoring and characterization techniques, it has not been evaluated across highly dynamic natural soil conditions. Consequently, GPR’s capacity to characterize a complete range of naturally occurring vadose zone conditions including wetting/drying and freeze/thaw cycles, is not yet fully understood. Further, the nature of GPR response during highly dynamic moisture periods has not been thoroughly investigated. The objective of this thesis is to examine the capacity of various surface GPR techniques and methodologies for the characterization of soil moisture dynamics in the upper few meters of vadose zone, and to develop measurement strategies capable of providing quantitative information about the current and future state of the shallow hydrologic system. To achieve this, an exhaustive soil moisture monitoring campaign employing a range of GPR antenna frequencies and survey acquisition geometries was initiated at three different agricultural field sites located in southern Ontario, Canada, between May 2006 and October 2008. This thesis represents the first attempt to evaluate multiple annual cycles of soil conditions and associated hydrological processes using high-frequency GPR measurements. Summaries of the seven major works embodied in this thesis are provided below. Direct ground wave (DGW) measurements obtained with GPR have been used in a number of previous studies to monitor volumetric water content changes in the root zone; however, these studies have involved controlled field experiments or measurements collected across limited ranges in soil moisture. To further investigate the capacity of the DGW method, multi-frequency (i.e., 225 MHz, 450 MHz and 900 MHz) common-midpoint (CMP) measurements were used to monitor a complete annual cycle of soil water content variations at three sites with different soil textures (i.e., sand, sandy loam and silt loam). CMP surveys permitted characterization of the nature and evolution of the near-surface electromagnetic wavefields, and their subsequent impact on DGW velocity measurements. GPR results showed significant temporal variations in both the near-surface wavefield and multi-frequency DGW velocities corresponding to both seasonal and shorter term variations in soil conditions. While all of the measurement sites displayed similar temporal responses, the rate and magnitude of these velocity variations corresponded to varying soil water contents which were primarily controlled by the soil textural properties. Overall, the DGW measurements obtained using higher frequency antennas were less impacted by near-surface wavefield interference due to their shorter signal pulse duration. The estimation of soil water content using GPR velocity requires an appropriate petrophysical relationship between the dielectric permittivity and volumetric water content of the soil. The ability of various empirical relationships, volumetric mixing formulae and effective medium approximations were evaluated to predict near-surface volumetric soil water content using high-frequency DGW velocity measurements obtained from CMP soundings. Measurements were collected using 225, 450 and 900 MHz antennas across sand, sandy loam and silt loam soil textures over a complete annual cycle of soil conditions. A lack of frequency dependence in the results indicated that frequency dispersion had minimal impact on the data set. However, the accuracy of soil water content predictions obtained from the various relationships ranged considerably. The best fitting relationships did exhibit some degree of textural bias that should be considered in the choice of petrophysical relationship for a given data set. Further improvements in water content estimates were obtained using a field calibrated third-order polynomial relationship and three-phase volumetric mixing formula. While DGW measurements provide valuable information within the root zone, the characterization of vertical moisture distribution and dynamics requires a different approach. A common approach utilizes normal-moveout (NMO) velocity analysis of CMP sounding data. To further examine this approach, an extensive field study using multi-frequency (i.e., 225 MHz, 450 MHz, 900 MHz) CMP soundings was conducted to monitor a complete annual cycle of vertical soil moisture conditions at the sand, sandy loam and silt loam sites. The use of NMO velocity analysis was examined for monitoring highly dynamic vertical soil moisture conditions consisting of wetting/drying and freeze/thaw cycles with varying degrees of magnitude and vertical velocity gradient. NMO velocity analysis was used to construct interval-velocity-depth models at a fixed location collected every 1 to 4 weeks. Time-lapse models were combined to construct temporal interval-velocity fields, which were converted into soil moisture content. These moisture fields were used to characterize the vertical distribution, and dynamics of soil moisture in the upper few meters of vadose zone. Although the use of multiple antenna frequencies provided varying investigation depths and vertical resolving capabilities, optimal characterization of soil moisture conditions was obtained with 900 MHz antennas. The integration of DGW and NMO velocity data from a single CMP sounding could be used to assess the nature of shallow soil moisture coupling with underlying vadose zone conditions; however, a more quantitative analyses of the surface moisture dynamics would require definitive knowledge of GPR sampling depth. Although surface techniques have been used by a number of previous researchers to characterize soil moisture content in the vadose zone, limited temporal sampling and low resolution near the surface in these studies impeded the quantitative analysis of vertical soil moisture distribution and its associated dynamics within the shallow subsurface. To further examine the capacity of surface GPR, an extensive 26 month field study was undertaken using concurrent high-frequency (i.e., 900 MHz) reflection profiling and CMP soundings to quantitatively monitor soil moisture distribution and dynamics within a sandy vadose zone environment. An analysis on the concurrent use of reflection and CMP measurements was conducted over two contrasting annual cycles of soil conditions. Reflection profiles provided high resolution traveltime data between four stratigraphic reflection events while cumulative results of the CMP sounding data set produced precise depth estimates for those reflecting interfaces, which were used to convert interval traveltime data into soil water content estimates. The downward propagation of episodic infiltration events associated with seasonal and transient conditions were well resolved by the GPR data. The GPR data also revealed variations in the nature of these infiltration events between contrasting annual cycles. The use of CMP soundings also permitted the determination of DGW velocities, which enabled better characterization of short-duration wetting/drying and freezing/thawing processes. This higher resolution information can be used to examine the nature of the coupling between shallow and deep moisture conditions. High-resolution surface GPR measurements were used to examine vertical soil moisture distribution and its associated dynamics within the shallow subsurface over a 26 month period. While the apparent ability of surface GPR methods to give high quality estimates of soil moisture distribution in the upper 3 meters of the vadose zone was demonstrated, the nature of these GPR-derived moisture data needed to be assessed in the context of other hydrological information. As a result, GPR soil moisture estimates were compared with predictions obtained from a well-accepted hydrological modeling package, HYDRUS-1D (Simunek et al., 2008). The nature of transient infiltration pulses, evapotranspiration episodes, and deep drainage patterns were examined by comparing them with vertical soil moisture flow simulations. Using laboratory derived soil hydraulic property information from soil samples and a number of simplifying assumptions about the system, very good agreement was achieved between measured and simulated soil moisture conditions without model calibration. The overall good agreement observed between forward simulations and field measurements over the vertical profile validated the capacity of surface GPR to provide detailed information about hydraulic state conditions in the upper few meters of vadose zone. A unique DGW propagation phenomenon was observed during early soil frost formation. High-frequency DGW measurements were used to monitor the seasonal development of a thin, high velocity frozen soil layer over a wet low velocity unfrozen substratum. During the freezing process, the progressive attenuation of a low velocity DGW and the subsequent development of a high velocity DGW were observed. Numerical simulations using GPRMAX2D (Giannopoulos, 2005) showed that low velocity DGW occurring after freezing commenced was due to energy leaking across the frozen layer from the spherical body wave in the unfrozen half space. This leaky phase progressively dissipated until the frozen layer reached a thickness equivalent to one quarter of the dominant wavelength in the frozen ground. The appearance of the high velocity DGW was governed by its destructive interference with the reflection events from the base of the frozen layer. This interference obscured the high velocity DGW until the frozen layer thickness reached one half of the dominant wavelength in the frozen ground. While GPR has been extensively used to study frozen soil conditions in alpine environments, its capacity to characterize highly dynamic shallow freeze-thaw processes typically observed in temperate environments is not well understood. High-frequency reflection profiles and CMP soundings were used to monitor the freezing and thawing process during the winter seasonal period at the sand and silt loam sites. Reflection profiles revealed the long-term development of a very shallow (<0.5 m) soil frost zone overlying unfrozen wet substratum. During the course of the winter season, long-term traveltime analysis yielded physical properties of the frozen and unfrozen layers as well as the spatial distribution of the base of the soil frost zone. Short-term shallow thawing events overlying frozen substratum formed a dispersive waveguide for both the CMP and reflection profile surveys. Inversion of the dispersive wavefields for the CMP data yielded physical property estimates for the thawed and frozen soils and thawed layer thickness. It was shown that GPR can be used to monitor very shallow freezing and thawing events by responding to changes in the relative dielectric permittivity of the soil water phase. The works embodied in this thesis demonstrate the effectiveness of high-frequency GPR as a non-invasive soil moisture monitoring tool under a full range of naturally occurring moisture conditions with the temporal and vertical resolution necessary to quantitatively examine shallow vadose zone moisture dynamics. Because this study encompassed an unprecedented range of naturally occurring soil conditions, including numerous short and long duration wetting/drying and freezing/thawing cycles, complex geophysical responses were observed during highly dynamic soil moisture processes. Analysis and interpretation of these geophysical responses yielded both qualitative and quantitative information about the state of the hydrologic system, and hence, provided a non-invasive means of characterizing soil moisture processes in shallow vadose zone environments. In the future, these GPR soil moisture monitoring strategies should be incorporated into advanced land-surface hydrological modeling studies to improve our understanding of shallow hydrologic systems and its impacts on groundwater resources.