Quasi-Saturated Layer: Implications for Estimating Recharge and Groundwater Modeling

This study presents an extension of the concept of “quasi-saturation” to a quasi-saturated layer, defined as the uppermost dynamic portion of the saturated zone subject to water table fluctuations. Entrapped air here may cause substantial reductions in the hydraulic conductivity (K) and fillable pore water. Air entrapment is caused by a rising water table, usually as a result of groundwater recharge. The most significant effects of entrapped air are recharge overestimation based on methods that use specific yield (S_y), such as the water table fluctuation method (WTF), and reductions in K values. These effects impact estimation of fluid flow velocities and contaminant migration rates in groundwater. In order to quantify actual groundwater recharge rates and the effects of entrapped air, numerical simulations with the FEFLOW (Version 7.0) groundwater flow model were carried out using a quasi-saturated layer for a pilot area in Rio Claro, Brazil. The calculated recharge rate represented 16% of the average precipitation over an 8-year period, approximately half of estimates using the WTF method. Air entrapment amounted to a fillable porosity of 0.07, significant lower that the value of 0.17 obtained experimentally for S_y. Numerical results showed that the entrapped air volume in the quasi-saturated layer can be very significant (0.58 of the air fraction) and hence can significantly affect estimates of groundwater recharge and groundwater flow rates near the water table.     

Patterns of Entrapped Air Dissolution in a Two‐Dimensional Pilot‐Scale Synthetic Aquifer

Past studies of entrapped air dissolution have focused on one‐dimensional laboratory columns. Here the multidimensional nature of entrapped air dissolution was investigated using an indoor tank (180 × 240 × 600 cm³) simulating an unconfined sand aquifer with horizontal flow. Time domain reflectometry (TDR) probes directly measured entrapped air contents, while dissolved gas conditions were monitored with total dissolved gas pressure (P_TDG) probes. Dissolution occurred as a diffuse wedge‐shaped front from the inlet downgradient, with preferential dissolution at depth. This pattern was mainly attributed to increased gas solubility, as shown by P_TDG measurements. However, compression of entrapped air at greater depths, captured by TDR and leading to lower quasi‐saturated hydraulic conductivities and thus greater velocities, also played a small role. Linear propagation of the dissolution front downgradient was observed at each depth, with both TDR and P_TDG, with increasing rates with depth (e.g, 4.1 to 5.7× slower at 15 cm vs. 165 cm depth). P_TDG values revealed equilibrium with the entrapped gas initially, being higher at greater depth and fluctuating with the barometric pressure, before declining concurrently with entrapped air contents to the lower P_TDG of the source water. The observed dissolution pattern has long‐term implications for a wide variety of groundwater management issues, from recharge to contaminant transport and remediation strategies, due to the persistence of entrapped air near the water table (potential timescale of years). This study also demonstrated the utility of P_TDG probes for simple in situ measurements to detect entrapped air and monitor its dissolution.     

Evidence that piezometers vent gas from peat soils and implications for pore‐water pressure and hydraulic conductivity measurements

Entrapped gas bubbles in peat can alter the buoyancy, storativity, void ratio and expansion/contraction properties of the peat. Moreover, when gas bubbles block water‐conducting pores they can significantly reduce saturated hydraulic conductivity and create zones of over‐pressuring, perhaps leading to an alteration in the magnitude and direction of groundwater flow and solute transport. Some previous researches have demonstrated that these zones of over‐pressuring are not observed by the measurements of pore‐water pressures using open‐pipe piezometers in peat; rather, they are only observed with pressure transducers sealed in the peat. In has been hypothesized that open‐pipe piezometers vent entrapped CH₄ to the atmosphere and thereby do not permit the natural development of zones of entrapped gas. Here we present findings of the study to investigate whether piezometers vent subsurface CH₄ to the atmosphere and whether the presence of piezometers alters the subsurface concentration of dissolved CH₄. We measured the flux of methane venting from the piezometers and also determined changes in pore‐water CH₄ concentration at a rich fen in southern Ontario and a poor fen in southern Quebec, in the summer of 2004. Seasonally averaged CH₄ flux from piezometers was 1450 and 37·8‐mg CH₄ m^?2 d^?1 at the southern Ontario site and Quebec site, respectively. The flux at the Ontario site was two orders of magnitude greater than the diffusive flux at the site. CH₄ pore‐water concentrations were significantly lower in open piezometers than in water taken from sealed samplers at both the Ontario and Quebec sites. The flux of CH₄ from piezometers decreased throughout the season suggesting that CH₄ venting through the piezometer exceeded the rate of methanogenesis in the peat. Consequently we conclude that piezometers may alter the gas dynamics of some peatlands. We suggest that less‐invasive techniques (e.g. buried pressure transducers, tracer experiments) are needed for the accurate measurement of pore‐water pressures and hydraulic conductivity in peatlands with a large entrapped gas component. Furthermore, we argue that caution must be made in interpreting results from previous peatland hydrology studies that use these traditional methods. Copyright © 2009 John Wiley & Sons, Ltd.     

Uncertainty in applying the linear poroelasticity model to field situations as a result of periodic loading in heterogeneous aquifers

The solid Earth's surface frequently experience changes in total stresses as a result of periodic loading. When the fluid‐saturated porous media deform in response to changes in stress, the induced variations in pore volume affect the pore water pressure. The fluid flow therefore occurs in response to the gradient in the induced excess pore water pressure. This work aims at quantifying the spatial variability in excess pressure head produced by the periodic loading accounting for the variation of log hydraulic conductivity (lnK). It is important for the rational management of groundwater resources. A closed‐form expression is developed by the nonstationary spectral approach to analyse the influence of the statistical properties of lnK process, the hydraulic parameters, and the spatial position. The general stochastic framework outlined in this work provides a basis for assessing the impact of statistical properties of input aquifer parameters on the output variability (or uncertainty). Copyright © 2014 John Wiley & Sons, Ltd.     

Near‐saturated surface soil hydraulic properties under different land uses in the St Denis National Wildlife Area,Saskatchewan, Canada

Surface soil hydraulic properties are key factors controlling the partition of rainfall and snowmelt into runoff and soil water storage, and their knowledge is needed for sound land management. The objective of this study was to evaluate the effects of three land uses (native grass, brome grass and cultivated) on surface soil hydraulic properties under near‐saturated conditions at the St Denis National Wildlife Area, Saskatchewan, Canada. For each land use, water infiltration rates were measured using double‐ring and tension infiltrometers at ?0·3, ?0·7, ?1·5 and ?2·2 kPa pressure heads. Macroporosity and unsaturated hydraulic properties of the surface soil were estimated. Mean field‐saturated hydraulic conductivity (K_fs), unsaturated hydraulic conductivity at ?0·3 kPa pressure head, inverse capillary length scale (α) and water‐conducting macroporosity were compared for different land uses. These parameters of the native grass and brome grass sites were significantly (p < 0·1) higher than that of the cultivated sites. At the ?0·3 kPa pressure head, hydraulic conductivity of grasslands was two to three times greater than that of cultivated lands. Values of α were about two times and values of K_fs about four times greater in grasslands than in cultivated fields. Water‐conducting macroporosity of grasslands and cultivated fields were 0·04% and 0·01% of the total soil volume, respectively. Over 90% of the total water flux at ?0·06 kPa pressure head was transmitted through pores > 1·36 × 10^?4 m in diameter in the three land uses. Land use modified near‐saturated hydraulic properties of surface soil and consequently may alter the water balance of the area by changing the amount of surface runoff and soil water storage. Copyright © 2004 John Wiley & Sons, Ltd.     

Determining hydraulic properties of a loam soil by alternative infiltrometer techniques

Testing infiltrometer techniques to determine soil hydraulic properties is necessary for specific soils. For a loam soil, the water retention and hydraulic conductivity predicted by the BEST (Beerkan Estimation of Soil Transfer parameters) procedure of soil hydraulic characterization was compared with data collected by more standard laboratory and field techniques. Six infiltrometer techniques were also compared in terms of saturated soil hydraulic conductivity, K_s. BEST yielded water retention values statistically similar to those obtained in the laboratory and K_s values practically coinciding with those determined in the field with the pressure infiltrometer (PI). The unsaturated soil hydraulic conductivity measured with the tension infiltrometer (TI) was reproduced satisfactorily by BEST only close to saturation. BEST, the PI, one‐potential experiments with both the TI and the mini disk infiltrometer (MDI), the simplified falling head (SFH) technique and the bottomless bucket (BB) method yielded statistically similar estimates of K_s, differing at the most by a factor of three. Smaller values were obtained with longer and more soil‐disturbing infiltration runs. Any of the tested infiltration techniques appears usable to obtain the order of magnitude of K_s at the field site, but the BEST, BB and PI data appear more appropriate to characterize the soil at some stage during a rainfall event. Additional investigations on both similar and different soils would allow development of more general procedures to apply infiltrometer techniques for soil hydraulic characterization. Copyright © 2015 John Wiley & Sons, Ltd.     

Peat deformation and biogenic gas bubbles control seasonal variations in peat hydraulic conductivity

The hydraulic conductivity (K) of peat beneath the water table varies over short (annual) periods. Biogenic gas bubbles block pores and reduce K, and seasonal changes in the water table position cause peat deformation, altering peat pore size distribution. Although it has been hypothesized that both processes reduce K during warm dry summer conditions, temporal variations in K under field conditions have been explained previously by peat volume changes (strain) alone. We determine the effect of both controls on K by monitoring changes in gas content (Δγ), strain and K within a poor fen. Over the growing season, K decreased by an order of magnitude. In the near‐surface peat (0.3–0.7 m), this reduction is more strongly correlated with Δγ, providing the first field‐based evidence that biogenic gas bubbles reduce K. In the deeper peat (0.7–1.3 m), K is correlated principally with strain. However, causality is uncertain because of multicollinearity between strain and Δγ. To mitigate for multicollinearity, we took advantage of a peatland drainage experiment where the water table was artificially dropped at the beginning of the growing season, reducing correlations between strain and Δγ. Δγ remained the primary cause of K variations just beneath the water table at a depth of 0.5–0.7 m, although further down through the peat profile (0.7–1.2 m) changes in K were controlled by strain. We suggest that the larger pore structure of the poorly decomposed peat just below the water table is impacted less by volume changes than that of the more decomposed peat at depth. However, within this poorly decomposed peat, K is reduced by the high gas contents that result from higher rates of methane production. Copyright © 2012 John Wiley & Sons, Ltd.     

Sensitivity of soil parameters in unsaturated zone modelling and the relation between effective,laboratory and in situ estimates

Simulation of soil moisture content requires effective soil hydraulic parameters that are valid at the modelling scale. This study investigates how these parameters can be estimated by inverse modelling using soil moisture measurements at 25 locations at three different depths (at the surface, at 30 and 60 cm depth) on an 80 by 20 m hillslope. The study presents two global sensitivity analyses to investigate the sensitivity in simulated soil moisture content of the different hydraulic parameters used in a one‐dimensional unsaturated zone model based on Richards' equation. For estimation of the effective parameters the shuffled complex evolution algorithm is applied. These estimated parameters are compared to their measured laboratory and in situ equivalents. Soil hydraulic functions were estimated in the laboratory on 100 cm³ undisturbed soil cores collected at 115 locations situated in two horizons in three profile pits along the hillslope. Furthermore, in situ field saturated hydraulic conductivity was estimated at 120 locations using single‐ring pressure infiltrometer measurements. The sensitivity analysis of 13 soil physical parameters (saturated hydraulic conductivity (K_s), saturated moisture content (θ_s), residual moisture content (θ_r), inverse of the air‐entry value (α), van Genuchten shape parameter (n), Averjanov shape parameter (N) for both horizons, and depth (d) from surface to B horizon) in a two‐layer single column model showed that the parameter N is the least sensitive parameter. K_s of both horizons, θ_s of the A horizon and d were found to be the most sensitive parameters. Distributions over all locations of the effective parameters and the distributions of the estimated soil physical parameters from the undisturbed soil samples and the single‐ring pressure infiltrometer estimates were found significantly different at a 5% level for all parameters except for α of the A horizon and K_s and θ_s of the B horizon. Different reasons are discussed to explain these large differences. Copyright © 2004 John Wiley & Sons, Ltd.     

Comparing hydraulic properties of different forest floors

The forest floor plays an important role in runoff rate, soil erosion and soil infiltration capacity by protecting mineral soils from the direct impact of falling raindrops. Forest floor consists of different kinds of litter with different hydraulic properties. In this study, the inverse method was used to estimate the hydraulic properties of three kinds of forest floor (broad‐leaved, needle‐leaved and mixed‐stand) at three replications in a completely random design. Forest floor samples were collected from the Gilan Province, Iran. The samples were piled up to make long columns 40.88 cm high with an inner diameter of 18.1 cm. Artificial rainfall experiments were conducted on top of the columns, and free drainage from the bottom of the columns was measured in the laboratory. Saturated hydraulic conductivity (K_s), saturated water content and water retention curve parameters (van Genuchten equation) were estimated by the inverse method. The results showed that the K_s of needle‐leaved samples differed significantly (p < 0.05) from those of broad‐leaved and mixed‐stand samples, whereas the latter two did not differ in this regard. No significant differences emerged in the water retention function parameters of van Genuchten (θ_r, β and α) in the three forest floor samples. The saturated water content of mixed‐stand samples was significantly different (p < 0.05) from that of broad‐leaved and needle‐leaved treatments with the latter two samples showing no significant difference. The good agreement between simulated and observed free drainage for all forest floor samples in the validation period indicates that the estimated hydraulic properties efficiently characterize the unsaturated water flow in the forest floor. Copyright © 2013 John Wiley & Sons, Ltd.     

Transient Recharge Estimability Through Field‐Scale Groundwater Model Calibration

The estimation of recharge through groundwater model calibration is hampered by the nonuniqueness of recharge and aquifer parameter values. It has been shown recently that the estimability of spatially distributed recharge through calibration of steady‐state models for practical situations (i.e., real‐world, field‐scale aquifer settings) is limited by the need for excessive amounts of hydraulic‐parameter and groundwater‐level data. However, the extent to which temporal recharge variability can be informed through transient model calibration, which involves larger water‐level datasets, but requires the additional consideration of storage parameters, is presently unknown for practical situations. In this study, time‐varying recharge estimates, inferred through calibration of a field‐scale highly parameterized groundwater model, are systematically investigated subject to changes in (1) the degree to which hydraulic parameters including hydraulic conductivity (K) and specific yield (S_y) are constrained, (2) the number of water‐level calibration targets, and (3) the temporal resolution (up to monthly time steps) at which recharge is estimated. The analysis involves the use of a synthetic reality (a reference model) based on a groundwater model of Uley South Basin, South Australia. Identifiability statistics are used to evaluate the ability of recharge and hydraulic parameters to be estimated uniquely. Results show that reasonable estimates of monthly recharge (<30% recharge root‐mean‐squared error) require a considerable amount of transient water‐level data, and that the spatial distribution of K is known. Joint estimation of recharge, S_y and K, however, precludes reasonable inference of recharge and hydraulic parameter values. We conclude that the estimation of temporal recharge variability through calibration may be impractical for real‐world settings.     

The high hydraulic conductivity of three wooded tropical peat swamps in northeast Peru: measurements and implications for hydrological function

The form and functioning of peatlands depend strongly on their hydrological status, but there are few data available on the hydraulic properties of tropical peatlands. In particular, the saturated hydraulic conductivity (K) has not previously been measured in neotropical peatlands. Piezometer slug tests were used to measure K at two depths (50 and 90 cm) in three contrasting forested peatlands in the Peruvian Amazon: Quistococha, San Jorge and Buena Vista. Measured K at 50 cm depth varies between 0.00032 and 0.11 cm s^?1, and at 90 cm, it varies between 0.00027 and 0.057 cm s^?1. Measurements of K taken from different areas of Quistococha showed that spatial heterogeneity accounts for ~20% of the within‐site variance and that depth is a good predictor of K. However, K did not vary significantly with depth at Buena Vista and San Jorge. Statistical analysis showed that ~18% of the variance in the K data can be explained by between‐site differences. Simulations using a simple hydrological model suggest that the relatively high K values could lead to lowering of the water table by >10 cm within ~48 m of the peatland edge for domed peatlands, if subjected to a drought lasting 30 days. However, under current climatic conditions, even with high K, peatlands would be unable to shed the large amount of water entering the system via rainfall through subsurface flow alone. We conclude that most of the water leaves these peatlands via overland flow and/or evapotranspiration. Copyright © 2013 John Wiley & Sons, Ltd.     

Effects of Multiscale Anisotropy on Basin and Hyporheic Groundwater Flow

Various subsurface flow systems exhibit a combination of small‐scale to large‐scale anisotropy in hydraulic conductivity (K). The large‐scale anisotropy results from systematic trends (e.g., exponential decrease or increase) of K with depth. We present a general two‐dimensional solution for calculation of topography‐driven groundwater flow considering both small‐ and large‐scale anisotropy in K. This solution can be applied to diverse systems with arbitrary head distribution and geometry of the water table boundary, such as basin or hyporheic flow. In a special case, this solution reduces to the well‐known Tóth model of uniform isotropic basin. We introduce an integral measure of flushing intensity that quantifies flushing at different depths. Using this solution, we simulate heads and streamlines and provide analyses of flow structure in the flow domain, relevant to basin analyses or hyporheic flow. It is shown that interactions between small‐scale anisotropy and large‐scale anisotropy strongly control the flow structure. In the classic Tóth flow model, the flushing intensity curves exhibit quasi‐exponential decrease with depth. The new measure is capable of capturing subtle changes in the flow structure. Our study shows that both small‐ and large‐scale anisotropy characteristics have substantial effects that need to be integrated into analysis of topography‐driven flow.     

Soil hydraulic conductivities of latosols under pasture,forest and teak in Rondonia,Brazil

We investigated the changes of saturated hydraulic conductivity, K_sat , with depth of latosols developed on Precambrian basement rocks under primary rainforest, pasture and teak. In all cases, K_sat decreased with depth, with most of the decrease occurring between the surface and a depth of 30 cm. In conjunction with prevailing rainfall intensities and frequencies, this anisotropy supports a pronounced lateral component of hillslope flow paths, and also of overland flow under pasture. Our results are at variance with data from other latosols where K_sat tends to increase with depth, and hence suggest that considerable restraint is needed in generalization and extrapolation until results from a co‐ordinated effort at hydrology‐oriented data collection become available. Copyright © 1999 John Wiley & Sons, Ltd.     

The usefulness of outcrop analogue air permeameter measurements for analyzing aquifer heterogeneity: quantifying outcrop hydraulic conductivity and its spatial variability

Saturated hydraulic conductivity (K) is one of the most important parameters determining groundwater flow and contaminant transport in both unsaturated and saturated porous media. Although several well‐established laboratory methods exist for determining K, in situ measurements of this parameter remain very complex and scale dependent. Often, the limited accessibility of subsurface sediments for sampling means an additional impediment to our ability to quantify subsurface K heterogeneity. One potential solution is the use of outcrops as analogues for subsurface sediments. This paper investigates the use of air permeameter measurements on outcrops of unconsolidated sediments to quantify K and its spatial heterogeneity on a broad range of sediment types. The Neogene aquifer in northern Belgium is used as a case study for this purpose. To characterize the variability in K, 511 small‐scale air permeability measurements were performed on outcrop sediments representative over five of the aquifer's lithostratigraphic units. From these measurements, outcrop‐scale equivalent K tensors were calculated using numerical upscaling techniques. Validation of the air permeameter‐based K values by comparison with laboratory constant head K measurements reveals a correlation of 0.93. Overall, the results indicate that hand‐held air permeameters are very efficient and accurate tools to characterize saturated K, as well as its small‐scale variability and anisotropy on a broad range of unconsolidated sediments. The studied outcrops further provided a qualitative understanding of aquifer hydrostratigraphy and quantitative estimates about K variability at the centimetre‐scale to metre‐scale. Copyright © 2013 John Wiley & Sons, Ltd.     

Unsaturated hydraulic properties of xerophilous mosses: towards implementation of moss covered soils in hydrological models

Evaporation from mosses and lichens can form a major component of the water balance, especially in ecosystems where mosses and lichens often grow abundantly, such as tundra, deserts and bogs. To facilitate moss representation in hydrological models, we parameterized the unsaturated hydraulic properties of mosses and lichens such that the capillary water flow through moss and lichen material during evaporation could be assessed. We derived the Mualem‐van Genuchten parameters of the drying retention and the hydraulic conductivity functions of four xerophilous moss species and one lichen species. The shape parameters of the retention functions (2.17 < n < 2.35 and 0.08 < α < 0.13 cm^?1) ranged between values that are typical for sandy loam and loamy sand. The shapes of the hydraulic conductivity functions of moss and lichen species diverged from those of mineral soils, because of strong negative pore‐connectivity parameters (?2.840 < l < ?2.175) and low hydraulic conductivities at slightly negative pressure heads (0.016 < K₀ < 0.280 cm/d). These K₀ values are surprisingly low, considering that mosses are very porous. However, during evaporation, large pores and voids were air filled and did not participate in capillary water flow. Small K₀ values cause mosses and lichens to be conservative with water during wet conditions, thus tempering evaporation compared to mineral soils. On the other hand, under dry conditions, mosses and lichens are able to maintain a moisture supply from the soil, leading to a higher evaporation rate than mineral soils. Hence, the modulating effect of mosses on evaporation possibly differs between wet and dry climates. Copyright © 2013 John Wiley & Sons, Ltd.     

Field Study of Subsurface Heterogeneity with Steady‐State Hydraulic Tomography

Remediation of subsurface contamination requires an understanding of the contaminant (history, source location, plume extent and concentration, etc.), and, knowledge of the spatial distribution of hydraulic conductivity (K) that governs groundwater flow and solute transport. Many methods exist for characterizing K heterogeneity, but most if not all methods require the collection of a large number of small‐scale data and its interpolation. In this study, we conduct a hydraulic tomography survey at a highly heterogeneous glaciofluvial deposit at the North Campus Research Site (NCRS) located at the University of Waterloo, Waterloo, Ontario, Canada to sequentially interpret four pumping tests using the steady‐state form of the Sequential Successive Linear Estimator (SSLE) ( Yeh and Liu 2000 ). The resulting three‐dimensional (3D) K distribution (or K‐tomogram) is compared against: ( 1 ) K distributions obtained through the inverse modeling of individual pumping tests using SSLE, and ( 2 ) effective hydraulic conductivity (K_eff) estimates obtained by automatically calibrating a groundwater flow model while treating the medium to be homogeneous. Such a K_eff is often used for designing remediation operations, and thus is used as the basis for comparison with the K‐tomogram. Our results clearly show that hydraulic tomography is superior to the inversions of single pumping tests or K_eff estimates. This is particularly significant for contaminated sites where an accurate representation of the flow field is critical for simulating contaminant transport and injection of chemical and biological agents used for active remediation of contaminant source zones and plumes.     

Spatial variability in streambed hydraulic conductivity of contrasting stream morphologies: channel bend and straight channel

Streambed hydraulic conductivity is one of the main factors controlling variability in surface water‐groundwater interactions, but only few studies aim at quantifying its spatial and temporal variability in different stream morphologies. Streambed horizontal hydraulic conductivities (K_h) were therefore determined from in‐stream slug tests, vertical hydraulic conductivities (K_v) were calculated with in‐stream permeameter tests and hydraulic heads were measured to obtain vertical head gradients at eight transects, each comprising five test locations, in a groundwater‐dominated stream. Seasonal small‐scale measurements were taken in December 2011 and August 2012, both in a straight stream channel with homogeneous elevation and downstream of a channel meander with heterogeneous elevation. All streambed attributes showed large spatial variability. K_h values were the highest at the depositional inner bend of the stream, whereas high K_v values were observed at the erosional outer bend and near the middle of the channel. Calculated K_v values were related to the thickness of the organic streambed sediment layer and also showed higher temporal variability than K_h because of sedimentation and scouring processes affecting the upper layers of the streambed. Test locations at the channel bend showed a more heterogeneous distribution of streambed properties than test locations in the straight channel, whereas within the channel bend, higher spatial variability in streambed attributes was observed across the stream than along the stream channel. Copyright © 2014 John Wiley & Sons, Ltd.     

Patterns and dynamics of river–aquifer exchange with variably-saturated flow using a fully-coupled model

The parallel physically-based surface–subsurface model PARFLOW was used to investigate the spatial patterns and temporal dynamics of river–aquifer exchange in a heterogeneous alluvial river–aquifer system with deep water table. Aquifer heterogeneity at two scales was incorporated into the model. The architecture of the alluvial hydrofacies was represented based on conditioned geostatistical indicator simulations. Subscale variability of hydraulic conductivities (K) within hydrofacies bodies was created with a parallel Gaussian simulation. The effects of subscale heterogeneity were investigated in a Monte Carlo framework. Dynamics and patterns of river–aquifer exchange were simulated for a 30-day flow event. Simulation results show the rapid formation of saturated connections between the river channel and the deep water table at preferential flow zones that are characterized by high conductivity hydrofacies. Where the river intersects low conductivity hydrofacies shallow perched saturated zones immediately below the river form, but seepage to the deep water table remains unsaturated and seepage rates are low. Preferential flow zones, although only taking up around 50% of the river channel, account for more than 98% of total seepage. Groundwater recharge is most efficiently realized through these zones. Subscale variability of K_sat slightly increased seepage volumes, but did not change the general seepage patterns (preferential flow zones versus perched zones). Overall it is concluded that typical alluvial heterogeneity (hydrofacies architecture) is an important control of river–aquifer exchange in rivers overlying deep water tables. Simulated patterns and dynamics are in line with field observations and results from previous modeling studies using simpler models. Alluvial heterogeneity results in distinct patterns and dynamics of river–aquifer exchange with implications for groundwater recharge and the management of riparian zones (e.g. river channel-floodplain connectivity via saturated zones).     

A Proposed Method to Estimate In Situ Dissolved Gas Concentrations in Gas‐Saturated Groundwater

Gas‐saturated groundwater forms bubbles when brought to atmospheric pressure, preventing precise determination of its in situ dissolved gas concentrations. To overcome this problem, a modeling approach called the atmospheric sampling method is suggested here to recover the in situ dissolved gas concentrations of groundwater collected ex situ under atmospheric conditions at the Horonobe Underground Research Laboratory, Japan. The results from this method were compared with results measured at the same locations using two special techniques, the sealed sampler and pre‐evacuated vial methods, that have been developed to collect groundwater under its in situ conditions. In gas‐saturated groundwater cases, dissolved methane and inorganic carbon concentrations derived using the atmospheric sampling method were mostly within ±4 and ±10%, respectively, of values from the sealed sampler and pre‐evacuated vial methods. In gas‐unsaturated groundwater, however, the atmospheric sampling method overestimated the in situ dissolved methane concentrations, because the groundwater pressure at which bubbles appear (P_critical) was overestimated. The atmospheric sampling method is recommended for use where gas‐saturated groundwater can be collected only ex situ under atmospheric conditions.     

Meta‐analysis of field‐saturated hydraulic conductivity recovery following wildland fire: Applications for hydrologic model parameterization and resilience assessment

Hydrologic recovery after wildfire is critical for restoring the ecosystem services of protecting of human lives and infrastructure from hazards and delivering water supply of sufficient quality and quantity. Recovery of soil‐hydraulic properties, such as field‐saturated hydraulic conductivity (K_fs), is a key factor for assessing the duration of watershed‐scale flash flood and debris flow risks after wildfire. Despite the crucial role of K_fs in parameterizing numerical hydrologic models to predict the magnitude of postwildfire run‐off and erosion, existing quantitative relations to predict K_fs recovery with time since wildfire are lacking. Here, we conduct meta‐analyses of 5 datasets from the literature that measure or estimate K_fs with time since wildfire for longer than 3‐year duration. The meta‐analyses focus on fitting 2 quantitative relations (linear and non‐linear logistic) to explain trends in K_fs temporal recovery. The 2 relations adequately described temporal recovery except for 1 site where macropore flow dominated infiltration and K_fs recovery. This work also suggests that K_fs can have low hydrologic resistance (large postfire changes), and moderate to high hydrologic stability (recovery time relative to disturbance recurrence interval) and resilience (recovery of hydrologic function and provision of ecosystem services). Future K_fs relations could more explicitly incorporate processes such as soil‐water repellency, ground cover and soil structure regeneration, macropore recovery, and vegetation regrowth.