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At least 37 records · Page 2Linked to original sources
USGS2006

Resolving structural influences on water-retention properties of alluvial deposits

With the goal of improving property-transfer model (PTM) predictions of unsaturated hydraulic properties, we investigated the influence of sedimentary structure, defined as particle arrangement during deposition, on laboratory-measured water retention (water content vs. potential [θ(ψ)]) of 10 undisturbed core samples from alluvial deposits in the western Mojave Desert, California. The samples were classified as having fluvial or debris-flow structure based on observed stratification and measured spread of particle-size distribution. The θ(ψ) data were fit with the Rossi–Nimmo junction model, representing water retention with three parameters: the maximum water content (θ max ), the ψ-scaling parameter (ψ o ), and the shape parameter (λ). We examined trends between these hydraulic parameters and bulk physical properties, both textural—geometric mean, M g , and geometric standard deviation, σ g , of particle diameter—and structural—bulk density, ρ b , the fraction of unfilled pore space at natural saturation, A e , and porosity-based randomness index, Φ s , defined as the excess of total porosity over 0.3. Structural parameters Φ s and A e were greater for fluvial samples, indicating greater structural pore space and a possibly broader pore-size distribution associated with a more systematic arrangement of particles. Multiple linear regression analysis and Mallow's C p statistic identified combinations of textural and structural parameters for the most useful predictive models: for θ max , including A e , Φ s , and σ g , and for both ψ o and λ, including only textural parameters, although use of A e can somewhat improve ψ o predictions. Textural properties can explain most of the sample-to-sample variation in θ(ψ) independent of deposit type, but inclusion of the simple structural indicators A e and Φ s can improve PTM predictions, especially for the wettest part of the θ(ψ) curve.

Vadose Zone Journal↗
USGS2006

Can basin-scale recharge be estimated reasonably with water-balance models?

We examine in-place recharge as an example of the complex, basin-scale hydrologic processes that are being represented with simplified numerical models. The rate and distribution of recharge depend on local meteorological conditions and hydrogeologic properties. The pattern of recharge is defined predominantly by the distribution of net precipitation (precipitation less evapotranspiration), but different pedotransfer functions (PTFs) predict different fractions of precipitation that become in-place recharge at a given location. At any single location, these differences can often be explained on the basis of the PTF characteristics, but because of the complex averaging that occurs across a basin, the combined effects of meteorological variation and soil textural variation on the basin-wide recharge rates cannot be predicted on the basis of the characteristics of different PTFs. In fact, we show that the same basin-scale numerical model, using identical inputs and modeling options, can produce almost an order of magnitude variation in predicted basin total recharge depending on the choice of PTF. This suggests that sensitivity analyses should be performed on the choice of constitutive relationship (e.g., PTF) when assessing the predictive capability of basin-scale hydrologic models. ?? Soil Science Society of America.

Vadose Zone Journal↗
USGS2006

Multiphase, multicomponent parameter estimation for liquid and vapor fluxes in deep arid systems using hydrologic data and natural environmental tracers

Multiphase, multicomponent numerical models of long-term unsaturated-zone liquid and vapor movement were created for a thick alluvial basin at the Nevada Test Site to predict present-day liquid and vapor fluxes. The numerical models are based on recently developed conceptual models of unsaturated-zone moisture movement in thick alluvium that explain present-day water potential and tracer profiles in terms of major climate and vegetation transitions that have occurred during the past 10 000 yr or more. The numerical models were calibrated using borehole hydrologic and environmental tracer data available from a low-level radioactive waste management site located in a former nuclear weapons testing area. The environmental tracer data used in the model calibration includes tracers that migrate in both the liquid and vapor phases (δD, δ 18 O) and tracers that migrate solely as dissolved solutes (Cl), thus enabling the estimation of some gas-phase as well as liquid-phase transport parameters. Parameter uncertainties and correlations identified during model calibration were used to generate parameter combinations for a set of Monte Carlo simulations to more fully characterize the uncertainty in liquid and vapor fluxes. The calculated background liquid and vapor fluxes decrease as the estimated time since the transition to the present-day arid climate increases. However, on the whole, the estimated fluxes display relatively little variability because correlations among parameters tend to create parameter sets for which changes in some parameters offset the effects of others in the set. Independent estimates on the timing since the climate transition established from packrat midden data were essential for constraining the model calibration results. The study demonstrates the utility of environmental tracer data in developing numerical models of liquid- and gas-phase moisture movement and the importance of considering parameter correlations when using Monte Carlo analysis to characterize the uncertainty in moisture fluxes.

Vadose Zone Journal↗
USGS2007

Climate variability controls on unsaturated water and chemical movement, High Plains aquifer, USA

Responses in the vadose zone and groundwater to interannual, interdecadal, and multidecadal climate variability have important implications for groundwater resource sustainability, yet they are poorly documented and not well understood in most aquifers of the USA. This investigation systematically examines the role of interannual to multidecadal climate variability on groundwater levels, deep infiltration (3–23 m) events, and downward displacement (>1 m) of chloride and nitrate reservoirs in thick (15–50 m) vadose zones across the regionally extensive High Plains aquifer. Such vadose zone responses are unexpected across much of the aquifer given a priori that unsaturated total-potential profiles indicate upward water movement from the water table toward the root zone, mean annual potential evapotranspiration exceeds mean annual precipitation, and millennia-scale evapoconcentration results in substantial vadose zone chloride and nitrate reservoirs. Using singular spectrum analysis (SSA) to reconstruct precipitation and groundwater level time-series components, variability was identified in all time series as partially coincident with known climate cycles, such as the Pacific Decadal Oscillation (PDO) (10–25 yr) and the El Niño/Southern Oscillation (ENSO) (2–6 yr). Using these lag-correlated hydrologic time series, a new method is demonstrated to estimate climate-varying unsaturated water flux. The results suggest the importance of interannual to interdecadal climate variability on water-flux estimation in thick vadose zones and provide better understanding of the climate-induced transients responsible for the observed deep infiltration and chemical-mobilization events. Based on these results, we discuss implications for climate-related sustainability of the High Plains aquifer.

Vadose Zone Journal↗
USGS2007

Research activities at U.S. Government agencies in subsurface reactive transport modeling

The fate of contaminants in the environment is controlled by both chemical reactions and transport phenomena in the subsurface. Our ability to understand the significance of these processes over time requires an accurate conceptual model that incorporates the various mechanisms of coupled chemical and physical processes. Adsorption, desorption, ion exchange, precipitation, dissolution, growth, solid solution, redox, microbial activity, and other processes are often incorporated into reactive transport models for the prediction of contaminant fate and transport. U.S. federal agencies use such models to evaluate contaminant transport and provide guidance to decision makers and regulators for treatment issues. We provide summaries of selected research projects and programs to demonstrate the level of activity in various applications and to present examples of recent advances in subsurface reactive transport modeling.

Vadose Zone Journal↗
USGS2007

Vertical spatial sensitivity and exploration depth of low-induction-number electromagnetic-induction instruments

Vertical spatial sensitivity and effective depth of exploration (d e) of low-induction-number (LIN) instruments over a layered soil were evaluated using a complete numerical solution to Maxwell's equations. Previous studies using approximate mathematical solutions predicted a vertical spatial sensitivity for instruments operating under LIN conditions that, for a given transmitter-receiver coil separation (s), coil orientation, and transmitter frequency, should depend solely on depth below the land surface. When not operating under LIN conditions, vertical spatial sensitivity and de also depend on apparent soil electrical conductivity (??a) and therefore the induction number (??). In this new evaluation, we determined the range of ??a and ?? values for which the LIN conditions hold and how de changes when they do not. Two-layer soil models were simulated with both horizontal (HCP) and vertical (VCP) coplanar coil orientations. Soil layers were given electrical conductivity values ranging from 0.1 to 200 mS m-1. As expected, de decreased as ??a increased. Only the least electrically conductive soil produced the de expected when operating under LIN conditions. For the VCP orientation, this was 1.6s, decreasing to 0.8s in the most electrically conductive soil. For the HCP orientation, de decreased from 0.76s to 0.51s. Differences between this and previous studies are attributed to inadequate representation of skin-depth effect and scattering at interfaces between layers. When using LIN instruments to identify depth to water tables, interfaces between soil layers, and variations in salt or moisture content, it is important to consider the dependence of de on ??a. ?? Soil Science Society of America.

Vadose Zone Journal↗
USGS2007

Nitrous oxide fluxes from cultivated areas and rangeland: U.S. High Plains

Concentration profiles of N2O, a greenhouse gas, and the conservative trace gases SF6 and the chlorofluorocarbons CFC-11, CFC-12, CFC-113, and were measured periodically through thick vadose zones at nine sites in the U.S. High Plains. The CFC and SF6 measurements were used to calibrate a one-dimensional gas diffusion model, using the parameter identification program UCODE. The calibrated model was used with N2O measurements to estimate average annual N2O flux from both the root zone and the deep vadose zone to the atmosphere. Estimates of root-zone N 2O fluxes from three rangeland sites ranged from near 0 to about 0.2 kg N2O-N ha-1 yr-1, values near the low end of the ranges determined for native grass from other studies. Estimates of root-zone N2O fluxes from two fields planted to corn (Zea mays L.) of about 2 to 6 kg N2O-N ha-1 yr-1 are similar to those determined for corn in other studies. Estimates of N2O flux from Conservation Reserve grassland converted from irrigated corn indicate that production of N2O is substantially reduced following conversion from cropland. Small N2O fluxes from the water table or from deep in the vadose zone occurred at three sites, ranging from 0.004 to 0.02 kg N 2O-N ha-1 yr-1. Our estimates of N2O flux represent space- and time-averaged values that should be useful to more fully evaluate the significance of instantaneous point flux measurements. ?? Soil Science Society of America.

Vadose Zone Journal↗
USGS2007

Physically based estimation of soil water retention from textural data: General framework, new models, and streamlined existing models

Numerous models are in widespread use for the estimation of soil water retention from more easily measured textural data. Improved models are needed for better prediction and wider applicability. We developed a basic framework from which new and existing models can be derived to facilitate improvements. Starting from the assumption that every particle has a characteristic dimension R associated uniquely with a matric pressure ψ and that the form of the ψ– R relation is the defining characteristic of each model, this framework leads to particular models by specification of geometric relationships between pores and particles. Typical assumptions are that particles are spheres, pores are cylinders with volume equal to the associated particle volume times the void ratio, and that the capillary inverse proportionality between radius and matric pressure is valid. Examples include fixed-pore-shape and fixed-pore-length models. We also developed alternative versions of the model of Arya and Paris that eliminate its interval-size dependence and other problems. The alternative models are calculable by direct application of algebraic formulas rather than manipulation of data tables and intermediate results, and they easily combine with other models (e.g., incorporating structural effects) that are formulated on a continuous basis. Additionally, we developed a family of models based on the same pore geometry as the widely used unsaturated hydraulic conductivity model of Mualem. Predictions of measurements for different suitable media show that some of the models provide consistently good results and can be chosen based on ease of calculations and other factors.

Vadose Zone Journal↗
USGS2008

Simulating water, solute, and heat transport in the subsurface with the VS2DI software package

The software package VS2DI was developed by the U.S. Geological Survey for simulating water, solute, and heat transport in variably saturated porous media. The package consists of a graphical preprocessor to facilitate construction of a simulation, a postprocessor for visualizing simulation results, and two numerical models that solve for flow and solute transport (VS2DT) and flow and heat transport (VS2DH). The finite-difference method is used to solve the Richards equation for flow and the advection-dispersion equation for solute or heat transport. This study presents a brief description of the VS2DI package, an overview of the various types of problems that have been addressed with the package, and an analysis of the advantages and limitations of the package. A review of other models and modeling approaches for studying water, solute, and heat transport also is provided. ?? Soil Science Society of America. All rights reserved.

Vadose Zone Journal↗
USGS2008

Susceptibility to enhanced chemical migration from depression-focused preferential flow, High Plains aquifer

Aquifer susceptibility to contamination is controlled in part by the inherent hydrogeologic properties of the vadose zone, which includes preferential-flow pathways. The purpose of this study was to investigate the importance of seasonal ponding near leaky irrigation wells as a mechanism for depression-focused preferential flow and enhanced chemical migration through the vadose zone of the High Plains aquifer. Such a mechanism may help explain the widespread presence of agrichemicals in recently recharged groundwater despite estimates of advective chemical transit times through the vadose zone from diffuse recharge that exceed the historical period of agriculture. Using a combination of field observations, vadose zone flow and transport simulations, and probabilistic neural network modeling, we demonstrated that vadose zone transit times near irrigation wells range from 7 to 50 yr, which are one to two orders of magnitude faster than previous estimates based on diffuse recharge. These findings support the concept of fast and slow transport zones and help to explain the previous discordant findings of long vadose zone transit times and the presence of agrichemicals at the water table. Using predictions of aquifer susceptibility from probabilistic neural network models, we delineated approximately 20% of the areal extent of the aquifer to have conditions that may promote advective chemical transit times to the water table of <50 yr if seasonal ponding and depression-focused flow exist. This aquifer-susceptibility map may help managers prioritize areas for groundwater monitoring or implementation of best management practices.

High Plains↗
USGS2009

Rapid measurement of field-saturated hydraulic conductivity for areal characterization

To provide an improved methodology for characterizing the field‐saturated hydraulic conductivity ( K fs ) over broad areas with extreme spatial variability and ordinary limitations of time and resources, we developed and tested a simplified apparatus and procedure, correcting mathematically for the major deficiencies of the simplified implementation. The methodology includes use of a portable, falling‐head, small‐diameter (∼20 cm) single‐ring infiltrometer and an analytical formula for K fs that compensates both for nonconstant falling head and for the subsurface radial spreading that unavoidably occurs with small ring size. We applied this method to alluvial fan deposits varying in degree of pedogenic maturity in the arid Mojave National Preserve, California. The measurements are consistent with a more rigorous and time‐consuming K fs measurement method, produce the expected systematic trends in K fs when compared among soils of contrasting degrees of pedogenic development, and relate in expected ways to results of widely accepted methods.

Vadose Zone Journal↗
USGS2009

Transport of tritium contamination to the atmosphere in an arid environment

Soil–plant–atmosphere interactions strongly influence water movement in desert unsaturated zones, but little is known about how such interactions affect atmospheric release of subsurface water-borne contaminants. This 2-yr study, performed at the U.S. Geological Survey's Amargosa Desert Research Site in southern Nevada, quantified the magnitude and spatiotemporal variability of tritium (3H) transport from the shallow unsaturated zone to the atmosphere adjacent to a low-level radioactive waste (LLRW) facility. Tritium fluxes were calculated as the product of 3H concentrations in water vapor and respective evaporation and transpiration water-vapor fluxes. Quarterly measured 3H concentrations in soil water vapor and in leaf water of the dominant creosote-bush [ Larrea tridentat a (DC.) Coville] were spatially extrapolated and temporally interpolated to develop daily maps of contamination across the 0.76-km2 study area. Maximum plant and root-zone soil concentrations (4200 and 8700 Bq L−1, respectively) were measured 25 m from the LLRW facility boundary. Continuous evaporation was estimated using a Priestley–Taylor model and transpiration was computed as the difference between measured eddy-covariance evapotranspiration and estimated evaporation. The mean evaporation/transpiration ratio was 3:1. Tritium released from the study area ranged from 0.12 to 12 μg d−1 and totaled 1.5 mg (8.2 × 1010 Bq) over 2 yr. Tritium flux variability was driven spatially by proximity to 3H source areas and temporally by changes in 3H concentrations and in the partitioning between evaporation and transpiration. Evapotranspiration removed and limited penetration of precipitation beneath native vegetation and fostered upward movement and release of 3H from below the root zone.

Vadose Zone Journal↗
USGS2009

Hydrologic characterization of desert soils with varying degrees of pedogenesis: 2. Inverse modeling for eff ective properties

To understand their relation to pedogenic development, soil hydraulic properties in the Mojave Desert were investi- gated for three deposit types: (i) recently deposited sediments in an active wash, (ii) a soil of early Holocene age, and (iii) a highly developed soil of late Pleistocene age. Eff ective parameter values were estimated for a simplifi ed model based on Richards' equation using a fl ow simulator (VS2D), an inverse algorithm (UCODE-2005), and matric pressure and water content data from three ponded infi ltration experiments. The inverse problem framework was designed to account for the eff ects of subsurface lateral spreading of infi ltrated water. Although none of the inverse problems converged on a unique, best-fi t parameter set, a minimum standard error of regression was reached for each deposit type. Parameter sets from the numerous inversions that reached the minimum error were used to develop probability distribu tions for each parameter and deposit type. Electrical resistance imaging obtained for two of the three infi ltration experiments was used to independently test fl ow model performance. Simulations for the active wash and Holocene soil successfully depicted the lateral and vertical fl uxes. Simulations of the more pedogenically developed Pleistocene soil did not adequately replicate the observed fl ow processes, which would require a more complex conceptual model to include smaller scale heterogeneities. The inverse-modeling results, however, indicate that with increasing age, the steep slope of the soil water retention curve shitis toward more negative matric pressures. Assigning eff ective soil hydraulic properties based on soil age provides a promising framework for future development of regional-scale models of soil moisture dynamics in arid environments for land-management applications. ?? Soil Science Society of America.

Vadose Zone Journal↗
USGS2009

Hydrologic characterization of desert soils with varying degrees of pedogenesis: 1. field experiments evaluating plant-relevant soil water behavior

To assess the eff ect of pedogenesis on the soil moisture dynamics infl uencing the character and quality of ecological habitat, we conducted infi ltration and redistribution experiments on three alluvial deposits in the Mojave National Preserve: (i) recently deposited active wash sediments, (ii) a soil of early Holocene age, and (iii) a highly developed soil of late Pleistocene age. At each, we ponded water in a 1-m-diameter infi ltration ring for 2.3 h and monitored soil water content and matric pressure during and atier infi ltration, using probes and electrical resistivity imaging (ERI). Infi ltration and downward fl ow rates were greater in younger material, favoring deep-rooted species. Deep-rooted species tend to colonize the margins of washes, where they are unaff ected by sediment transport that inhibits colonization. The ERI results support important generalizations, for example that shallower than 0.5 m, infi ltrated water persists longer in highly developed soil, favoring shallow-rooted species. Soil moisture data for the two youngest soils suggested that saturation overshoot, which may have signifi cant but unexplored hydroecologic and pedogenic eff ects, occurred at the horizontally advancing weting front. Spatial heterogeneity of soil properties generally increased with pedogenic development. Evidence suggested that some early-stage developmental processes may promote uniformity; the intermediate- age soil appeared to have the least heterogeneity in terms of textural variation with depth, and also the least anisotropy. Lateral heterogeneity was pronounced in older soil, having a multitude of eff ects on the distribution and retention of soil water, and may facilitate certain water-conserving strategies of plants over what would be possible in a laterally homogeneous soil. ?? Soil Science Society of America.

California↗
USGS2009

Effect of grain-coating mineralogy on nitrate and sulfate storage in the unsaturated zone

Unsaturated-zone sediments and the chemistry of shallow groundwater underlying a small (∼8-km 2 ) watershed were studied to identify the mechanisms responsible for anion storage within the Miocene Bridgeton Formation and weathered Coastal Plain deposits in southern New Jersey. Lower unsaturated-zone sediments and shallow groundwater samples were collected and concentrations of selected ions (including NO 3 − and SO 4 2− ) from 11 locations were determined. Grain size, sorting, and color of the lower unsaturated-zone sediments were determined and the mineralogy of these grains and the composition of coatings were analyzed by petrographic examination, scanning electron microscopy and energy dispersive analysis of x-rays, and quantitative whole-rock x-ray diffraction. The sediment grains, largely quartz and chert (80–94% w/w), are coated with a very fine-grained (<20 μm), complex mixture of kaolinite, halloysite, goethite, and possibly gibbsite and lepidocrocite. The mineral coatings are present as an open fabric, resulting in a large surface area in contact with pore water. Significant correlations between the amount of goethite in the grain coatings and the concentration of sediment-bound SO 4 2− were observed, indicative of anion sorption. Other mineral–chemical relations indicate that negatively charged surfaces and competition with SO 4 2− results in exclusion of NO 3 − from inner sphere exchange sites. The observed NO 3 − storage may be a result of matrix forces within the grain coatings and outer sphere complexation. The results of this study indicate that the mineralogy of grain coatings can have demonstrable effects on the storage of NO 3 − and SO 4 2− in the unsaturated zone.

Vadose Zone Journal↗
USGS2009

Comment on "Evaluating interactions between groundwater and vadose zone using the HYDRUS-based flow package for MODFLOW" by Navin Kumar C. Twarakavi, Jirka Šimůnek and Sophia Seo

Twarakavi et al (2008) compared four packages that can be used to estimate recharge for regional-scale groundwater flow simulations using MODFLOW (Harbaugh, 2005). This comment is focused on the comparisons made between two of these packages, namely, UZF1 (Niswonger et al., 2006) and a derivative of HYDRUS referred to herein as HYDRUS (Seo et al., 2007). In their paper, Twarakavi et al. (2008) stated that HYDRUS more accurately simulates unsaturated flow processes and groundwater recharge as compared to UZF1. However, Twarakavi et al. (2008) did not address several important differences between these models that undermine the advantages of HYDRUS as compared to UZF1 for simulating recharge. These differences were not revealed by the comparisons presented by Twarakavi et al. because the test simulations used to compare the models were too simple

Vadose Zone Journal↗
USGS2010

Theory for source-responsive and free-surface film modeling of unsaturated flow

A new model explicitly incorporates the possibility of rapid response, across significant distance, to substantial water input. It is useful for unsaturated flow processes that are not inherently diffusive, or that do not progress through a series of equilibrium states. The term source-responsive is used to mean that flow responds sensitively to changing conditions at the source of water input (e.g., rainfall, irrigation, or ponded infiltration). The domain of preferential flow can be conceptualized as laminar flow in free-surface films along the walls of pores. These films may be considered to have uniform thickness, as suggested by field evidence that preferential flow moves at an approximately uniform rate when generated by a continuous and ample water supply. An effective facial area per unit volume quantitatively characterizes the medium with respect to source-responsive flow. A flow-intensity factor dependent on conditions within the medium represents the amount of source-responsive flow at a given time and position. Laminar flow theory provides relations for the velocity and thickness of flowing source-responsive films. Combination with the Darcy-Buckingham law and the continuity equation leads to expressions for both fluxes and dynamic water contents. Where preferential flow is sometimes or always significant, the interactive combination of source-responsive and diffuse flow has the potential to improve prediction of unsaturated-zone fluxes in response to hydraulic inputs and the evolving distribution of soil moisture. Examples for which this approach is efficient and physically plausible include (i) rainstorm-generated rapid fluctuations of a deep water table and (ii) space- and time-dependent soil water content response to infiltration in a macroporous soil. ?? Soil Science Society of America.

Vadose Zone Journal↗
USGS2011

Interacting vegetative and thermal contributions to water movement in desert soil

Thermally driven water-vapor flow can be an important component of total water movement in bare soil and in deep unsaturated zones, but this process is often neglected when considering the effects of soil–plant–atmosphere interactions on shallow water movement. The objectives of this study were to evaluate the coupled and separate effects of vegetative and thermal-gradient contributions to soil water movement in desert environments. The evaluation was done by comparing a series of simulations with and without vegetation and thermal forcing during a 4.7-yr period (May 2001–December 2005). For vegetated soil, evapotranspiration alone reduced root-zone (upper 1 m) moisture to a minimum value (25 mm) each year under both isothermal and nonisothermal conditions. Variations in the leaf area index altered the minimum storage values by up to 10 mm. For unvegetated isothermal and nonisothermal simulations, root-zone water storage nearly doubled during the simulation period and created a persistent driving force for downward liquid fluxes below the root zone (total net flux ~1 mm). Total soil water movement during the study period was dominated by thermally driven vapor fluxes. Thermally driven vapor flow and condensation supplemented moisture supplies to plant roots during the driest times of each year. The results show how nonisothermal flow is coupled with plant water uptake, potentially influencing ecohydrologic relations in desert environments.

Vadose Zone Journal↗