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The guest editors introduce the contributions to this special section devoted to work at the Danish hydrological observatory—HOBE. The research projects at this site are part of a global effort to refine our understanding of hydrological processes in the face of land-use impacts and climate change.

Vadose Zone Journal February 01, 2011, Vol.10, 1-7. doi:https://doi.org/10.2136/vzj2011.0006

A methodology for quantitative precipitation estimation (QPE) was developed. The cumulative effects of radar QPE on hydrologic predictions at a long time scale were assessed. The results show that the influence of radar QPE on the accuracy of a designated hydrologic model is highly sensitive to the size of the catchment.

Vadose Zone Journal February 01, 2011, Vol.10, 8-24. doi:https://doi.org/10.2136/vzj2010.0034

The impact of spatial resolution of precipitation input on the hydrological response predicted by a distributed model is analyzed. Little impact is found on the annual water balance and on the discharge hydrograph at catchment outlet. On the other hand both groundwater head and recharge are affected. The impact of spatial resolution of precipitation input is reduced with increasing catchment size.

Vadose Zone Journal February 01, 2011, Vol.10, 25-36. doi:https://doi.org/10.2136/vzj2009.0186

By introducing different model input scenarios in a systematic way, the impact of precipitation correction on catchment-scale hydrologic modeling results has been elucidated probably for the very first time. The results show that great improvement in model performance can be achieved by applying a dynamic rain gauge catch correction.

Vadose Zone Journal February 01, 2011, Vol.10, 37-53. doi:https://doi.org/10.2136/vzj2010.0001

Surface control of evapotranspiration is explored using eddy-covariance measurements of energy fluxes above a forest, a wet grassland, and an agricultural field. Substantial differences are found in the annual evapotranspiration sum among the surfaces, and a range of different processes that control the intersurface variation are identified.

Vadose Zone Journal February 01, 2011, Vol.10, 54-66. doi:https://doi.org/10.2136/vzj2009.0181

How different are the greenhouse gas fluxes above different land surface types facing the same climatic conditions? Eddy covariance measurements of carbon dioxide, methane and nitrous oxide above forest, grass and agricultural land demonstrate the importance of land use for the greenhouse gas budget of a small catchment.

Vadose Zone Journal February 01, 2011, Vol.10, 67-77. doi:https://doi.org/10.2136/vzj2010.0058

Daily root zone soil moisture depletion, measured using time domain reflectometry (TDR), was compared with daily accumulated water vapor loss, using the eddy covariance (EC) technique. The two approaches were in good agreement during dry periods, indicating that TDR-based evapotranspiration rates can be used for evaluating EC-measured evapotranspiration rates.

Vadose Zone Journal February 01, 2011, Vol.10, 78-83. doi:https://doi.org/10.2136/vzj2010.0060

Infiltration of dyed water into sandy alluvial sediments revealed that preferential flow occurs at several scales and dominates the flow field. The infiltration flow field was imaged by reflection ground penetrating radar (GPR), the results of which demonstrated the ability of reflection GPR to nondestructively image the distribution of moisture in preferential flow fields.

Vadose Zone Journal February 01, 2011, Vol.10, 84-97. doi:https://doi.org/10.2136/vzj2009.0188

Time series of temperature profiles from multilevel probes installed within a streambed cross-section showed nonuniform discharge with strongly increasing vertical fluxes within the top 20 cm of the streambed–aquifer interface. Differences in conditions for lateral inflow to the stream could be a possible cause for the discharge variation.

Vadose Zone Journal February 01, 2011, Vol.10, 98-109. doi:https://doi.org/10.2136/vzj2010.0005

Seepage distribution at a flow-through lake was investigated using onshore and offshore geophysical, geochemical, and seepage measurements. Three-dimensional modeling results are in agreement with apparent groundwater ages and seepage rates. Offshore ground-penetrating radar combined with deep profiles of the natural abundance of oxygen-18 revealed an unexpected offshore recharge pattern from the lake.

Vadose Zone Journal February 01, 2011, Vol.10, 110-124. doi:https://doi.org/10.2136/vzj2010.0017

Understanding hydrogeologic processes in coastal areas is very important for, e.g., groundwater management and can be a very complicated task. Such settings are often governed by complex processes that can be very difficult to map. We demonstrated how airborne electromagnetic methods can be used to map the salinity distribution in such an area.

Vadose Zone Journal February 01, 2011, Vol.10, 125-135. doi:https://doi.org/10.2136/vzj2010.0038

Two methods to perturb daily precipitation, temperature, and evapotranspiration data from a regional climate model for use in hydrological simulations of climate change are compared. Simulated groundwater recharge and stream discharge from the methods differed only slightly, showing that climate data can be used directly in hydrological models.

Vadose Zone Journal February 01, 2011, Vol.10, 136-150. doi:https://doi.org/10.2136/vzj2010.0112

After a brief introduction of trace gas emissions from the soil to the atmosphere and relevant processes involved, the seven papers dealing with novel modeling approaches, experimental data analysis tools, and improved concepts for the mathematical description of key subprocesses are introduced.

Vadose Zone Journal February 01, 2011, Vol.10, 151-155. doi:https://doi.org/10.2136/vzj2010.0117

Nitrous oxide emissions were modeled on a daily basis with the process-based model combination SWAP-ANIMO. Deviations between observed and simulated emissions point out the need to incorporate the effect of soil structure on diffusion, storage time, and further reduction of nitrous oxide to nitrogen gas.

Vadose Zone Journal February 01, 2011, Vol.10, 156-168. doi:https://doi.org/10.2136/vzj2010.0029

Stochastic finite elements to fit soil gas concentration profiles were applied to quantify subsoil production of carbon dioxide and nitrous oxide in a forest soil. A comparative test with synthetic data revealed superiority to existing methods as long as the assumptions, mainly that homogeneity exists within the soil layers, is fulfilled.

Vadose Zone Journal February 01, 2011, Vol.10, 169-183. doi:https://doi.org/10.2136/vzj2009.0193

Two modeling approaches were tested for their ability to describe and quantify seasonal variations of nitrous oxide fluxes in a potato-cropped soil. While both modeling approaches were able to simulate the observed seasonal dynamics of nitrous oxide emissions, the inclusion of a gas transport module in the second model resulted in smoother transient behavior.

Vadose Zone Journal February 01, 2011, Vol.10, 184-194. doi:https://doi.org/10.2136/vzj2009.0194

Modeling of C turnover is a widely accepted tool for the prediction of C stocks in soils and the carbon dioxide efflux into the atmosphere; however, the implications of variable time discretization for model input have not yet been analyzed. Our results indicate that averaging temperature inputs will lead to errors in the prediction of cumulative as well as instantaneous carbon dioxide efflux.

Vadose Zone Journal February 01, 2011, Vol.10, 195-205. doi:https://doi.org/10.2136/vzj2009.0157

A new model coupling soil, vegetation, and atmospheric processes was developed in a macroscopic cellular automata simulation environment directly compatible with parallel programming. The model was tested to show the effects of simulation dimensionality (one or three dimensions) and the degree of soil heterogeneity on carbon dioxide, water, and heat fluxes at the field scale.

Vadose Zone Journal February 01, 2011, Vol.10, 206-225. doi:https://doi.org/10.2136/vzj2010.0033

Models for predicting the soil gas diffusion coefficient and the soil air permeability from only air-filled and total porosities and which are corrected for soil dry bulk density are presented. These models were derived based on measurements on 150 intact soils representing a wide range of soil texture, compaction, and land use.

Vadose Zone Journal February 01, 2011, Vol.10, 226-238. doi:https://doi.org/10.2136/vzj2009.0137

Soil carbon dioxide efflux is often computed as an area average of successive chamber measurements at many points. Because the deviation of each point from this average remains stable for a certain time period, subtraction of an expected value for each point can be used to estimate a time series of the area average at the temporal resolution of the raw measurements.

Vadose Zone Journal February 01, 2011, Vol.10, 239-251. doi:https://doi.org/10.2136/vzj2009.0152

Counteracting hydrodynamic and adhesive torques determine whether a colloid will immobilize or roll on a solid. We use pore-scale simulations and scaling methods to predict the cumulative density function of hydrodynamic torque in sphere packs, and quantify the fraction of and locations on the solid surfaces that contribute to colloid retention.

Vadose Zone Journal February 01, 2011, Vol.10, 252-261. doi:https://doi.org/10.2136/vzj2010.0064

How does water actually move in sloped soil areas—especially when affected by soil water repellency? We “animated” real data to show how water actually moves in a sloping sandy soil exhibiting water-repellent behavior—and the effect of soil surfactants on the same area.

Vadose Zone Journal February 01, 2011, Vol.10, 262-269. doi:https://doi.org/10.2136/vzj2010.0051

Estimation of soil moisture using ground-penetrating radar requires a petrophysical relationship between dielectric permittivity and volumetric water content. We examined the capacity of various empirical relationships, volumetric mixing formulae, and effective medium approximations to predict moisture content under natural field conditions.

Vadose Zone Journal February 01, 2011, Vol.10, 270-285. doi:https://doi.org/10.2136/vzj2010.0040

Observations from a dryland ecosystem link the distribution of vegetation with patterns of subterranean termite nests. In this laboratory study, we developed strategies to capture the general behavior of transient soil moisture dynamics generated in macropores. The strategies may help elucidate ecosystem behavior on larger spatial and temporal scales.

Vadose Zone Journal February 01, 2011, Vol.10, 286-298. doi:https://doi.org/10.2136/vzj2010.0031

A numerical solution scheme capable of expressing dependent-variable jump discontinuities at material interfaces is presented. This scheme is consistent, stable, and convergent for advective–diffusive transport in heterogeneous porous media. Less refinement is needed to produce accurate results compared with conventional solution schemes.

Vadose Zone Journal February 01, 2011, Vol.10, 299-312. doi:https://doi.org/10.2136/vzj2010.0015

Two models, a mixing-coefficient model to estimate the porosity and a power-averaging model to determine the effective grain diameter of sediments, were developed. The models were used to predict the saturated hydraulic conductivity of binary mixtures for a full range of gravel contents and were tested with laboratory experiments.

Vadose Zone Journal February 01, 2011, Vol.10, 313-321. doi:https://doi.org/10.2136/vzj2009.0138

Water percolation and solute transport through the vadose zone of a sandy formation were monitored. Nonequilibrium between sediment and aqueous salt concentrations indicated preferential transport, but flow measurements indicated nonpreferential flow. The results suggest that pore-scale dual-domain flow controls water flow and solute transport.

Vadose Zone Journal February 01, 2011, Vol.10, 322-331. doi:https://doi.org/10.2136/vzj2009.0113

Experience with the performance of soil cover systems over waste rock dumps in cool and humid climates is limited. We measured lateral subsurface flow from a multilayer soil cover system located in Southeast Alaska and used the numerical model HYDRUS-2D to better understand internal flow processes and partitioning of water.

Vadose Zone Journal February 01, 2011, Vol.10, 332-344. doi:https://doi.org/10.2136/vzj2010.0094

This field study revealed distinct differences in retention of tracers along macropores in and between drain trenches. It was found that substances can accumulate along drain pipes during intense water application events. Good connectivity from the surface to the drain pipe through macropores leads to drainage flow with high tracer concentrations.

Vadose Zone Journal February 01, 2011, Vol.10, 345-353. doi:https://doi.org/10.2136/vzj2010.0078

We measured management-induced changes in water retention and hydraulic conductivity for two soil management systems, mixed crop farming and pasture farming. The results were incorporated into an existing model describing the evolution of the pore size distribution as the soil management changes from mixed crop farming to pasture.

Vadose Zone Journal February 01, 2011, Vol.10, 354-366. doi:https://doi.org/10.2136/vzj2010.0035

Upscaling flow and transport processes from hillslope to catchment scale requires connecting hillsope soil structure quantification to its impact on those processes at the landscape scale. We address this by integrating classical pedological soil-landscape pattern description with the process knowledge gained from soil physics and hydrology.

Vadose Zone Journal February 01, 2011, Vol.10, 367-377. doi:https://doi.org/10.2136/vzj2010.0076

An episodic solute transit time distribution model that accounts for the effects of antecedent soil moisture and rainfall intensity on preferential flow is presented. Synthetic drain flow and Oxygen-18 data generated with the MACRO model were used to test the model. Comparisons were excellent for soils with a high potential for preferential flow.

Vadose Zone Journal February 01, 2011, Vol.10, 378-385. doi:https://doi.org/10.2136/vzj2010.0071

Effects of rock fragment content and size on solute transport in soil columns was studied with a tracer. The tracer breakthrough data were used to determine transport parameters for the conventional convection–dispersion equation and for a two-region transport model. The effect of rock fragment content and size on those parameters was quantified.

Vadose Zone Journal February 01, 2011, Vol.10, 386-393. doi:https://doi.org/10.2136/vzj2009.0195

Solute transport experiments in a soil core suggested the flow regime to be a convective-dispersive process at low flow rates, and a stochastic-convective process at high flow rates. A dye tracer experiment showed that preferential flow and the associated decrease of flow path tortuosity leads to decreases of solute mixing between stream tubes.

Vadose Zone Journal February 01, 2011, Vol.10, 394-402. doi:https://doi.org/10.2136/vzj2010.0046

A two-dimensional lab flow cell and HYDRUS-2D with concentration-dependent functions, were used to investigate a coupled unsaturated flow and transport phenomenon referred to as solute-dependent capillarity-induced focused flow. Highly focused vertical flow was characteristic of water infiltration into a sand initially containing n-Butanol.

Vadose Zone Journal February 01, 2011, Vol.10, 403-411. doi:https://doi.org/10.2136/vzj2009.0081

The depletion of soil water by barley plants cropped on an undisturbed soil monolith was measured with electrical resistance tomography (ERT). The soil water content derived with ERT was compared at specific locations with TDR probes. A global water balance on the monolith and the comparison to TDR validated the ERT measurements.

Vadose Zone Journal February 01, 2011, Vol.10, 412-424. doi:https://doi.org/10.2136/vzj2010.0079

This study focused on controlling capillary pressure to induce horizontal components of water flux in the vadose zone. Flow over short distances was proven in the laboratory. Modeling showed that flow over larger distances is possible, but at slow velocities and for simple heterogeneities, suggesting technical limitations to flow induction.

Vadose Zone Journal February 01, 2011, Vol.10, 425-436. doi:https://doi.org/10.2136/vzj2010.0048

This study evaluated soil-moisture dynamics associated with Avena barbata, a species found in annual grasslands. The objective was to assess the hydrologic response to two precipitation regimes and contrasting soil types. Of particular interest are the temporal trends in soil moisture and the role of vegetation in controlling soil moisture.

Vadose Zone Journal February 01, 2011, Vol.10, 437-449. doi:https://doi.org/10.2136/vzj2010.0055

Hydraulic properties of heterogeneous soil horizons are hard to determine using classical laboratory experiments. One possibility is to consider each horizon as a homogeneous structure and to determine the effective hydraulic properties at this scale. An upscaling approach based on the “scaleway” scheme is proposed using numerical simulation or analytical bounds.

Vadose Zone Journal February 01, 2011, Vol.10, 450-458. doi:https://doi.org/10.2136/vzj2010.0008

Prediction of porous medium hydraulic properties is based on measured air permeability values, a pore-size distribution model, and a scheme to optimize the pore-size distribution so that calculated air permeability values closely match the measured values. The ensuing pore-size distribution is then used to predict the hydraulic properties.

Vadose Zone Journal February 01, 2011, Vol.10, 459-465. doi:https://doi.org/10.2136/vzj2010.0053

In a vertical displacement, saturation overshoot is the macroscopic manifestation of a microscopically sharp front. Experiments on saturation overshoot involving various fluids showed that contact angle plays a role in determining the overshoot flux. A correction for contact angle allows the transition flux to be scaled with fluid properties.

Vadose Zone Journal February 01, 2011, Vol.10, 466-468. doi:https://doi.org/10.2136/vzj2010.0047
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