Issues

A special section on modeling fully integrated surface, subsurface, and land-surface processes is introduced, composed of five invited contributions, from the 17th Computational Methods in Water Resources 2008 Meeting. These papers present theoretical, numerical, and process-description advancements, all current research topics in coupled modeling.

Characteristic response times for streamflow, overland runoff, and subsurface flow can differ by orders of magnitude. A subtiming strategy can significantly improve the efficiency of an integrated flow simulation with minimal loss in accuracy by applying smaller subtime steps only to the subdomain where the accuracy requirements are greater.

The ensemble Kalman filter and the Newtonian nudging data assimilation methods are compared for a distributed, physically based model of coupled surface and subsurface flow. The capabilities of the two assimilation techniques to retrieve the correct watershed response and the tradeoffs between the two approaches are assessed for a small catchment.

The influence of simplifying assumptions of subsurface heat transport in land surface models was studied, showing that rain sensible heat and the process of convection may impact the land surface energy balance. The lower temperature boundary condition predetermines the behavior of the subsurface as an energy source-sink in long-term simulations.

Rapid growth has led to challenges for salinity management in surface-water and groundwater in parts of Southern California. A fully integrated groundwater/surface-water flow and transport model was developed to evaluate chloride loadings and impacts and facilitate constructive dispute resolution based on objective scientific analysis.

A physics-based, fully coupled, constrained unstructured grid-based hydrologic model is presented. The model explores the influence of initial hydrologic states, soil properties, capillarity, anisotropy and heterogeneity in determining the hydrologic coupling behavior between the surface, the vadose, and saturated zones.

Neutron radiography recorded 2-d infiltrations into fine, medium, and coarse sands. A water-content wave model based on gravity-driven and viscosity-controlled free-surface film flow approached the spatially heterogeneous water content distributions.

A conceptual model was developed that classifies soil susceptibility to macropore flow from easily available soil and site factors. The model was designed to support hydropedological upscaling approaches to predict vadose zone leaching. A preliminary test using literature tracer breakthrough data suggests that it shows some promise for this purpose.

Earthworm channels increase leaching risks for agricultural pollutants. A decision tree was developed from literature data that classifies the abundance of earthworms from information on land use, management practices, and soil texture with 71% accuracy. The scheme is designed to support regional-scale models of vadose zone leaching.

A vadose zone monitoring system, developed to allow continuous tracking of water flow and contaminant transport in the vadose zone, was implemented in a multitracer experiment. The observed temporal variation in the vadose zone water content profiles together with tracer breakthrough curves at various depths allowed detailed visualization of the percolation and transport processes.

Electromagnetic (EM) sensors (capacitance and time domain) used in access tubes reported significantly more variation of soil water content than did a neutron moisture meter and were less accurate. The EM sensors responded not only to the mean water content in the sampling volume but to the smaller scale structure of soil electrical properties.

The effects of bulk density on the solute diffusion coefficient were investigated for soils with different textures, including Andisols. Contrary to the behavior for sandy soils, the solute diffusion coefficient for Andisols decreased markedly with increasing bulk density under wet conditions but increased with increasing bulk density under dry conditions.

Gas pressures measured within a 15-m-high waste rock pile show a clear response to wind speed and wind direction. The results suggest that wind-driven advection can be an important oxygen transport mechanism contributing to sulfide oxidation. The physical heterogeneity of the pile appears to have a strong control on gas pressure gradients.

In this study, we have shown that the dye tracer Brilliant Blue can be visualized in undisturbed soil using electrical resistivity tomography (ERT). We compared optically and ERT-derived Brilliant Blue images. We concluded that the joint application of both imaging methods provides additional information for quantifying transport phenomena.

Aqueous foam generated from a surfactant solution containing remedial amendment calcium polysulfide was used to deliver the amendment to sediments under vadose zone conditions for Cr(VI) immobilization. This foam-delivery approach was demonstrated to minimize the mobilization of Cr(VI) from the sediments.

The effects of average particle size and soil compaction on the gas diffusion coefficient and air permeability for six different sandy soils were investigated. Under variably saturated conditions, soil compaction simultaneously caused reduced water blockage effects and a reduction in large-pore space, resulting in higher gas diffusion coefficient but lower air permeability.

Infiltration is approached as a gravity-driven and viscosity-controlled water content wave. Capillarity abstracts water from the wave into unsaturated pores. Abstraction lengths from 215 water content waves varied from 0.2 to greater than 10 m.

Soil biogeochemistry develops within, and as a result of, a framework of intricate physical structure. A novel diffusion cell was used to explore Fe transformations within an idealized micropore. We reveal that diffusion-controlled gradients of Fe ions result in a complex array of spatially distributed secondary mineral phases.

A new model was developed that calculates solar radiation, canopy and surface energy balance, snowpack dynamics, soil water flow, and heat exchange among snow, soil, and bedrock, including soil water freezing. Model results for a brush-dominated mountain slope near Boise, ID, showed that evapotranspiration consumes about 60% of yearly precipitation.

An analytical solution for two-dimensional, steady-state solute transport under unsaturated flow conditions is presented. A specific case with two sources is solved but may be generalized to any combination of sources. The analytical results complement numerical solutions, which were previously required to solve this class of problems.

A program THERMOX was developed for multiphase reactive transport modeling in waste rock piles with oxygen transport by convection and/or by diffusion. The program was tested with data from the Mine Doyon site, Quebec, Canada, and results were found satisfactory. The program can be used for prediction of acid mine drainage behavior and remediation.

Fifteen automated equilibrium tension lysimeters were installed at five depths in a volcanic vadose zone in New Zealand, allowing tracing of water and contaminant movement from the soil surface down to the permanent saturated zone. Three-dimensional model simulations are used to investigate “dry zones” developing in the vicinity of the lysimeters.

A sensitivity analysis of the latest dual-porosity, transient, mobile-immobile (MIM) version of HYDRUS-1D was performed to identify key parameters governing pesticide concentrations at 1-m depth in the soil profile. Saturated mobile and immobile water contents were found to be important as well as pesticide sorption and degradation parameters.

Using a capillary model, we show that the unsaturated hydraulic conductivity function is highly sensitive to the spatial distribution of biofilms within the pore space. The analysis yielded relations for conductivity versus water content, and these indicated the counterintuitive result that conductivity may actually increase with biofilm growth.