Gypsies in the palace: experimentalist's view on the use of 3‐D physics‐based simulation of hillslope hydrological response

As a fundamental unit of the landscape, hillslopes are studied for their retention and release of water and nutrients across a wide range of ecosystems. The understanding of these near‐surface processes is relevant to issues of runoff generation, groundwater–surface water interactions, catchment export of nutrients, dissolved organic carbon, contaminants (e.g. mercury) and ultimately surface water health. We develop a 3‐D physics‐based representation of the Panola Mountain Research Watershed experimental hillslope using the TOUGH2 sub‐surface flow and transport simulator. A recent investigation of sub‐surface flow within this experimental hillslope has generated important knowledge of threshold rainfall‐runoff response and its relation to patterns of transient water table development. This work has identified components of the 3‐D sub‐surface, such as bedrock topography, that contribute to changing connectivity in saturated zones and the generation of sub‐surface stormflow. Here, we test the ability of a 3‐D hillslope model (both calibrated and uncalibrated) to simulate forested hillslope rainfall‐runoff response and internal transient sub‐surface stormflow dynamics. We also provide a transparent illustration of physics‐based model development, issues of parameterization, examples of model rejection and usefulness of data types (e.g. runoff, mean soil moisture and transient water table depth) to the model enterprise. Our simulations show the inability of an uncalibrated model based on laboratory and field characterization of soil properties and topography to successfully simulate the integrated hydrological response or the distributed water table within the soil profile. Although not an uncommon result, the failure of the field‐based characterized model to represent system behaviour is an important challenge that continues to vex scientists at many scales. We focus our attention particularly on examining the influence of bedrock permeability, soil anisotropy and drainable porosity on the development of patterns of transient groundwater and sub‐surface flow. Internal dynamics of transient water table development prove to be essential in determining appropriate model parameterization. Copyright © 2010 John Wiley & Sons, Ltd.     

A comparison of continuous mass‐lumped finite elements with finite differences for 3‐D wave propagation

The finite‐difference method on rectangular meshes is widely used for time‐domain modelling of the wave equation. It is relatively easy to implement high‐order spatial discretization schemes and parallelization. Also, the method is computationally efficient. However, the use of finite elements on tetrahedral unstructured meshes is more accurate in complex geometries near sharp interfaces. We compared the standard eighth‐order finite‐difference method to fourth‐order continuous mass‐lumped finite elements in terms of accuracy and computational cost. The results show that, for simple models like a cube with constant density and velocity, the finite‐difference method outperforms the finite‐element method by at least an order of magnitude. Outside the application area of rectangular meshes, i.e., for a model with interior complexity and topography well described by tetrahedra, however, finite‐element methods are about two orders of magnitude faster than finite‐difference methods, for a given accuracy.     

Computational simulation of slab vibration and horizontal‐vertical coupling in a full‐scale test bed subjected to 3D shaking at E‐Defense

This paper focuses on slab vibration and a horizontal‐vertical coupling effect observed in a full‐scale 5‐story moment frame test bed building in 2 configurations: isolated with a hybrid combination of lead‐rubber bearings and cross‐linear (rolling) bearings, and fixed at the base. Median peak slab vibrations were amplified—relative to the peak vertical shake table accelerations—by factors ranging from 2 at the second floor to 7 at the roof, and horizontal floor accelerations were significantly amplified during 3D (combined horizontal and vertical) motions compared with 2D (horizontal only) motions of comparable input intensity. The experimentally observed slab accelerations and the horizontal‐vertical coupling effect were simulated through a 3D model of the specimen using standard software and modeling assumptions. The floor system was modeled with frame elements for beams/girders and shell elements for floor slabs; the insertion point method with end joint offsets was used to represent the floor system composite behavior, and floor mass was finely distributed through element discretization. The coupling behavior was partially attributed to the asymmetry of the building that was intensified by asymmetrically configured supplemental mass at the roof. Horizontal‐vertical coupled modes were identified through modal analysis and verified with evaluation of floor spectral peaks.     

A 27‐point scheme for a 3D frequency‐domain scalar wave equation based on an average‐derivative method

Based on an average‐derivative method and optimization techniques, a 27‐point scheme for a 3D frequency‐domain scalar wave equation is developed. Compared to the rotated‐coordinate approach, the average‐derivative optimal method is not only concise but also applies to equal and unequal directional sampling intervals. The resulting 27‐point scheme uses a 27‐point operator to approximate spatial derivatives and the mass acceleration term. The coefficients are determined by minimizing phase velocity dispersion errors and the resultant optimal coefficients depend on ratios of directional sampling intervals. Compared to the classical 7‐point scheme, the number of grid points per shortest wavelength is reduced from approximately 13 to approximately 4 by this 27‐point optimal scheme for equal directional sampling intervals and unequal directional sampling intervals as well. Two numerical examples are presented to demonstrate the theoretical analysis. The average‐derivative algorithm is also extended to a 3D frequency‐domain viscous scalar wave equation.     

A temporal stable isotopic (δ18O, δD,d‐excess) comparison in glacier meltwater streams,Taylor Valley,Antarctica

In this paper, we describe the importance of hyporheic dynamics within Andersen Creek and Von Guerard Stream, Taylor Valley, Antarctica, from the 2010–2011 melt season using natural tracers. Water collection started at flow onset and continued, with weekly hyporheic‐zone sampling. The water δ¹⁸O and δD values were isotopically lighter in the beginning and heavier later in the season. D‐excess measurements were used as an indicator of mixing because an evaporative signature was evident and distinguishable between 2 primary end‐members (glacier meltwater and hyporheic zone). Hyporheic‐zone influence on the channel water was variable with a strong control on streamwater chemistry, except at highest discharges. This work supports previous research indicating that Von Guerard Stream has a large, widespread hyporheic zone that varies in size with time and discharge. Andersen Creek, with a smaller hyporheic zone, displayed hyporheic‐zone solute interaction through the influence from subsurface hypersaline flow. Overall, the evolution of Taylor Valley hyporheic‐zone hydrology is described seasonally. In mid‐December, the hyporheic zone is a dynamic system exchanging with the glacier meltwater in the channel, and with diminishing flow in January, the hyporheic zone drains back into the channel flow also impacting stream chemistry. This work adds new information on the role of hyporheic zone–stream interaction in these glacier meltwater streams.     

A tale of two isotopes: differences in hydrograph separation for a runoff event when using δD versus δ18O

It is often assumed that stable water isotopes (δD and δ¹⁸O) provide redundant information for a given sample of water. In this note we illustrate that the choice of isotope used may influence the resultant hydrograph separation. This is especially true in light of the spatial and temporal variability in the isotopic composition of rainfall water at the catchment scale. We present several possible hydrograph separations based on both δD and δ¹⁸O observed in rainfall for a single runoff event occurring in the southwest USA. This study demonstrates the potential of using both stable water isotopes by showing that δD and δ¹⁸O may provide unique information for catchment hydrologists. We also report on the utility of new technology capable of simultaneous measurements of both δD and δ¹⁸O using off‐axis integrated cavity output spectroscopy (OA‐ICOS) methods. This may be of interest to catchment hydrologists seeking to incorporate this type of equipment into their laboratory. Copyright © 2009 John Wiley & Sons, Ltd.     

A modelling framework for evaluation of the hydrological impacts of nature‐based approaches to flood risk management,with application to in‐channel interventions across a 29‐km2 scale catchment in the United Kingdom

Nature‐based approaches to flood risk management are increasing in popularity. Evidence for the effectiveness at the catchment scale of such spatially distributed upstream measures is inconclusive. However, it also remains an open question whether, under certain conditions, the individual impacts of a collection of flood mitigation interventions could combine to produce a detrimental effect on runoff response. A modelling framework is presented for evaluation of the impacts of hillslope and in‐channel natural flood management interventions. It couples an existing semidistributed hydrological model with a new, spatially explicit, hydraulic channel network routing model. The model is applied to assess a potential flood mitigation scheme in an agricultural catchment in North Yorkshire, United Kingdom, comprising various configurations of a single variety of in‐channel feature. The hydrological model is used to generate subsurface and surface fluxes for a flood event in 2012. The network routing model is then applied to evaluate the response to the addition of up to 59 features. Additional channel and floodplain storage of approximately 70,000 m³ is seen with a reduction of around 11% in peak discharge. Although this might be sufficient to reduce flooding in moderate events, it is inadequate to prevent flooding in the double‐peaked storm of the magnitude that caused damage within the catchment in 2012. Some strategies using features specific to this catchment are suggested in order to improve the attenuation that could be achieved by applying a nature‐based approach.