Evaluating a modified point-based method to downscale cell-based climate variable data to high-resolution grids

To address the demand for high spatial resolution gridded climate data, we have advanced the Daymet point-based interpolation algorithm for downscaling global, coarsely gridded data with additional output variables. The updated algorithm, High-Resolution Climate Downscaler (HRCD), performs very good downscaling of daily, global, historical reanalysis data from 1° input resolution to 2.5 arcmin output resolution for day length, downward longwave radiation, pressure, maximum and minimum temperature, and vapor pressure deficit. It gives good results for monthly and yearly cumulative precipitation and fair results for wind speed distributions and modeled downward shortwave radiation. Over complex terrain, 2.5 arcmin resolution is likely too low and aggregating it up to 15 arcmin preserves accuracy. HRCD performs comparably to existing daily and monthly US datasets but with a global extent for nine daily climate variables spanning 1948–2006. Furthermore, HRCD can readily be applied to other gridded climate datasets.     

Phase transitions and path dependence in urban evolution

There is a long history of recognising and interpreting discontinuous change—phase transitions—in urban systems. In this paper, we use the aggregate retail model as an archetype to explore some new ideas. For example, we argue that the dependence of paths of development on initial conditions has been understudied, and we offer a new graphical analysis that demonstrates explicitly their influence on discrete change. We introduce an order parameter, and we plot this on a ‘results grid’ to facilitate the discovery of possible phase transitions. We illustrate the use of these new developments with an application to London. We show how retail developers can change the ‘initial conditions’ at a point in time and possibly bring about phase transitions by their actions. This also shows that it should be possible to account for the history of urban development as a sequence of initial conditions, illustrating path dependence. Secondly, the model could be deployed in relation to a new shopping zone, and the tools developed here used to calculate the minimum size of a new development in order for it to compete. We explore the ‘minimum size’ idea in relation to a new shopping zone in London.     

Recent advances in accessibility research: Representation,methodology and applications

 In this article we examine recent advances in accessibility research and their implications for future studies. We base our discussion on three intersecting dimensions that are useful for evaluating the contribution of recent studies: representation, methodology and applications. Various examples are selected to show that research concerned with representation and methodological problem solving is often applied to issues of broad concern in policy and planning. It is, however, not clear that the simultaneous treatment of representation, methodological and application issues has ever been fully worked out. The questions raised in this article may serve as a foundation for addressing issues pertinent to accurate representation, improved model building, and more rigorous applications in accessibility research. Received: 9 December 2002 / Accepted: 10 February 2003     

Modeling planform evolution of a mud‐dominated meandering river: Quinn River,Nevada, USA

A depth‐averaged linearized meander evolution model was calibrated and tested using the field data collected at the Quinn River in the Black Rock Desert, Nevada. Two approaches used to test the model were: (1) simulating meander evolution and comparing the results with the observed 38 year migration pattern; and (2) fitting the model parameters to present bank asymmetry (the ratio of the maximum bank gradients on opposite sides of the channel). The data required as input were collected in the field during a high flow in May 2011 and from aerial photographs and LiDAR data. Both approaches yielded similar results for the best fit parameter values. The bank asymmetry analysis showed that the bank asymmetry and the velocity perturbation have high correlation at close to zero spatial lag while the maximum correlation between the bank asymmetry and maximum bend curvature is offset by about 25 m. The model sufficiently replicated 38 years of channel migration, with a few locations significantly under‐ or over‐predicted. Inadequacies of the flow model and/or variation in bank properties unaccounted for are most likely the causes for these discrepancies. Flow through the Quinn River was also simulated by a more general 3D model. The downstream pattern of near‐bank shear stresses simulated by the 3D model is nearly identical to those resulting from the linearized flow model. Topographic profiles across interior bends are essentially invariant over a wide range of migration rates, suggesting that the traditional formulation that cut bank erosion processes govern migration rates is appropriate for the Quinn River. Copyright © 2014 John Wiley & Sons, Ltd.     

Significance of geomorphological and subsurface drainage controls on failures of peat‐covered hillslopes triggered by extreme rainfall

On 19 September 2003, 40 landslides of 140–18 000 m³ volume occurred within 2·5 km² on the slopes of Dooncarton Mountain (Republic of Ireland) during a storm that may have exceeded 90 mm within 90 minutes. The landslides were investigated to determine the reasons for such a high density of slope failures. All of the landslides were surveyed within four months, and nine of them were investigated in detail. The six largest landslides, all peat failures, accounted for 57% of the more than 100 000 m³ of material displaced during the event. A consistent sequence of superficial materials was found on the failed hillslopes, including an extensive iron pan at the base of a buried soil horizon 0·3 m below the base of the peat. Morphologically, almost all of the landslides occurred on steep planar slopes or around sharp convexities, with the latter failures developing retrogressively upslope. The only significant relationship found from analysis of 371 subsurface pipes and 142 seepage cracks (defined here as contiguous fissures conducting concentrated subsurface flow) across all the failures was that the thinner the peat cover, the deeper the pipes and seepage cracks occurred below the base of peat. It is concluded that most of the landslides were probably caused by a combination of excess water pressures in the buried soil horizon and the thinner overburden of peat or peaty soil associated with the steeper slope segments. Pipes and seepage cracks formed on the iron pan probably existed prior to the failure event and may have contributed to the high water pressures as rainwater inputs exceeded their discharge capacities. One large peat slide was probably triggered by excess water pressures developed within and between artificial tine cuts. The properties of the blanket peat were generally of little consequence in the occurrence of the landslides, but relict desiccation cracks and other structural weaknesses through the peat mass were probably highly significant. Although several aspects of the peat failures correspond to previously published examples, the context of these failures in terms of the topography and upland catena is distinctive. Copyright © 2007 John Wiley & Sons, Ltd.     

On the importance of non-hydrostatic processes in determining tidally induced mixing in sill regions

The importance of using a non-hydrostatic model to compute tidally induced mixing and flow in the region of a sill is examined using idealized topography representing the sill at the entrance to Loch Etive. This site is chosen since detailed measurements were recently made there. Calculations are performed with and without the inclusion of non-hydrostatic dynamics using a vertical slice model for a range of sill widths corresponding to typical sill regions. Initial non-hydrostatic calculations showed that the model could reproduce the observed flow characteristics in the region. However, when calculations were performed using the model in hydrostatic form, the significant artificial convective mixing that occurred in order to remove density inversions led to excessively high vertical mixing. This influenced the computed temperature field and the intensity of the current jet that separated from the sill on its lee side. In addition it affected the magnitude and spatial characteristics of the lee waves generated on the lee side of the sill. Calculations with a range of sill widths, showed that as the sill width decreased the difference between the solution computed with the non-hydrostatic and hydrostatic model increased.     

A new technique for determining the surface level of salt marshes and other uncompacted sedimentary environments

The method described involves the use of an electro-optical distance measurer mounted on a theodolite. Observations are taken onto a series of acrylic reflectors, aligned at right angles to the line of sight, and slope distances and vertical and horizontal angles are recorded. From these data it is possible to obtain surface heights to an accuracy of ±0·5 cm at 200 m and to avoid any direct disturbance of the site under investigation.     

New instrument for measuring water discharge by the salt dilution method

The instantaneous salt dilution method for water discharge measurements in open channels has been improved by the development of a new instrument measuring conductivity. The salt method consists of two parts: the calibration and the actual measurement in the stream. The calibration aims to calculate the linear relationship between electrical conductivity and salt concentration at various degrees of dilution in a salt solution. The original undiluted solution is injected into the water of a stream and the conductivity is measured downstream from the injection site. When measuring, the new instrument integrates the conductivity over time. From the value obtained on the instrument's display, the water discharge can easily be calculated on a hand-PC in the field. The instrument has eliminated the subsequent calculation work formerly necessary. It has increased the accuracy of the method and also reduced the need for field personnel during measurements.