Gamma-ray bursts and Swift

Now that the Swift satellite is up and running, Paul O'Brien, Julian Osborne and Keith Mason report on the ideas and observations that this fast-moving autonomous observatory is investigating.     

Arsenic Management Through Well Modification and Simulation

Arsenic concentrations can be managed with a relatively simple strategy of grouting instead of completely destroying a selected interval of well. The strategy of selective grouting was investigated in Antelope Valley, California, where groundwater supplies most of the water demand. Naturally occurring arsenic typically exceeds concentrations of 10 µg/L in the water produced from these long-screened wells. The vertical distributions of arsenic concentrations in intervals of the aquifer contributing water to selected supply wells were characterized with depth-dependent water-quality sampling and flow logs. Arsenic primarily entered the lower half of the wells where lacustrine clay deposits and a deeper aquifer occurred. Five wells were modified by grouting from below the top of the lacustrine clay deposits to the bottom of the well, which reduced produced arsenic concentrations to less than 2 µg/L in four of the five wells. Long-term viability of well modification and reduction of specific capacity was assessed for well 4-54 with AnalyzeHOLE, which creates and uses axisymmetric, radial MODFLOW models. Two radial models were calibrated to observed borehole flows, drawdowns, and transmissivity by estimating hydraulic-conductivity values in the aquifer system and gravel packs of the original and modified wells. Lithology also constrained hydraulic-conductivity estimates as regularization observations. Well encrustations caused as much as 2 µg/L increase in simulated arsenic concentration by reducing the contribution of flow from the aquifer system above the lacustrine clay deposits. Simulated arsenic concentrations in the modified well remained less than 3 µg/L over a 20-year period.     

Homogenization of spatial patterns of hydrologic response in artificially drained agricultural catchments

Anthropogenic modifications to the landscape, with agricultural activities being a primary driver, have resulted in significant alterations to the hydrologic cycle. Artificial drainage, including surface and subsurface drainage (tile drains), is one of the most extensive manipulations in agricultural landscapes and thus is expected to provide a distinct signature of anthropogenic modification. This study adopts a data synthesis approach in an effort to characterize the signature of artificial subsurface drainage. Daily discharge data from 24 basins across the state of Iowa, which encapsulate a range of anthropogenic modifications, are assessed using a variety of flow metrics. Results indicate that the presence of artificial subsurface drainage leads to a homogenization of landscape hydrologic response. Non‐tiled watersheds exhibit a decrease in the area‐normalized peak discharge and an increase in the baseflow ratio (baseflow/streamflow) with increases in the spatial scale, while scale invariance is apparent in tiled basins. Within‐basin variability in hydrograph recession coefficients also appears to decrease with increases in the proportion of the catchment that is artificially drained. Finally, the differences between tiled and non‐tiled landscapes disappear at scales greater than approximately 2200 km², indicating that this may be a threshold scale for studying the effects of tile drainage. This decrease in within‐basin variability and the scale invariance of hydrologic metrics in artificially drained watersheds are attributed to the creation of a bypass flow hydrologic pathway that bypasses the complexity of the catchment travel paths. Spatial homogeneity in responses implies that it may be possible to develop more parsimonious hydrologic models for these regions. Copyright © 2013 John Wiley & Sons, Ltd.     

Experimental Study of the Brittle Behavior of Clay shale in Rapid Unconfined Compression

The mechanical behavior of clay shales is of great interest in many branches of geo-engineering, including nuclear waste disposal, underground excavations, and deep well drilling. Observations from test galleries (Mont Terri, Switzerland and Bure, France) in these materials have shown that the rock mass response near the excavation is associated with brittle failure processes combined with bedding parallel shearing. To investigate the brittle failure characteristics of the Opalinus Clay recovered from the Mont Terri Underground Research Laboratory, a series of 19 unconfined uniaxial compression tests were performed utilizing servo-controlled testing procedures. All specimens were tested at their natural water content with loading approximately normal to the bedding. Acoustic emission (AE) measurements were utilized to help quantify stress levels associated with crack initiation and propagation. The unconfined compression strength of the tested specimens averaged 6.9 MPa. The crack initiation threshold occurred at approximately 30% of the rupture stress based on analyzing both the acoustic emission measurements and the stress–strain behavior. The crack damage threshold showed large variability and occurred at approximately 70% of the rupture stress.     

Consumer shopping strategies and purchasing activity: An exploratory investigation

Geographers have relied on somewhat simplistic stimulus-response models to explain shopping expenditure patterns. It is argued that this has resulted in an emphasis on the end-points of the shopping process (places of purchase), rather than on the process itself; so failing to recognise that the same end-points can be reached along a variety of paths. More attention should be given to process and hence to the strategies consumers adopt when engaging in purchasing activity. In this exploratory study, set in a North Yorkshire parish, it was found that the criteria consumers used to select amongst retail outlets were associated with particlar styles of shopping activity, which were in turn tied to particular spatial patterns of purchasing.     

CO2 Mitigation by Agriculture: An Overview

Agriculture currently contributes significantly to the increase of CO2 in the atmosphere, primarily through the conversion of native ecosystems to agricultural uses in the tropics. Yet there are major opportunities for mitigation of CO2 and other greenhouse gas emissions through changes in the use and management of agricultural lands. Agricultural mitigation options can be broadly divided into two categories: (I) strategies to maintain and increase stocks of organic C in soils (and biomass), and (ii) reductions in fossil C consumption, including reduced emissions by the agricultural sector itself and through agricultural production of biofuels to substitute for fossil fuels.Reducing the conversion of new land to agriculture in the tropics could substantially reduce CO2 emissions, but this option faces several difficult issues including population increase, land tenure and other socio-political factors in developing countries. The most significant opportunities for reducing tropical land conversions are in the humid tropics and in tropical wetlands. An important linkage is to improve the productivity and sustainability of existing agricultural lands in these regions.Globally, we estimate potential agricultural CO2 mitigation through soil C sequestration to be 0.4-0.9 Pg C y-1, through better management of existing agricultural soils, restoration of degraded lands, permanent "set-asides" of surplus agricultural lands in temperate developed countries and restoration of 10-20% of former wetlands now being used for agriculture. However, soils have a finite capacity to store additional C and therefore any increases in C stocks following changes in management would be largely realized within 50-100 years.Mitigation potential through reducing direct agricultural emissions is modest, 0.01-0.05 Pg C y-1. However, the potential to offset fossil C consumption through the use of biofuels produced by agriculture is substantial, 0.5-1.6 Pg C y-1, mainly through the production of dedicated biofuel crops with a smaller contribution (0.2-0.3 Pg C y-1) from crop residues.Many agricultural mitigation options represent "win-win" situations, in that there are important side benefits, in addition to CO2 mitigation, that could be achieved, e.g. improved soil fertility with higher soil organic matter, protection of lands poorly suited for permanent agriculture, cost saving for fossil fuel inputs and diversification of agricultural production (e.g. biofuels). However, the needs for global food production and farmer/societal acceptability suggest that mitigation technologies should conform to: (I) the enhancement of agricultural production levels in parts of the world where food production and population demand are in delicate balance and (ii) the accrual of additional benefits to the farmer (e.g., reduced labor, reduced or more efficient use of inputs) and society at large.     

Development of a non-linear mixed spectral finite difference model for turbulent boundary-layer flow over topography

Further development of the non-linear mixed spectral finite difference (NLMSFD) model of turbulent boundary-layer flow over topography is documented. This includes modifications and refinements to the solution procedure, the incorporation of second-order turbulence closures to the model and the three-dimensional extension of the model. Based on these higher order closures, linear limitations, boundary-layer approximation and non-linear effects are discussed. The impact of different turbulence closures on the prediction of the NLMSFD model is also demonstrated. Furthermore, sample results for 3D idealized topography (sinusoidal) are presented. The parameterization of drag over small-scale topography is also addressed.     

Quasi-Wavelet Model of Von Kármán Spectrum of Turbulent Velocity Fluctuations

The von Kármán spectra of turbulent temperature andvelocity fluctuations have been widely used in the literature on turbulenceand electromagnetic, seismic, and acoustic wave propagation in random media.In this paper we provide a phenomenological motivation for the vonKármán velocity spectrum in terms of the quasi-wavelet model ofturbulence developed recently. In this model, turbulence is represented as asuperposition of self-similar localized eddies of many different scales. Wefind a functional form for these eddies that yields the von Kármán velocity spectrum exactly. We also show that other eddy functions producevelocity spectra that have the same general form as the von Kármán spectrum, and we consider possible quasi-wavelet representations of the`Kansas' spectrum and the `-1' spectrum. We also present asystematic determination, based on turbulence similarity theories, of theparameters of the von Kármán spectra of temperature and velocityfluctuations in an unstable atmospheric boundary layer.     

When is it appropriate to combine expert judgments?

Summary Rather than seeking improved methodologies, difficulty in combining expert opinion should serve as a warning flag that causes us to seek alternative modes of policy analysis. These alternatives are usually more appropriate for the real audience for our analyses.Policy analysis of climate change is too often framed in terms that amount to preparing the tools with which a benevolent world dictator could do cost-benefit analysis. This tends to overemphasize end-to-end analysis that must rely on the combined opinions of experts. This framing is unrealistic and encourages omission of important aspects of the climate problem such as its heterogeneity. Rejecting this framing in favor of alternate, less all encompassing, forms of policy analysis permits more robust results, and reduces the emphasis on combining expert opinion.While the opinions expressed here are my own, I thank Hadi Dowlatabadi, M. Granger Morgan, Ted Parson, and James Risbey for their perceptive comments.