Laboratory and field assessment of an infrared turbidity probe and its response to particle size and variation in suspended sediment concentration

Abstract

Using a combination of laboratory and field experiments, the performance of a Partech Instruments Ltd IR40-C active head suspended solids sensor has been tested with respect to changing particle size distribution and rapid variation in suspended sediment transport rate. The sensor, which utilizes light attenuation in the infrared waveband, has several advantages both over visible light optical designs and over nuclear or optical back-scatter (OBS) devices. Three sets of laboratory tests are reported: calibration experiments using estuarine, pro-glacial and control media; flume simulations of pulsed sediment supply at a variety of frequencies, concentrations and ambient flow velocities; and simple tests for the effect of air bubbles within the sensor light path. The sensor was also deployed in the field, together with an electromagnetic current meter, to monitor flow and suspended sediment transport fluctuations in the turbulent frequency range. Although the sensor performed well with respect to drift, noise and stability of calibration, significant variation occurred with respect to suspensions of differing particle sizes. A simple correction procedure involving the weighting of sensor output by specific particle surface area may enable comparison of turbidity data from different locations and times, and with standard calibration solutions. At high frequencies, the sensor is capable of resolving fluctuations in the transport rate to c. 0.5 s, which appears to correspond well to field observations of significant velocity fluctuation and transport ‘events’ and is comparable with results obtained using OBS devices. However, the resolution of transport fluctuations is again dependent upon particle characteristics, and the presence of air bubbles in the flow whose size approximates the path width of the sensor may mimic a fluctuating transport process. Further interpretation of high frequency measurement awaits research into the fundamentals of rapidly varying flow and sediment transport characteristics.     

Genetics and the Environment: Understanding Geographical Variations in the Incidence of Childhood Diabetes

Abstract. Interest in the geography of childhood diabetes has flourished worldwide as a result of the recognition that the place where a child lives is one of the most important determinants of risk. Childhood diabetes is thought to result from exposure to environmental factors which trigger a pathogenic response in genetically susceptible children. The paper reviews evidence of marked spatial and temporal variation in the incidence of childhood diabetes around the world, and examines the potential role of environmental and immunogenetic risk factors which may give rise to these patterns.     

A Petrogenetic Grid for Reactions Producing Biotite and other Al-Fe-Mg Silicates in the Greenschist Facies

This report presents an analysis of phase relations among biotite,muscovite, chlorite, stilpnomelane, actinolite and K-feldsparin a ten component system within the greenschist facies. Itis based on study of the chemical composition of these minerals,on calculations to balance chemical equations-among them, andon their field distribution. A petrogenetic grid resulting fromthis treatment consists of a single invariant point and multipleunivariant lines, the number depending on what part of the assemblageset is held constant. The reactions which involve biotite aresimilar to previously proposed reactions for the biotite isograd.At high pressure, biotite is produced from muscovite+stilpnomelane+actinolite?K-feldspar.At low pressure, chlorite+K-feldspar?stilpnomelane?actinolitereact to form biotite. A biotite-free reaction, not previouslyidentified, divides the chlorite zone into high pressure andlow pressure fields, characterized by the assemblages muscovite+stilpnomelane+actinoliteand chlorite+K-feldspar, respectively. In the blueschist facies,muscovite plus stilpnomelane and/or actinolite are stable insteadof biotite.     

EM COUPLING IN MULTIFREQUENCY IP AND A GENERALIZATION OF THE COLE-COLE IMPEDANCE MODEL*

The Cole-Cole relaxation model has been found to provide good fits to multifrequency IP data and is derivable mathematically from a reasonable, albeit greatly simplified, physical model of conduction in porous rocks. However, the Cole-Cole model is used to represent the mutual impedance due to inductive or electromagnetic coupling on an empirical basis: this use has not been similarly justified by derivation from any simple physical representation of, say, a half-space, layered or uniform. A uniform conductive half-space can be represented as a simple subsurface loop with particular resistive and inductive properties. Based upon this, a mathematical expression for the mutual impedance between the two pairs of electrodes of a dipole-dipole array is derived and designated “model I”. It is seen that a degenerate case of model I is the Cole-Cole model with frequency exponent c= 1. Model I is thus more general than the Cole-Cole expression and must provide at least as good a fit to a set of field data. Provision for variation of c from unity could be made in model I equally well as for the Cole-Cole model although, at present, this would be a purely empirical alteration. Model I contains four parameters, one of which is, in effect, the resistivity of the half-space. Therefore only three parameters are involved in the model I expressions for normalized amplitude and for phase of the EM-coupling mutual impedance. Model I is compared with previously published “standard” values for two different dipole separations. Under particular constraints, model I is shown to provide better fits than the Cole-Cole model (with c= 1) over particular frequency ranges, specifically at very low frequencies and at moderately high frequencies where the model I phase curve follows the standard phase curve across the axis to positive values (negative coupling).     

SAMPLING METHODS AND APPROACHES USING RADIONUCLIDE TRACERS IN THE STUDY OF SEDIMENT RESUSPENSION AND CROSS MARGIN TRANSPORT IN NEARSHORE OF THE LAURENTIAN GREAT LAKES

1 INTRODUCTION Sediment resuspension, particularly in response to episodic storm events, is a common characteristic of coastal and nearshore environments. The resulting 揵enthic storms?may play a significant role in the dynamics of benthic systems, including structuring the physical characteristics of the bottom, providing a mechanism for the recycling of biogeochemically important materials to the water column, and driving horizontal and cross margin transport of suspended particulates …     

Black Spot and Rain: A Markov Chain Model for Appleby, New Zealand

Black spot (Venturia inaequalis) is a serious fungal disease of apples associated with rain. Control is by use of agrichemicals, but infection still occurs. A first‐order Markov chain probability model was applied to the distribution of spring wet spells for Appleby, for Sep–Dec, 1982–1990. Comparison with risk periods for primary black spot infection showed that, while 1‐day wet spells were the most frequent (41% probability of occurrence), the most frequent occurrence of risk periods was in association with 2‐day wet spells. Long wet spells (up to 18 days) were almost always associated with risk periods, but were infrequent.     

EXPORTS OF ORGANIC CARBON IN BRITISH RIVERS

This study provides the first detailed estimate of riverine organic carbon fluxes in British rivers, as well as highlighting major gaps in organic carbon data in national archives. Existing data on organic carbon and suspended solids concentrations collected between 1989 and 1993, during routine monitoring by the River Purification Boards (RPBs) in Scotland and the National River Authorities (NRAs) in England and Wales, were used with annual mean flows to estimate fluxes of dissolved and particulate organic carbon (DOC and POC) in British rivers. Riverine DOC exports during 1993 varied from 7·7–103·5 kg ha⁻¹ year⁻¹, with a median flux of 31·9 kg ha⁻¹ year⁻¹ in the 85 rivers for which data were available. There was a trend for DOC fluxes to increase from the south and east to the north and west. A predictive model based on mean soil carbon storage in 17 catchments, together with regional precipitation totals, explained 94% of the variation in the riverine DOC exports in 1993. This model was used to predict riverine DOC fluxes in regions where no organic carbon data were available. Calculated and predicted fluxes were combined to produce an estimate for exports of DOC to tidal waters in British rivers during 1993 of 0·68±0·07 Mt. Of this total, rivers in Scotland accounted for 53%, England 38% and Wales 9%. Scottish blanket peats would appear to be the largest single source of DOC exports in British rivers. An additional 0·20 Mt of organic carbon were estimated to have been exported in particulate form in 1993, approximately two–thirds of which was contributed by English rivers. It is suggested that riverine losses of organic carbon have the potential to affect the long-term dynamics of terrestrial organic carbon pools in Britain and that rivers may regulate increases in soil carbon pools brought about by climate change. © 1997 by John Wiley & Sons, Ltd.     

REAL-TIME FILTERING OF DATA FROM MOBILE,PASSIVE REMOTE INFRARED SENSORS WITH PRINCIPAL COMPONENT MODELS OF BACKGROUND

Real-time monitoring of pollutant levels from a mobile measuring platform requires fast,flexible dataanalysis methods.This paper reports a method for rapid analysis of passive remotely sensed infrared datawith the aid of a Kalman filter.The background spectra produced by emission from the atmosphere aremodelled at the start of the data collection sequence with a simple principal components model obtainedby eigenanalysis of the initial‘blank’data taken with the spectrometer.The species of interest areincluded in the state space model by a separate measurement of their infrared spectra.It is demonstratedthat for best filter performance in detecting the simulated pollutant species SF_6 in the atmosphere,a filtermodel with two principal components describing the emission background works best.The filter‘maps’of SF_6 closely follow the integrated spectral intensities measured after removal of suitable backgrounds.     

The dynamics of vulnerability: locating coping strategies in Kenya and Tanzania

We investigate how smallholder farmers at two sites in Kenya and Tanzania cope with climate stress and how constraints and opportunities shape variations in coping strategies between households and over time during a drought. On the basis of this analysis, we draw out implications for adaptation and adaptive policy. We find that households where an individual was able to specialize in one favoured activity, such as employment or charcoal burning, in the context of overall diversification by the household, were often less vulnerable than households where each individual is engaged in many activities at low intensity. Many households had limited access to the favoured coping options due to a lack of skill, labour and/or capital. This lack of access was compounded by social relations that led to exclusion of certain groups, especially women, from carrying out favoured activities with sufficient intensity. These households instead carried out a multitude of less favoured and frequently complementary activities, such as collecting indigenous fruit. While characterized by suitability to seasonal environmental variations and low demands on time and cash investments, these strategies often yielded marginal returns. Both the marginalization of local niche products and the commercialization of forest resources exemplify processes leading to differential vulnerability. We suggest that vulnerability can usefully be viewed in terms of the interaction of such processes, following the concept of locality. We argue that coping is a distinct component of vulnerability and that understanding the dynamism of coping and vulnerability is critical to developing adaptation measures that support people as active agents.     

Image‐intensified video results from the 1998 Leonid shower: I. Atmospheric trajectories and physical structure

Abstract— Two‐station electro‐optical observations of the 1998 Leonid shower are presented. Precise heights and light curves were obtained for 79 Leonid meteors that ranged in brightness (at maximum luminosity) from +0.3 to +6.1 astronomical magnitude. The mean photometric mass of the data sample was 1.4 × 10^?6 kg. The dependence of astronomical magnitude at peak luminosity on photometric mass and zenith angle was consistent with earlier studies of faint sporadic meteors. For example, a Leonid meteoroid with a photometric mass of ～1.0 × 10‐⁷ kg corresponds to a peak meteor luminosity of about +4.5 astronomical magnitudes. The mean beginning height of the Leonid meteors in this sample was 112.6 km and the mean ending height was 95.3 km. The highest beginning height observed was 144.3 km. There is relatively little dependence of either the first or last heights on mass, which is indicative of meteoroids that have clustered into constituent grains prior to the onset of intensive grain ablation. The height distribution, combined with numerical modelling of the ablation of the meteoroids, suggests that silicate‐like materials are not the principal component of Leonid meteoroids and hints at the presence of a more volatile component. Light curves of many Leonid meteors were examined for evidence of the physical structure of the associated meteoroids: similar to the 1997 Leonid meteors, the narrow, nearly symmetric curves imply that the meteoroids are not solid objects. The light curves are consistent with a dustball structure.