bluuebunny/crossref_metadata_2025 huggingface dataset:
Case/pseudocontrol analysis in genetic association studies: A unified framework for detection of genotype and haplotype associations, gene‐gene and gene‐environment interactions, and parent‐of‐origin effects
Authors: Cordell Heather J. , Barratt Bryan J. , Clayton David G.
Published: March 2004

Estimation and testing of genetic effects (genotype relative risks) are often performed conditionally on parental genotypes, using data from case‐parent trios. This strategy avoids having to estimate nuisance parameters such as parental mating type frequencies, and also avoids generating spurious results due to confounding causes of association such as population stratification. For effects at a single locus, the resulting analysis is equivalent to matched case/control analysis via conditional logistic regression, using the case and three “pseudocontrols” derived from the untransmitted parental alleles. We previously showed that a similar approach can be used for analyzing genotype and haplotype effects at a set of closely linked loci, but with a required adjustment to the conditioning argument that results in varying numbers of pseudocontrols, depending on the disease model that is to be fitted. Here we extend this method to include the analysis of epistatic effects (gene‐gene interactions) at unlinked loci, to include parent‐of‐origin effects at one or more loci, and to allow additional incorporation of gene‐environment interactions. The conditional logistic approach provides a natural and flexible framework for incorporating these additional effects. By relaxing the conditioning on parental genotypes to allow exchangeability of parental genotypes, we show how the power of this approach can be increased when studying parent‐of‐origin effects. Simulations suggest that there is limited power to distinguish between parent‐of‐origin effects and effects due to interaction between genotypes of mother and child. © 2004 Wiley‐Liss, Inc.
Multipoint linkage analysis of quantitative traits on sex‐chromosomes
Authors: Ekstrøm Claus Thorn
Published: March 2004

Variance component models form a powerful and flexible tool for multipoint linkage analysis of quantitative traits. Estimates of genetic similarity are needed for the variance component model to detect linkage and to locate genes, and two methods are commonly used to calculate multipoint identity‐by‐descent (IBD) estimates for autosomes. Fulker et al. ([1995] Am. J. Hum. Genet. 56: 1229–1233) and Almasy and Blangero ([1998] Am. J. Hum. Genet. 62: 119–121) used multiple regression to estimate the IBD sharing along a chromosome, while the approach of Kruglyak and Lander ([1995] Am. J. Hum. Genet. 57: 439–454) is based on a hidden Markov model. In this paper, we modify the variance component model to accommodate sex‐chromosomes, and we extend both multipoint IBD estimation methods to accommodate sex‐linked loci. Simulation studies demonstrate the power and precision of the variance component model to detect QTLs located on the sex‐chromosome. The two multipoint IBD estimation methods have the same accuracy to identify QTL position, but the hidden Markov model yields a larger average maximum LOD score to detect linkage than the regression model. The extension of the multipoint IBD estimation methods and the variance component model to the X chromosome shows that the variance component model is a powerful and flexible tool for linkage analysis of quantitative traits on both autosomes and sex‐chromosomes. © 2004 Wiley‐Liss, Inc.
Latent class and genetic analysis does not support migraine with aura and migraine without aura as separate entities
Authors: Nyholt Dale R. , Gillespie Nathan G. , Heath Andrew C. , Merikangas Kathleen R. , Duffy David L. , Martin Nicholas G.
Published: March 2004

Latent class and genetic analyses were used to identify subgroups of migraine sufferers in a community sample of 6,265 Australian twins (55% female) aged 25–36 who had completed an interview based on International Headache Society (IHS) criteria. Consistent with prevalence rates from other population‐based studies, 703 (20%) female and 250 (9%) male twins satisfied the IHS criteria for migraine without aura (MO), and of these, 432 (13%) female and 166 (6%) male twins satisfied the criteria for migraine with aura (MA) as indicated by visual symptoms. Latent class analysis (LCA) of IHS symptoms identified three major symptomatic classes, representing 1) a mild form of recurrent nonmigrainous headache, 2) a moderately severe form of migraine, typically without visual aura symptoms (although 40% of individuals in this class were positive for aura), and 3) a severe form of migraine typically with visual aura symptoms (although 24% of individuals were negative for aura). Using the LCA classification, many more individuals were considered affected to some degree than when using IHS criteria (35% vs. 13%). Furthermore, genetic model fitting indicated a greater genetic contribution to migraine using the LCA classification (heritability, h2=0.40; 95% CI, 0.29–0.46) compared with the IHS classification (h2=0.36; 95% CI, 0.22–0.42). Exploratory latent class modeling, fitting up to 10 classes, did not identify classes corresponding to either the IHS MO or MA classification. Our data indicate the existence of a continuum of severity, with MA more severe but not etiologically distinct from MO. In searching for predisposing genes, we should therefore expect to find some genes that may underlie all major recurrent headache subtypes, with modifying genetic or environmental factors that may lead to differential expression of the liability for migraine. © 2004 Wiley‐Liss, Inc.
Evaluations of maximization procedures for estimating linkage parameters under heterogeneity
Authors: Biswas Swati , Lin Shili
Published: March 2004

Locus heterogeneity is a major problem plaguing the mapping of disease genes responsible for complex genetic traits via linkage analysis. A common feature of several available methods to account for heterogeneity is that they involve maximizing a multidimensional likelihood to obtain maximum likelihood estimates. The high dimensionality of the likelihood surface may be due to multiple heterogeneity (mixing) parameters, linkage parameters, and/or regression coefficients corresponding to multiple covariates. Here, we focus on this nontrivial computational aspect of incorporating heterogeneity by considering several likelihood maximization procedures, including the expectation maximization (EM) algorithm and the stochastic expectation maximization (SEM) algorithm. The wide applicability of these procedures is demonstrated first through a general formulation of accounting for heterogeneity, and then by applying them to two specific formulations. Furthermore, our simulation studies as well as an application to the Genetic Analysis Workshop 12 asthma datasets show that, among other observations, SEM performs better than EM. As an aside, we illustrate a limitation of the popular admixture approach for incorporating heterogeneity, proved elsewhere. We also show how to obtain standard errors (SEs) for EM and SEM estimates, using methods available in the literature. These SEs can then be combined with the corresponding estimates to provide confidence intervals of the parameters. © 2004 Wiley‐Liss, Inc.
Oblique rainfall and contemporary geomorphological dynamics (Serra da Estrela, Portugal)
Authors: Vieira Gonçalo , Mora Carla , Gouveia Maria Manuel
Published: March 2004

Coarse sand accumulations are polygenic microforms that attain a width of several metres, a height up to 30–40 cm, a gradient of 8–12° and a slope length up to 1 m. These accumulations are frequent in the gruss‐covered plateaus of the granite mountains of central and northern Portugal, but they have been described in other mountain areas (i.e. Cairngorms, Scotland). Though these microforms are frequent features, studies on them are rare. They have been attributed to complex genesis controlled primarily by aeolian processes, but also by wash and cryogenic dynamics. Results presented here add new insights into the origin of the sand accumulations and emphasize the importance of rainsplash‐saltation induced by oblique rainfall as the main transportation mechanism. The study was conducted in the Serra da Estrela, a granite mountain in central Portugal (1993 m above sea level) and is supported by a detailed mapping of the orientation of the accumulations, monitoring of the surface material and analysis of meteorological data. The results are particularly significant since they indicate that the coarse sand accumulations are very active features that show a clear climatic and ecological signal. Copyright © 2004 John Wiley & Sons, Ltd.
Sources of weathering‐derived solutes in two granitic catchments with contrasting forest growth
Authors: Asano Yuko , Ohte Nobuhito , Uchida Taro
Published: March 2004

Chemical weathering is one of the controlling factors for landform evolution. Here, we present deductions about the location and rate of chemical weathering within catchments based on chemical observations of soil water, groundwater, springs, and streams in two granitic catchments with contrasting forest growth conditions (forested and non‐vegetated) and soil development. The two catchments are located in the Tanakami Mountains of central Japan, and share the same climate. In both catchments, SiO2 and Na+ are the dominant weathering‐derived solutes in the first‐order stream. Using hydrological evidence, the contribution to the SiO2 and Na+ fluxes from soil and bedrock were partitioned. Despite the difference in mean soil depth, net fluxes of SiO2 and Na+ from the soil profile do not differ much between the two catchments, suggesting low levels of easily weatherable minerals in the forest soil. Net SiO2 and Na+ fluxes from bedrock are about three times greater in the forested catchment than in the bare catchment, and these differences contribute largely to the difference in net fluxes from the catchments. Net fluxes of SiO2 and Na+ from the forested catchment are about twice those from the bare catchment. Part of this increase may be related to the greater proton production in forested catchment. Consequently, about 60 to 70% of the net SiO2 and Na+ fluxes from the non‐vegetated catchment, and 70 to 90% from the forested catchment, are derived from bedrock, suggesting that the contribution of bedrock to the export of these weathering products increased with forest growth and soil formation. Copyright © 2004 John Wiley & Sons, Ltd.
Coupling of runoff processes and sediment transport in mountainous watersheds underlain by different sedimentary rocks
Authors: Hattanji Tsuyoshi , Onda Yuichi
Published: March 2004

Runoff and sediment transport were observed in two mountainous watersheds with different lithologies. One watershed is underlain by chert and the other by sandstone and shale. The chert watershed consists of bedrock cliffs and shallow regolith (0–2 m), whereas the sandstone–shale watershed is covered with a deeper regolith (1–5 m). In the sandstone–shale watershed, a large spring located in the channel head contributed to storm runoff. Tensiometric response in regolith above the spring showed negative pressure head throughout storm runoff, thus implying that the subsurface flow in bedrock predominated even during storm runoff. In the chert watershed, the runoff coefficient was larger than in the sandstone–shale watershed. Tensiometric response in regolith showed positive pressure head during storm runoff, indicating generation of rapid subsurface stormflow. Concentration of subsurface flow from side slopes increased peak streamflow in the downstream direction. In these two watersheds, different subsurface flowpaths, including bedrock fractures, facilitated different modes of sediment transport. Groundwater outflow at the spring caused seepage erosion in the sandstone–shale watershed, whereas peak streamflow transported bedload supplied by rockfall in the chert watershed. Copyright © 2004 John Wiley & Sons, Ltd.
Sources of suspended sediment within a tropical catchment recovering from selective logging
Authors: Chappell Nick A. , Douglas Ian , Hanapi Jamal Mohd , Tych Wlodek
Published: March 2004

Quantification of the source of suspended sediments generated by selective forestry activities is central to the development of sustainable forestry guidelines. This assessment is hampered by a dearth of available studies, particularly from the tropics. This study involved the monitoring (at 10 s intervals) of surface discharge and turbidity from 15 contributory areas of a 44 ha catchment. The catchment is located within lowland dipterocarp rain forest on Borneo Island, in a region recovering from the first episode of selective timber removal that took place some 5 years previously. Both the within‐storm dynamics of the sediment flux and the time‐integrated sediment yields were analysed to link the source landforms to the catchment behaviour.

A 10 year rainfall event of 167 mm occurred during the monitoring period and triggered a debris slide and several log‐culvert collapses along the area's main timber haulage road. The sampling design captured this event's dynamics and allowed lumped catchment response to be traced to the new landforms. During the 1 day period of the 10 year event, some 33 t of suspended sediment were transported from one debris slide, comprising a significant proportion of the 105 t discharging from the whole catchment, which itself constituted 40% of the annual yield of 592 t km−2. The contributory areas with only ephemeral waterflows, including former haulage roads and tracks, generated relatively little sediment during this 10 year event or in other storms. This work suggests that though some sediment sources recover from the impacts of forest road construction and harvesting, collapse of roadfill materials or more local log‐culvert failure persists for several years after harvesting. Sustainable forestry guidelines that do not focus on ameliorating these persistent instabilities may not significantly mitigate the geomorphic impacts of conventional, selective harvesting. Copyright © 2004 John Wiley & Sons, Ltd.
Theoretical research on the relationship between return period of rainfall and shallow landslides
Authors: Iida Tomoyuki
Published: March 2004

A hydrogeomorphological model for shallow landslide prediction that considers both the stochastic character of rainfall intensity and duration and the deterministic aspects controlling slope stability is presented. The probability of shallow subsurface flow (i.e. the process triggering shallow landsliding) is expressed by a lognormal distribution. The short‐term probability of landsliding is defined as the probability that the saturated depth exceeds a critical value. The average recurrence interval of landsliding Tav can be defined as the expected value of the recurrence interval of landsliding. This model was applied to a field site in Japan where extensive shallow landsliding occurred as a consequence of a heavy rainstorm in 1988. A digital elevation model was utilized to calculate Tav for every grid cell. It was found that Tav is a useful index to correlate to the susceptibility to shallow landsliding. A time series of landsliding was then simulated using this model. A series of values of return periods of rainfall (and, consequently, of saturated throughflow) were randomly generated for 10 000 years (one rainstorm event per year). It was found that the spatial extent of landsliding (expressed as a percentage of scar area) associated with each rainstorm event was strongly influenced not only by rainfall, but also by the historical sequence of landsliding. The effective return period of rainfall for shallow landsliding was then estimated using the same model. It was found that rainfall events with return periods <500 years and <1000 years trigger about 50% and 65% respectively of the total number of landslides over the long term. Copyright © 2004 John Wiley & Sons, Ltd.
Hydrogeomorphology: overview of an emerging science
Authors: Sidle Roy C. , Onda Yuichi
Published: March 2004

Here, we introduce a series of 14 papers generated from a symposium related to hydrogeomorphology that was part of the Fifth International Conference on Geomorphology. Additionally, recent developments in hydrogeomorphology are highlighted, particularly with respect to research in Japan. Linkages are drawn between natural hazards and the necessity to focus on hydrogeomorphic processes. Future research needs are discussed in the areas of temporal and spatial patterns of water and sediment dynamics, including the effects of distributed land management practices and interactions with natural hazards. Copyright © 2004 John Wiley & Sons, Ltd.
Suspended solids discharge from a small forested basin in the humid tropics
Authors: Sammori Toshiaki , Yusop Zulkifli , Kasran Baharuddin , Noguchi Shoji , Tani Makoto
Published: March 2004

Suspended solids (SS) discharge from a tropical rain forest was observed at the Bukit Tarek Experimental Watershed in Peninsular Malaysia in order to elucidate mechanisms of SS production and transport. Peaks of water discharge and electrical conductivity (EC) lagged further behind rainfall peaks than did dissolved oxygen (DO), indicating that the discharge in the early stage of a storm is mainly formed by rain water with high DO. Stream water showing a high value of EC originating from subsurface water formed the main storm flow and lagged behind the rainfall. SS concentrations rose to a peak quickly and, like DO, the peak preceded that of water discharge. A clockwise hysteresis loop in the relationship between SS and water discharge exists, and the magnitude of hysteresis loop is in proportion to storm size. The values of SS concentration correlate positively with the values of rainfall intensity on logarithmic axes. The time intervals between peaks of rainfall and SS concentrations are assumed to be a delivery term expressing distance from sediment source to measuring point. Immediate transport of SS from the source to the sampling site, the short time gap between the peaks of rainfall and SS concentrations, and the high rate of infiltration on the hillslope suggest that the sources of high SS concentrations are located close to the stream. The calculated source area is located at a gentler part of the stream, where wet riparian areas exist. The strong relationship between water discharge and SS concentration during the small storm proves that the source areas of SS and water were the same. In contrast, the source area of SS disappeared when rainfall ceased, whereas the source area of water discharge was still expanding in the larger storm. These phenomena may produce clockwise loops in the SS concentration–water discharge relationship. Copyright © 2004 John Wiley & Sons, Ltd.
The role of subsurface water flow paths on hillslope hydrological processes, landslides and landform development in steep mountains of Japan
Authors: Onda Yuichi , Tsujimura Maki , Tabuchi Hidekazu
Published: March 2004

Hydrological monitoring was conducted in high‐relief watersheds in the Japan Alps to investigate the relationship between hillslope hydrological processes and landform evolution in steep granite and shale mountains. In the Koshibu watershed, underlain by Mesozoic shale, the drainage density and frequency was significantly lower than in the Yotagiri watershed underlain by granite. Drainage micro‐morphology analysis showed that hillslopes in the watersheds K1 and K6 (Koshibu basin) are mostly combinations of talus and bedrock exposures. In contrast, watershed Y1 (Yotagiri basin) is composed of several zero‐order streams with hollows. Infinite slope stability analysis indicates that the regolith shear strength in the K6 watershed (Koshibu basin) is lower than that of the Y1 hillslope, but groundwater levels were higher in the Y1 hillslope than in the K6 hillslope during storm events. These data suggest that, although the shear strength of the soil is stronger in the Yotagiri watershed, the slopes are unstable because of the groundwater conditions, whereas deep‐seated landslides may occur episodically in the Koshibu watershed associated with extreme storms and very high antecedent soil moisture. These differences would strongly contribute to the different observed hillslope processes and drainage characteristics. Copyright © 2004 John Wiley & Sons, Ltd.
Landslides in Rio de Janeiro: The role played by variations in soil hydraulic conductivity
Authors: Vieira Bianca Carvalho , Fernandes Nelson Ferreira
Published: March 2004

Landslides are natural processes triggered by complex relationships between many conditioning factors. In Rio de Janeiro city, intense rainstorms combined with steep slopes, usually occupied by slums, create favourable conditions for landslide generation. In general, slope failures are related to high positive pore‐pressures inside soil mantles, which are associated with topographic hollows and hydraulic discontinuities inside the deep weathered profile.

This study aims to characterize the role played by spatial variations in saturated soil hydraulic conductivity Ksat on landsliding in the Papagaio Basin (2·5 km2), triggered by intense rainstorms (250 mm in less than 2 days) in February 1996. Field measurements, using the Guelph permeameter, were conducted along soil profiles (up to 5·0 m depth), around and inside landslide scars representing different geomorphological domains. Also, soil physical properties (grain size, total porosity, macroporosity, microporosity, etc.) were evaluated at the 87 sites where Ksat was measured.

Despite the many geological and pedological variations inside the basin, including changes in lithology, fracture density, vegetation type and succession stage, soil characteristics and weathering stage, and human activities, 94% of the Ksat values varied by only two orders of magnitude (1·0 × 10−6–9·0 × 10−5 m s−1). However, abrupt changes in Ksat were observed along some of the soil profiles studied, e.g. an increase of two orders of magnitude in only 0·30 m. These hydraulic discontinuities may contribute locally to the development of high pore pressures during intense rainstorms, leading to landsliding. The results also suggest that different Ksat variation patterns coexist inside the basin, which should be considered in mathematical models of landslide prediction. Copyright © 2004 John Wiley & Sons, Ltd.
Sediment pathways in a tropical forest: effects of logging roads and skid trails
Authors: Sidle Roy C. , Sasaki Shozo , Otsuki Mieko , Noguchi Shoji , Rahim Nik Abdul
Published: March 2004

Significant erosion occurred from recently constructed forest logging roads and skid trails in a small headwater catchment in Peninsular Malaysia. Soil loss was estimated by measuring dimensions of all significant rills and gullies along the road, as well as by measuring height of preserved soil pedestals in sidecast and fill material and on skid trails. Estimates of surface erosion from logging roads and skid trails were 272 ± 20 t ha−1year−1and 275 ± 20 t ha−1year−1respectively. However, owing to lack of connectivity of skid trails to the stream, much of the sediment mobilized on skid trails was stored either on adjacent hillslopes or the trails themselves, rather than being transported to the stream system, as was the case for the road. Steeper skid trails (>20% gradient) had slightly higher erosion rates (320 ± 24 t ha−1year−1) than trails with gentler gradients (245–264 t ha−1year−1). Some 60% of the soil loss on logging roads comes from erosion of the running surface. Disturbed cut and fill material along the road supplied the remaining 40% of the soil loss from roads. Roads and skid trails had no designed drainage systems; runoff discharged onto the hillslope at 25 major discharge nodes from the logging road (690 m total length) and at 34 nodes from skid trails (2300 m). Sediment pathways were either fully or moderately connected to headwater channels at 64% of the logging road nodes, but at only 26% of the nodes emanating from skid trails. A detailed sediment budget revealed that 78% of the soil loss from the road system (including log landings) was delivered to the stream in the first 16 months after logging began. Most (90%) of the deposition from skid trails occurred below just three discharge nodes. Runoff from and onto skid trails often exacerbated the sediment connectivity to channels. Clearly, sediment discharge from logging roads was more highly connected to the stream than discharge from skid trails. Once in the channel, much of this sediment was temporarily stored in the floodplain and behind woody debris. Copyright © 2004 John Wiley & Sons, Ltd.
Distributed simulations of landslides for different rainfall conditions
Authors: Dhakal Amod S. , Sidle Roy C.
Published: March 2004

A physically based distributed slope stability model is described that utilizes a combined surface–subsurface kinematic wave module to assess groundwater fluctuations related to slope stability. A total of 82 major rainstorms from 1972 to 1990 in Carnation Creek, British Columbia, were examined to determine the influence of different characteristics of rainstorms (such as mean and maximum hourly intensity, duration, and rainfall amount) on the slope stability. These rainstorms vary in mean intensity from 1·6 to 11·2 mm h−1, storm duration from 11 to 93 h, and maximum hourly intensity from 3·4 to 35 mm h−1. Four synthetic ‘uniform intensity’ rainstorms were also tested against real storms to assess the effect of short‐term hourly rainfall intensity peaks on landslide occurrence. Altogether, 602 simulations were conducted. The combined influence of mean and maximum hourly intensity, duration, and total rainfall amount of rainstorms were important in generating landslides. The temporal distribution of short‐term intensity also influenced the landslide occurrence. When saturated hydraulic conductivity of the soil was lowered or soil depth was raised, most rainstorms produced larger numbers of landslides. For the most part, actual rainstorms produced less stable conditions than their synthetic ‘uniform intensity’ counterparts. For all landslide‐producing storms, slope failure usually occurred after some threshold of cumulative rainfall and maximum hourly rainfall intensity. These simulations provide insights into the distributed behaviour of landslide occurrence during large rainstorms with varying characteristics. Copyright © 2004 John Wiley & Sons, Ltd.
Hydrogeomorphic linkages of sediment transport in headwater streams, Maybeso Experimental Forest, southeast Alaska
Authors: Gomi Takashi , Sidle Roy C. , Swanston Douglas N.
Published: March 2004

Hydrogemorphic linkages related to sediment transport in headwater streams following basin wide clear‐cut logging on Prince of Wales Island, southeast Alaska, were investigated. Landslides and debris flows transported sediment and woody debris in headwater tributaries in 1961, 1979, and 1993. Widespread landsliding in 1961 and 1993 was triggered by rainstorms with recurrence intervals (24 h precipitation) of 7·0 years and 4·2 years respectively. Occurrence, distribution, and downstream effects of these mass movements were controlled by landform characteristics such as channel gradient and valley configuration. Landslides and channelized debris flows created exposed bedrock reaches, log jams, fans, and abandoned channels. The terminus of the deposits did not enter main channels because debris flows spread and thinned on the unconfined bottom of the U‐shaped glaciated valley. Chronic sediment input to channels included surface erosion of exposed till (rain splash, sheet erosion, and freeze–thaw action) and bank failures. Bedload sediment transport in a channel impacted by 1993 landslides and debris flows was two to ten times greater and relatively finer compared with bedload transport in a young alder riparian channel that had last experienced a landslide and debris flow in 1961. Sediment transport and storage were influenced by regeneration of riparian vegetation, storage behind recruited woody debris, development of a streambed armour layer, and the decoupling of hillslopes and channels. Both spatial and temporal variations of sediment movement and riparian condition are important factors in understanding material transport within headwaters and through channel networks. Copyright © 2004 John Wiley & Sons, Ltd.
Numerical calculation of soil pipe flow and its effect on water dynamics in a slope
Authors: Kosugi Ken'ichirou , Uchida Taro , Mizuyama Takahisa
Published: March 2004

A chain of connected macropores, which develops nearly parallel to the impermeable subsurface layer, is commonly found in hillslopes and referred to as a soil pipe. Field observations have revealed that water flow in such a pipe (pipe flow) has large hydrogeomorphic effects, since it contributes to rapid transfer of storm water and subsurface soil erosion. This study proposed a simple simulation method to describe water dynamics in a piped hillslope. The applicability of the method was examined using data obtained from a bench‐scale experiment using pseudo soil pipes. Results of the experiment showed that (1) the flume‐bed pore water pressure became small around the upper ends of the pipes; (2) when pipe outlets were connected to a stream channel (the open pipe condition), the flume‐bed pore water pressures in the whole slope were much lower than those under the ‘no pipe’ condition; (3) when pipe outlets were not connected to a stream channel (the closed pipe condition), the flume‐bed pore water pressure at the pipe outlets was higher than that under the ‘no pipe’ condition. These results were reproduced by solving the two‐dimensional Richards equation assuming an equivalent soil matrix layer for the original ‘pipes plus soil matrix’ part, which had a larger saturated hydraulic conductivity than the surrounding soil matrix. This simple simulation method has an advantage over previous pipe flow models: both the pipe flow and the interaction between the soil pipes and the surrounding soil matrix can be computed by solving the Richards equation without using any further assumptions for the boundary conditions between the soil pipes and the soil matrix. Results of numerical experiments using the simple method showed that (1) under the closed pipe condition, the flume‐bed pore water pressure at the pipe outlets increased as the pipe length increased, and (2) once the open pipes clogged, flume‐bed pore water pressures in the whole slope increased, and the greatest increase occurred at the boundary between the clogging and non‐clogging parts. Copyright © 2004 John Wiley & Sons, Ltd.
A geomorphological explanation of the unit hydrograph concept
Authors: Cudennec C. , Fouad Y. , Sumarjo Gatot I. , Duchesne J.
Published: March 2004

The water path from any point of a basin to the outlet through the self‐similar river network was considered. This hydraulic path was split into components within the Strahler ordering scheme. For the entire basin, we assumed the probability density functions of the lengths of these components, reduced by the scaling factor, to be independent and isotropic. As with these assumptions, we propose a statistical physics reasoning (similar to Maxwell's reasoning) that considers a hydraulic length symbolic space, built on the self‐similar lengths of the components. Theoretical expressions of the probability density functions of the hydraulic length and of the lengths of all the components were derived. These expressions are gamma laws expressed in terms of simple geomorphological parameters. We validated our theory with experimental observations from two French basins, which are different in terms of size and relief. From the comparisons, we discuss the relevance of the assumptions and show how a gamma law structure underlies the river network organization, but under the influence of a strong hierarchy constraint. These geomorphological results have been translated into travel time probability density functions, through the hydraulic linear hypothesis. This translation provides deterministic explanations of some famous a priori assumptions of the unit hydrograph and the geomorphological unit hydrograph theories, such as the gamma law general shape and the exponential distribution of residence time in Strahler states. Copyright © 2004 John Wiley & Sons, Ltd.
Explaining the internal behaviour of artificial neural network river flow models
Authors: Sudheer K. P. , Jain Ashu
Published: March 2004

A novel method of visualizing and understanding the internal functional behaviour of an artificial neural network (ANN) river flow model is presented. The method hypothesizes that an ANN is able to map a function similar to the flow duration curve while modelling the river flow. A mathematical analysis of the hypothesis is presented, and a case study of an ANN river flow model confirms its significance. The proposed approach is also useful within other models that improve the performance of an ANN. The reasons why these models improve a raw ANN can be clearly understood using this approach. While the field of ANN knowledge‐extraction is one that continues to attract considerable interest, it is anticipated that the current approach will initiate further research and make ANNs more useful to the hydrologic community. Copyright © 2004 John Wiley & Sons, Ltd.
Palaeoenvironmental reconstruction and OSL dating of terrestrial Eemian deposits in the southeastern Netherlands
Authors: Schokker Jeroen , Cleveringa Piet , Murray Andrew S.
Published: March 2004

The Roer Valley Graben in the southeastern Netherlands is a subsiding area situated just south of the maximum extent of the Pleistocene glaciations. In this area, Eemian (marine isotope stage 5e) clastic and organic deposits have been preserved in a terrestrial sedimentary environment. Deposition took place in the early Eemian (regional pollen zones E2–E3) and late Eemian (E5–E6A–E6B). The sedimentary sequence contains a hiatus during the E4A–E4B–E5 regional pollen zones. Sedimentary and palynological data reveal that both the presence and extent of the Eemian deposits are intimately linked to the local groundwater‐level history and clastic sediment flux. Changes in these environmental factors are likely to be related to changes in the global climate. Optically stimulated luminescence (OSL) dating has been applied to provide an absolute time frame for the Late Quaternary deposits in the area. This study is thereby the first to present a reliable quartz OSL dating (114±12 ka) for terrestrial Eemian deposits in The Netherlands. Copyright © 2004 John Wiley & Sons, Ltd.
Luminescence dating of quaternary sediments: recent advances
Authors: Duller G. A. T.
Published: March 2004

Luminescence dating is unique amongst geochronological methods in dating the deposition of sediments based upon the properties of the constituent minerals. For Quaternary sediments the event being dated is the last exposure of the grains to daylight, and an implicit assumption is that this exposure was sufficient to remove any pre‐existing signal. Until recently it has been difficult to test this assumption, other than by dating samples from a given depositional context with an age that is known from independent methods. There have been a series of technological and methodological developments in the past 5–10 yr that make it possible to undertake many replicate measurements of the luminescence from a single sample and hence to explicitly test whether for a specific sample all the grains had their luminescence signal reset at deposition. This allows the reliability of the luminescence age to be assessed. Where all the grains in a sample were not exposed to sufficient daylight to reset their luminescence signal, the apparent age will be an average value of the luminescence from all the grains measured at one time. Where many grains are measured simultaneously, this will overestimate the depositional age. The distribution of apparent age can be made clear by reducing the number of grains being measured in a given experiment, ultimately to the point of measuring individual sand‐sized mineral grains. Copyright © 2004 John Wiley & Sons, Ltd.
Towards a Holocene tephrochronology for Sweden: geochemistry and correlation with the North Atlantic tephra stratigraphy
Authors: Boygle Jane
Published: March 2004

Northern Europe has been affected by fallout of tephra from volcanic eruptions in Iceland throughout at least the Late‐glacial and Holocene. Microscopic volcanic ash horizons found in the British Isles, northern Germany and Scandinavia (e.g. Hekla 4) now provide important regional isochrons. Over recent years analytical techniques such as electron probe microanalysis (EPMA) have helped identify these distal tephra layers by glass geochemistry. Early tephra work in Sweden used refractive indices and bracketing radiocarbon dates for identification, resulting in tentative correlation. This paper presents geochemical data for Icelandic Holocene tephra in Sweden. Several sites in central Sweden have four distinct mid‐Holocene tephra layers. Correlation with the Icelandic record indicates that the following tephra layers are present: Hekla 4 (ca. 3830 yr BP), Kebister tephra (ca. 3600 yr BP), Hekla 3 (ca. 2880 yr BP) and Askja ad 1875. Preliminary analyses suggest that more, previously unidentified, tephra layers are present in low concentrations in the region, indicating potential for expanding the Swedish tephrochronology for palaeoecological and palaeoenvironmental work in northern Europe. Copyright © 2004 John Wiley & Sons, Ltd.
A coherent high‐precision radiocarbon chronology for the Late‐glacial sequence at Sluggan Bog, Co. Antrim, Northern Ireland
Authors: Lowe J. J. , Walker M. J. C. , Scott E. M. , Harkness D. D. , Bryant C. L. , Davies S. M.
Published: March 2004

Seventy‐five radiocarbon dates are presented from Sluggan Bog in Co. Antrim, Northern Ireland. The Holocene peats are underlain by Late‐glacial sediments, which also appear to have accumulated largely in a mire environment. The radiocarbon dates, from the Late‐glacial and early Holocene part of the profile, were obtained from the humic and humin fractions of the sedimentary matrix, and from plant macrofossils. The last‐named were dated by AMS and the sediment samples by radiometric (beta counting) methods. Age–depth models for the three dating series show a very high level of agreement between the two fractions and the macrofossils. No statistically significant difference is found between the beta counting and AMS results. Three tephras were located in the profile, the uppermost of which is in a stratigraphical position suggestive of the Vedde Ash, but the geochemical and radiocarbon evidence do not support this interpretation. The lower ashes are in the correct stratigraphical position for the Laacher See and Borrobol tephras, attributions substantiated by the radiocarbon evidence, but not by the geochemical data. The Sluggan sequence has generated one of the most internally consistent radiocarbon chronologies for any Late‐glacial site in the British Isles, and it is suggested that in future more effort should be devoted to the search for, and analysis of, Late‐glacial mire sequences, rather than the limnic records that have formed the principal focus of Late‐glacial investigations hitherto. Copyright © 2004 John Wiley & Sons, Ltd.
River terrace sequences: templates for Quaternary geochronology and marine–terrestrial correlation
Authors: Bridgland David , Maddy Darrel , Bates Martin
Published: March 2004

Fluvial sequences, particularly terrace staircases, represent archives of Quaternary palaeoclimatic fluctuation and can serve as stratigraphical frameworks for geochronology and for correlation with other depositional environments, in particular, the global marine oxygen isotope record. Fluvial lithostratigraphical frameworks also provide contexts for records, from fossils and artefacts, of faunal evolution and human occupation; conversely, both records can be means of relative dating of riverine sequences.

Three fluvial sequences are examined as case studies. First is the Severn–Avon system in the English Midlands, which has biostratgraphical evidence and an amino acid geochronology, together with marker inputs from three different glaciations. The Somme sequence of northern France, famous for its Palaeolithic artefact assemblages, again has biostratigraphy and an amino acid geochronology and has also been dated with reference to overlying loess/palaeosols sequences. The fluvial terraces of the River Arun, the final case study, lack dating evidence but are interspersed within the Sussex raised beach staircase. Although various lines of evidence suggest that the rivers discussed have formed terraces in response to climatic fluctuation, an intriguing difference is that interglacial sediments occur at the bases of terrace formations in the Severn–Avon, whereas in the Somme they occur at the tops of sequences, beneath loessic overburden. Copyright © 2004 John Wiley & Sons, Ltd.
Bayesian analysis of radiocarbon chronologies: examples from the European Late‐glacial
Authors: Blockley S. P. E. , Lowe J. J. , Walker M. J. C. , Asioli A. , Trincardi F. , Coope G. R. , Donahue R. E.
Published: March 2004

Although there are many Late‐glacial (ca. 15 000–11 000 cal. yr BP) proxy climate records from northwest Europe, some analysed at a very high temporal resolution (decadal to century scale), attempts to establish time‐stratigraphical correlations between sequences are constrained by problems of radiocarbon dating. In an attempt to overcome some of these difficulties, we have used a Bayesian approach to the analysis of radiocarbon chronologies for two Late‐glacial sites in the British Isles and one in the Adriatic Sea. The palaeoclimatic records from the three sites were then compared with that from the GRIP Greenland ice‐core. Although there are some apparent differences in the timing of climatic events during the early part of the Late‐glacial (pre‐14 000 cal. yr BP), the results suggest that regional climatic changes appear to have been broadly comparable between Greenland, the British Isles and the Adriatic during the major part of the Late‐glacial (i.e. between 14 000 and 11 000 cal. yr BP). The advantage of using the Bayesian approach is that it provides a means of testing the reliability of Late‐glacial radiocarbon chronologies that is independent of regional chronostratigraphical (climatostratigraphical) frameworks. It also uses the full radiocarbon inventory available for each sequence and makes explicit any data selection applied. Potentially, therefore, it offers a more ‘objective’ basis for comparing regional radiocarbon chronologies than the conventional approaches that have been used hitherto. Copyright © 2004 John Wiley & Sons, Ltd.
The age and chronostratigraphical significance of North Atlantic Ash zone II
Authors: Austin William E. N. , Wilson Lindsay J. , Hunt John B.
Published: March 2004

Rhyolitic tephra with the geochemical characteristics of North Atlantic Ash Zone (NAAZ) II are described from the giant piston core MD95‐2006 from the northeast Atlantic Ocean. Shard size distribution indicates that the tephra represent a wind‐sorted, primary air‐fall deposit, but with a mode close to 200 μm they are too coarse to be air‐fall deposits at the core site, which lies ca. 1000 km from the possible tephra source. Randomly sampled shards reveal a single geochemical population belonging to the Icelandic transitional alkali magma series, suggesting that they are unlikely to represent ice rafted debris derived from Icelandic icebergs. The tephra probably represent air‐fall deposits, transported to the core site by sea ice within the northeast Atlantic gyre. The NAAZ II peak coincides with the rapid climate transition (cooling) at the end of interstadial 15, which can be assigned an age of 53 260±2660 yr BP from direct correlation with the Greenland ice‐core (GISP2) record. A comparison of the MD95‐2006 Neogloboquadrina pachyderma (sinistral) relative abundance and GISP2 δ18O records, relative to the NAAZ II isochron, suggests that this climatic event was synchronous across the North Atlantic. By direct correlation of interstadial maxima between the two records, the GISP2 time‐scale is transferred to MD95‐2006. Comparison of corrected and calibrated radiocarbon ages derived from monospecific foraminifers with the GISP2 ages at the same stratigraphical horizon suggest major age differences. These probably result from large variations in atmospheric 14C concentration and highlight the significant uncertainties associated with radiocarbon calibration during marine isotope stage 3. Copyright © 2004 John Wiley & Sons, Ltd.
Tephrochronology of last termination sequences in Europe: a protocol for improved analytical precision and robust correlation procedures (a joint SCOTAV–INTIMATE proposal)
Authors: Turney Chris S. M. , Lowe J. John , Davies Siwan M. , Hall Valerie , Lowe David J. , Wastegård Stefan , Hoek Wim Z. , Alloway Brent
Published: March 2004

The precise sequence of events during the Last Termination (18 000–9000 ka 14C yr BP), and the extent to which major environmental changes were synchronous, are difficult to establish using the radiocarbon method alone because of serious distortions of the radiocarbon time‐scale, as well as the influences of site‐specific errors that can affect the materials dated. Attention has therefore turned to other methods that can provide independent tests of the chronology and correlation of events during the Last Termination. With emphasis on European sequences, we summarise here the potential of tephrostratigraphy and tephrochronology to fulfil this role. Recent advances in the detection and analysis of ‘hidden’ tephra layers (cryptotephra) indicate that some tephras of Last Termination age are much more widespread in Europe than appreciated hitherto, and a number of new tephra deposits have also been identified. There is much potential for developing an integrated tephrochronological framework for Europe, which can help to underpin the overall chronology of events during the Last Termination. For that potential to be realised, however, there needs to be a more systematic and robust analysis of tephra layers than has been the practice in the past. We propose a protocol for improving analytical and reporting procedures, as well as the establishment of a centralised data base of the results, which will provide an important geochronological tool to support a diverse range of stratigraphical studies, including opportunities to reassess volcanic hazards. Although aimed primarily at Europe, the protocol proposed here is of equal relevance to other regions and periods of interest. Copyright © 2004 John Wiley & Sons, Ltd.
Hydrocarbon type analysis by thin‐layer chromatography with flame‐ionization detection: vacuum gas oils, heavy feeds, and hydroprocessed products
Authors: Barman Bhajendra N.
Published: March 2004

Thin‐layer chromatography with flame‐ionization detection (TLC‐FID) provides quantitative hydrocarbon type data as well as distribution of aromatics by ring number. This method has been applied to obtain amounts of saturates, aromatics, and polars in heavy oil distillates such as light vacuum gas oils and heavy vacuum gas oils derived from different crude sources. TLC‐FID chromatograms and resultant quantitative hydrocarbon type data show that these distillates vary markedly in aromatic contents and aromatic ring types. Similar observations are made with several fluid catalytic cracking feeds. Effects of process parameters such as operating pressure and temperature on hydroconversion of aromatics and polars from a heavy oil are assessed by TLC‐FID. It has been demonstrated that there is a preferential reduction of higher polycyclic aromatic hydrocarbons and polars with an increase of both hydrogen partial pressure and reactor temperature.