Topographic Discontinuities Research Articles

Excess topography and outburst flood: Geomorphic imprint of October 2023 extreme flood event in the Teesta catchment of Eastern Himalayas

The 2023 South-Lohnak Lake outburst flood event across the upstream Teesta catchment triggered short-lived, high-magnitude flooding and substantial socioeconomic disruption downstream. We performed landscape analysis across the Teesta catchment using topographic metrics to understand the geomorphic response to this extreme flood event. We estimated the various topographic metrics such as ksn anomaly (normalized channel steepness index) (∼500–2500 m0.9), SL-index (stream-length gradient index) (∼5000–7000 m), and KsnQ (channel steepness-weighted discharge) along the trunk channel draining through the South-Lohnak Lake, which shows high magnitude in flooded regions and numerous spikes in their longitudinal profiles across mass-wasting regions downstream. We used the KsnQ metric as an event characteristic to investigate the changes in the channel morphology due to high-magnitude flooding. There was a sudden increase in the magnitude of the KsnQ metric when the river traversed through the Chungthang dam over the high ranges of excess topography (∼0–1700 m). This shows that the downstream channel morphology changes rapidly during this flood event, and KsnQ is a significant indicator of geomorphic change. To understand the spatial relationship between the physical drivers that trigger this outburst flood, we quantified the causal relationship between glacial-hydrological drivers over the upstream Zemu-sub catchment. Our observations suggest that precipitation intensity and surface temperature had a significant direct causal influence on snowmelt and snow depth (in terms of water equivalent) over the event duration with a 5-day and 1-day lag composite. Through the inclusion of lagged composites and causal linkages, the drivers across the glaciated landscape serve as event anomalies, which trigger the glacier hillslope failure and subsequent high-magnitude outburst flooding. Therefore, we suggest that the interaction of topographic discontinuity with hydrological extremes and their causal interdependencies influences this outburst event in the upper reaches, followed by high-magnitude flooding in the downstream reaches of the Teesta River. However, the long-term geomorphic consequences of such events on the evolution of the regional landscape remain unclear.

Along-Strike Morphologic Variations of the Kyushu-Palau Ridge at 13°–17° N and Their Tectonic Implications

The paper provides a detailed description of the topographical distribution characteristics, along-strike morphologic variations, and tectonic implications of the Kyushu-Palau Ridge (KPR) in the 13°–17° N regions. The description is based on multibeam bathymetric data that was recently collected in the mid-southern segment of KPR and its surrounding areas. The findings indicate that the KPR in the studied region comprises several discontinuous seamounts and seamount chains. Along the ridge running from east to west, there is significant variation in the topography of the seafloor. Visible on this ridge are numerous deep-sea depressions and apparent topographical discontinuities. Geological tectonic processes such as plate subduction and seafloor spreading largely determine the topography of the seafloor. During the beginning of the Parece Vela Basin (PVB) spreading, the pre-existing volcanic arcs fractured under east-west extensional stress, resulting in the development of prominent seamounts, sharp ridges, relief alternations, and rift basins in the KPR. The east-west stress was subsequently transmitted to the weak and inactive mid-ocean ridge in the West Philippine Basin (WPB), which was reactivated and resulted in an episode of magma-poor spreading during 30–26 Ma. This episode caused the KPR to move eastward, stretched, and destroyed its pre-existing structure.

Morpho-structural signatures of neotectonic activity along the HFT bound Himalayan mountain front in Kathgodam-Chorgallia sector of NW Himalaya, India

Thrusting supports the development of diverse landforms and deformational structures in the Himalayan Frontal Thrust (HFT) region between Kathgodam and the Chorgallia sector, according to this study. The influence of the active tectonic condition in the area was analyzed using seven geomorphic indices, which also underlined the importance of morpho-structural study utilizing remote sensing, geomorphic indices, tectono-geomorphic landforms, field evidence, and luminescence dating. The investigation of sub-basins surrounding the HFT reveals moderate to high tectonic activity, with tectonic activity concentrating in the eastern half and decreasing toward the west. A topographic discontinuity was also observed in the region as indirect active tectonic evidence. Hanging wall rocks have well-preserved features that govern tectonic transport direction, and the HFT in this region is emergent. Shear sense indications point to southerly tectonic migration, with a few modest northward shear senses indicating northward movement, which might be attributable to reverse thrusting along the HFT at some time during the Outer Himalaya's tectonic history. Petrographic investigations reveal sustained frontal deformation at various intervals, with well-preserved thrust-induced deformation fingerprints in deformed rocks throughout the Jam Raula and Sukhi nadi sections. Micro thrust duplexes, or micro-scale asymmetrical drag folds, are shown in thin section investigations as a result of HFT-related brittle-ductile deformation at moderate temperature and pressure. The sandy unit from the Quaternary sediments yields 10 ± 0.8 ka, 11.8 ± 0.7 ka, 17 ± 1 ka, and 56 ± 3 ka quartz OSL dates, demonstrating the migration of hanging wall rocks on footwall post-Siwalik fluvial deposits along the HFT plane and reactivation of the HFT in the study area.

Real-Time Study of Surface-Guided Nanowire Growth by In Situ Scanning Electron Microscopy.

Surface-guided growth has proven to be an efficient approach for the production of nanowire arrays with controlled orientations and their large-scale integration into electronic and optoelectronic devices. Much has been learned about the different mechanisms of guided nanowire growth by epitaxy, graphoepitaxy, and artificial epitaxy. A model describing the kinetics of surface-guided nanowire growth has been recently reported. Yet, many aspects of the surface-guided growth process remain unclear due to a lack of its observation in real time. Here we observe how surface-guided nanowires grow in real time by in situ scanning electron microscopy (SEM). Movies of ZnSe surface-guided nanowires growing on periodically faceted substrates of annealed M-plane sapphire clearly show how the nanowires elongate along the substrate nanogrooves while pushing the catalytic Au nanodroplet forward at the tip of the nanowire. The movies reveal the timing between competing processes, such as planar vs nonplanar growth, catalyst-selective vapor-liquid-solid elongation vs nonselective vapor-solid thickening, and the effect of topographic discontinuities of the substrate on the growth direction, leading to the formation of kinks and loops. Contrary to some observations for nonplanar nanowire growth, planar nanowires are shown to elongate at a constant rate and not by jumps. A decrease in precursor concentration as it is consumed after long reaction time causes the nanowires to shrink back instead of growing, thus indicating that the process is reversible and takes place near equilibrium. This real-time study of surface-guided growth, enabled by in situ SEM, enables a better understanding of the formation of nanostructures on surfaces.

Spatial and temporal patterns of soil salinization in shallow groundwater environments of the Bahía Blanca estuary: Influence of topography and land use

AbstractSoil salinization is considered an environmental threat as high salt content hinders plant growth. Soil salinity is usually associated with climate aridity at a regional scale, while topography, subsurface hydrology, and land use combine with climate to produce salinization patterns at landscape scale. In this study, the first such assessment for the study area, we investigated the relationship between climate, groundwater level (GWL) changes, and soil salinity. For comparison, two adjacent vegetation units used as marginal grazing lands were selected: thickets of Allenrolfea patagonica (Ap thicket), located at the base of a scarp, and bushes of Cyclolepis genistoides (Cg bush), located away from the topographic discontinuity. Electrical conductivity (ECrs) maps derived from LANDSAT‐8/OLI were used to analyze salinity patterns. The results showed that GWL rose in coincidence with precipitation events and fell during dryer and warmer periods. GWL was always closer to the surface in the Ap thicket, and level changes were more likely to occur there. ECrs showed a seasonal pattern with significantly higher (p < 0.05) values in the Ap thicket during summer. Within the Cg bush, relatively higher ECrs appeared around a permanent water trough. Within this zone, ECrs values during summer were significantly higher (p < 0.05). Coupled evaluation of GWL dynamics and soil salinization patterns showed that GWL changes interact with land use practices to produce local‐scale patterns of salinization. Topography played an important role in controlling soil surface properties through its influence on groundwater dynamics and small topographic changes produced largely different conditions for vegetation growth.

Experimental investigations on complex block propagation for the assessment of propagation models quality

Rockfall propagation models are routinely used for the quantitative assessment of rockfall hazard. Their capacities and limitations remain difficult to assess due to the limited amount of exhaustive experimental data at the slope scale. This article presents experiments of block propagation performed in a quarry located in Authume (France). This study site was chosen for its complexity, related with the presence of topographical discontinuities and of configurations promoting block rolling. A total of more than one hundred blocks were released on two propagation paths. The propagation of the blocks was assessed by measuring the block stopping points as well as their kinematics at specific locations of the paths, called evaluation screens. Significant variability of the stopping points and of the block kinematics at the evaluation screens was observed and preferential transit and deposit zones were highlighted. The analysis of the results showed predominant effect of topography, in particular that related to topographical discontinuities. Significant influence of local and small scale parameters (e.g. block orientation, local topography) was also highlighted. These conclusions are of particular interest for researchers or practitioners who would like to assess the relevance of propagation modelling tools considering this complex study site. In this configuration, the quality of block propagation simulations should notably rely on the accuracy of digital terrain models, and on the integration of local conditions effects using physically based approaches.

Deciphering the known unknowns in the behavioural ecology of the intertidal gastropod Littorina littorea

A considerable amount of attention has been devoted to understand the role of chemoreception in the behavioural ecology of gastropods. There is still, however, a limited amount of information related to their sensory ecology and their subsequent ability to navigate through topographically complex landscapes, such as intertidal rocky shores, despite the documented role of substrate spatial structure in the movement rates and directionality of gastropods. Responses to gravity and visual cues have been documented in a range of intertidal gastropods, but often led to contradictory conclusions. We examined the response of the gastropod Littorina littorea, a keystone grazer on most temperate intertidal ecosystems of the northern hemisphere, to substrate slope, topographic discontinuities and light using purpose-designed experimental set-ups under laboratory-controlled conditions. In contrast to previous results, we showed that L. littorea did not exhibit a strict geotactic behaviour. Instead, they were predominantly thigmotactic following an encounter with a topographic discontinuity, and we suggest that the observed seemingly negative geotactic responses are only a by-product of thigmotaxis, i.e. a thigmotactically-induced negative geotactic response. L. littorea also showed a strong preference for dark areas and shapes. Taken together, our results suggest that thigmotaxis and scototaxis are evolutionary advantageous to navigate through topographically complex and thermally stressful environments, and that the behavioural repertoire of gastropod may be much wider than previously thought.

Transition of the Tsushima Warm Current Path Observed over Toyama Trough, Japan

AbstractMooring, CTD, and ADCP observations were made in 2012 in and around the Toyama Trough (TT) cutting across a continental shelf along the Japanese coast of the Japan Sea between Noto Peninsula (NP) and Sado Island (SI) to investigate spatiotemporal characteristics of path transition of the coastal branch of the Tsushima Warm Current (CBTWC). Around SI, downstream of the TT boundary, a wavelike alongshore current perturbation, accompanied by sea level rise, was observed. This perturbation occurred after the seasonal amplification of the CBTWC around the NP on the upstream boundary of the TT. This process was delineated by the results of numerical experiments performed with a two-layer model using idealized topography. The model showed that a current path of the CBTWC shifted from alongshore mode to offshore mode bridged over the TT in association with the lee eddy development behind the NP toward the SI over the TT. This lee eddy is generated by positive vorticity induced over topographic discontinuity between the continental shelf off the northern coast of the NP and deeper region of the TT. The model indicated the period of eddy formation is 60–90 days if the volume transport is 1 Sv (1 Sv ≡ 106 m3 s−1), whereas the observations showed the formation period was only 47 days at 1.2 Sv of volume transport. To explain this discrepancy, temporal variation of the CBTWC, vortex supply from preexisting eddies, or eddies caused by the scattering of coastal-trapped waves were suggested as new processes that accelerate the growth rate of the lee eddy.

反射法地震探査および重力探査から明らかになった仙台平野南部の伏在活断層

The southern part of the Sendai Plain is located between the Futaba active fault zone and the Nagamachi-Rifu active fault zone. In the existing research, only 6.5km long of the active fault trace was estimated along the eastern foot of Medeshima Hills because the geomorphic surface of the Medeshima Hills is warping toward east. Besides the above, no other active fault trace had been clearly identified in this area. To reveal the subsurface structure, we conducted 5.3-km-long of seismic reflection survey across the southern part of the Sendai Plain, near the mouth of the Abukuma river, which is the northern extension of Futaba active fault zone. The result of seismic profile analyzed with a standard CMP method shows the existence of concealed active fault beneath the Sendai Plain and 30-40m of dislocations in seismic reflectors from Miocene to Quaternary layer. The activity of the concealed fault is estimated as a C-class (Long-term rate of faulting, 0.01-0.1mm/yr). The survey of relative gravity measurement at intervals of 200 m was also conducted, including the seismic reflection survey line. Steep gradient of the Bouguer gravity anomaly were observed in the vicinity of the concealed active fault that is revealed from the seismic reflection survey. Analysis with the 2-D density structure analysis demonstrates that steep gradient of Bouguer gravity anomaly was responsible for large deformation of pre-Miocene basement rocks by repeated faulting of the concealed active fault. In addition, to discuss a tectonic evolution of the Sendai Plain, we made topographic profiles, using 1m-DEM and 2m-DEM data across the plain. Topographies of these profiles show that discontinuity of 1.5-2.2m on both sides of the westernmost beach ridge in the southern part of the Sendai Plain. Active faulting of the concealed fault has possibly contributed to these topographic discontinuities.

Cloud-based Lineament Extraction of Topographic Lineaments from NASA Shuttle Radar Topography Mission Data

This paper presents initial results of a JavaTM based, feature extraction tool, which represents a standard implementation of a hill-shading algorithm that transforms a 2D image to pseudo 3D image to enhance edge contrast in combination with an edge detection Canny algorithm that performs segmentation to produce multidirectional sun-shaded images and their edges. Our goal is to firstly automate this processes in JavaTM to obtain multidirectional optimization of edge discovery and secondly scale this algorithm to the complete SRTM raster collection at multiple pixel resolutions to document the distribution of Earth topographic discontinuities from continental to regional and local scales, respectively on the order of 1000s, 100s and 10s of kilometers. This tool will support the automatic extraction of lineaments of the transformed images to predict the existence of linear features that can be often found in association with ore deposits and landslides, if they represent tectonic lineaments. The collection of processed big data, represents a multi-scale data repository that may find use for these and other geological and environmental applications. We present preliminary outputs from a case study conducted in the Flin-Flon greenstone belt in Canada, which is well known for its base-metal endowment. In this study, two main shaded relief images with multidirectional illumination were created in Java each with four azimuth angles of the light sources and from which our developed tool extracts automatically multiple lineaments. The extracted lineaments represent both positive and negative elevation breaks, due to sudden slope inversions identifying dominantly crest lines and valleys. Preliminary results show good agreement with drainage networks, mapped fault lines and orientations of structures measured in the field. The main trends of the extracted lineaments of both images are NW-SE, N-S, E-W and NE-SW.

Geophysical characteristics of Aswa shear, Nagasongola discontinuity and ring dyke complex in Uganda

During the years 2008–2012, the geology of most of Uganda was studied within the framework of the Sustainable Management of Mineral Resources Project (SMMRP). During the project, comprehensive airborne magnetic and radiometric surveys were flown over the entire country and geological, petrophysical, geochemical sampling, geological field studies and detailed geophysical field profiles were undertaken in selected sub-areas. This report concentrates on the geophysical properties of three major geophysical structures in the area considered during the project: the Pan-African (0.6–0.7Ga) Aswa shear zone and Nagasongola discontinuity (suture), and the 1.36Ga Uganda–Tanzania semi-circular ‘ring dyke’ complex.The geophysical profiles across the Aswa shear indicate that the fault zone dips steeply, at about 60° to NE. The structure represents a magnetic, gravimetric (density), radiometric and topographic discontinuity, all diminishing from SW to NE across the zone. The zone is also characterized by complex radiometric anomalies. A schematic reconstruction of the evolution of the Aswa shear zone on the magnetic map suggests a nearly 60km sinistral horizontal component of displacement along the zone. The Nakasongola zone is another distinct magnetic, gravimetric and radiometric discontinuity, interpreted to represent a collision (suture) zone, where the northern, low-magnetic block has been thrust over the southern, denser and more magnetic block. Modeling of gravity and magnetic data are consistent with a geometry in which the southern, magnetic and high-density block dips gently to great depth beneath the northern block. Bedrock exposures in both the Aswa shear zone and Nagasongola zone areas indicate a very protracted and complex history of tectonic processes commencing in the Archaean–Paleoproterozoic era and culminating in Pan-African orogenies. Both, the Aswa shear zone and Nagasongola discontinuity are cut by continuous younger dykes that show no signs of disruption, indicating that these deformation zones are presently relatively inactive. Thus, the dating of these dykes can constrain estimates of minimum activation ages of the structures that they transect.The semicircular ring dyke complex in SW Uganda, which continues southwards across Lake Victoria to NW Tanzania, is most clearly visible in magnetic anomaly maps as curved anomalies with a diameter of about 450–650km. Magnetic profile interpretations across the dykes in Tanzania and Uganda indicate that the dykes dip at about 30–40° towards the geometric center of the concentric rings, implying a cone shaped geometry for the dykes in three dimensions. Minimal offsets in anomalies cut by dykes indicate that the dykes were not intruded within fault zones associated with major dislocations, rather that they were emplaced within extensional fractures due to uplift above a mantle plume below the conical dyke complex.The present study, based on the results of the SMMRP, gives valuable, but still preliminary new information concerning large scale tectonic processes operating in the bedrock of Uganda. The structures described here can all be potentially related to hydrothermal events and enrichment of elements of economic interest.

Grain Boundary Responses to Heterogeneous Deformation in Tantalum Polycrystals

The evolution of heterogeneous deformation in a tantalum polycrystal was examined during a three-point bending experiment using electron backscatter pattern mapping. Slip bands formed at strains as low as 1%, and they became more intense with strain. Heterogeneous deformation was evident as intragranular orientation gradients as large as 30° were observed after a strain of about 8%. Nonmonotonic changes in the local average misorientation distribution were observed, implying that dislocation substructure developed in a complex manner. Slip bands were analyzed using plane traces computed from local orientation information. With the assumption of uniaxial stress, Schmid factors for favorable slip systems were identified for each grain and compared with observations, showing evidence for macroscopic activity on both {110} and {112} slip systems. Reconstructed boundary data were used to estimate the geometric potential for slip transfer at grain boundaries. The correlations indicated that when active slip systems were favorably oriented for slip transfer across the boundary, it was often observed in the form of continuous slip bands aligned across the boundary. In boundaries where geometrical alignment and Schmid factors were not favorable for slip transfer, there was a higher likelihood to form ledges (topographic discontinuities) along the grain boundaries. Dislocation pileups at grain boundaries were also correlated with a low potential for slip transfer.

Microbial communities in the respiratory tract of patients with interstitial lung disease

BackgroundMolecular methods based on phylogenetic differences in the 16S rRNA gene are able to characterise the microbiota of the respiratory tract in health and disease.ObjectivesOur goals were (1) to characterise...

Terrain representation impact on periurban catchment morphological properties

Modelling the hydrological behaviour of suburban catchments requires an estimation of environmental features, including land use and hydrographic networks. Suburban areas display a highly heterogeneous composition and encompass many anthropogenic elements that affect water flow paths, such as ditches, sewers, culverts and embankments. The geographical data available, either raster or vector data, may be of various origins and resolutions. Urban databases often offer very detailed data for sewer networks and 3D streets, yet the data covering rural zones may be coarser. This study is intended to highlight the sensitivity of geographical data as well as the data discretisation method used on the essential features of a periurban catchment, i.e. the catchment border and the drainage network. Three methods are implemented for this purpose. The first is the DEM (for digital elevation model) treatment method, which has traditionally been applied in the field of catchment hydrology. The second is based on urban database analysis and focuses on vector data, i.e. polygons and segments. The third method is a TIN (or triangular irregular network), which provides a consistent description of flow directions from an accurate representation of slope. It is assumed herein that the width function is representative of the catchment’s hydrological response. The periurban Chezine catchment, located within the Nantes metropolitan area in western France, serves as the case study. The determination of both the main morphological features and the hydrological response of a suburban catchment varies significantly according to the discretization method employed, especially on upstream rural areas. Vector- and TIN-based methods allow representing the higher drainage density of urban areas, and consequently reveal the impact of these areas on the width function, since the DEM method fails. TINs seem to be more appropriate to take streets into account, because it allows a finer representation of topographical discontinuities. These results may help future developments of distributed hydrological models on periurban areas.

Longitudinal Dispersion in River Flows Characterized by Random Large-Scale Bed Irregularities: First-Order Analytical Solution

AbstractA stochastic Lagrangian approach is proposed for the analytical derivation of a longitudinal dispersion coefficient that accounts for both transverse and longitudinal flow field variability in straight-axis open channels characterized by longitudinal large-scale bed heterogeneity, that is, by longitudinal bed irregularities over a wide range of representative lengths, from the simple grain roughness to large undulations (up to order of kilometers), possibly related to topographical discontinuities or to extended and inhomogeneous depositional processes carried out by natural or anthropogenic agents. The resulting dimensionless expression, involving the average Chezy coefficient and the ratios of river width and longitudinal heterogeneity correlation length to the average flow depth, is obtained at the first order in the depth fluctuations as a time-dependent function given by the sum of three distinct components. The first main component is related to the transverse velocity distribution and would...

The geomorphological features and continuity of the Kyushu-Palau Ridge (KPR)

The Philippine Basin, surrounded by a series of oceanic trenches, is an independent deep ocean basin in the West Pacific Ocean. Its middle part is divided into three marginal sea sub-basins by the Kyushu-Palau and West Mariana Ridges, namely, the West Philippine Basin, the Shikoku and Parece Vela Basins and the Mariana Trough. This paper, through the analysis of the geomorphologic features and gravity and magnetic characteristics of the basin and identification of striped magnetic anomalies, suggests that the entire Philippine Basin developed magnetic lineation of oceanic nature, and therefore, the entire basin is of the nature of oceanic crust. The basin has developed a series of special geomorphic units with different shapes. The KPR runs through the entire Philippine Basin. From the view of geomorphologic features, the KPR is a discontinuous seamount chain (chain-shaped seamounts) and subduction beneath the Japanese Island arc at the Nankai Trough which is the natural boundary between the basin and the Japanese Island arc. At the positions of 25°N, 24°N, 23°N and 18°N, obvious discontinuity is shown, which belongs to natural topographic discontinuity. Therefore, the KPR is topographically discontinuous.

Exhumational variability within the Himalaya of northwest India

In the Himalaya of Chamba, NW India, a major orographic barrier in front of the Greater Himalayan Range extracts a high proportion of the monsoonal rainfall along its southern slopes and effectively shields the orogen interior from moisture-bearing winds. Along a ~ 100-km-long orogen perpendicular transect, 28 new apatite fission track (AFT) and 30 new zircon (U–Th)/He (ZHe) cooling ages reveal marked variations in age distributions and long-term exhumation rates between the humid frontal range and the semi-arid orogen interior. On the southern topographic front, very young, elevation-invariant AFT ages of < 4 Ma have been obtained that are concentrated in a ~ 30-km-wide zone; 1-D-thermal modeling suggests a Plio–Pleistocene mean erosion rate of 0.8–1.9 mm yr − 1 . In contrast, AFT and ZHe ages within the orogen interior are older (4–9 and 7–18 Ma, respectively), are positively correlated with sample elevation, and yield lower mean erosion rates (0.3–0.9 mm yr − 1 ). Protracted low exhumation rates within the orogen interior over the last ~ 15 Myr prevailed contemporaneously with overall humid conditions and an effective erosional regime within the southern Himalaya. This suggests that the frontal Dhauladar Range was sufficiently high during this time to form an orographic barrier, focusing climatically enhanced erosional processes and tectonic deformation there. Thrusting along the two frontal range-bounding thrust, the Main Central Thrust and the Main Boundary Thrusts, was initiated at least ~ 15 Ma ago and has remained localized since then. The lack of evidence for localized uplift farther north indicates either a rather flat décollement with no ramp or the absence of active duplex systems beneath the interior of Chamba. Exhumational variability within Chamba is best explained as the result of continuous thrusting along a major basal décollement, with a flat beneath the slowly exhuming internal compartments and a steep frontal ramp at the rapidly exhuming frontal range. The pattern in Chamba contrasts with what is observed elsewhere along the Himalaya, where exhumation is focused in a zone ~ 150 km north of the orogenic front. In the NW Himalaya, preserved High Himalayan Crystalline nappes and Lesser Himalayan windows alternate on a relatively small scale of < 100 km; these alternations are closely correlated with the pattern of exhumation. Although the spatial distribution of high-exhumation zones varies considerably between individual Himalayan sectors, all of these zones are closely correlated with locally higher rock-uplift rates, sharp topographic discontinuities, and focused orographic precipitation, suggesting strong feedbacks between tectonically driven rock uplift, orographically enhanced precipitation, and erosional processes.

Modeling edge effects of tillage erosion

Tillage erosion has been recognized as an important factor in redistribution of soil over time and in the development of morphological changes within agricultural fields. Field borders, fences, and vegetated strips that interrupt soil fluxes lead to the creation of topographic discontinuities or lynchets. When tillage tools that preferentially throw soil to one side are used repeatedly to move soil in one direction, rather than alternating with each pass, they create berms at the receiving side of the tilled domain and a “dead furrow” or channel at the contributing side. However, even tillage implements that are symmetrical in throwing soil equally in both lateral directions on flat surfaces may throw some soil beyond the implement width and so contribute to soil berms formation just beyond the tilled zone that can affect water flow paths. We developed a two-dimensional Tillage Erosion and Landscape Evolution Model that allows complex internal boundaries to be defined within the simulation domain. In this paper we develop and demonstrate techniques and tools to allow prediction of the formation of edge-of-field berms by defining alternative boundary conditions. The derivation and assumptions of the model are presented and then it is applied and compared to survey results from two field studies: one an experimental field in Coffeeville, Mississippi, where grass hedges were planted close to field elevation contours to evaluate their effectiveness as an erosion control measure and were monitored over a 16-year period; and the other a set of 0.1ha plots located near Holly Springs, MS where the effect of edge-of-field berm formation on runoff partitioning was evaluated during an 8-year study. Results demonstrate the ability of the model to correctly reproduce the location and magnitude of soil loss and accumulation. At Coffeeville, erosion averaging over 20cm in the downslope side of each grass hedge and deposition taking place near the slope ends led to the formation of lynchets up to 0.8m high, and the average slope steepness in the cropped areas between hedges decreased from an average of 7.2% in 1993 to about 3.7% in 2009. In Holly Springs, repeated tillage conducted next to grass hedges planted along the hillslope bottoms led to the formation of berms with average height of 13cm, which may significantly alter field-scale hydrological, erosion, and sediment transport processes.

WHEN GAPS REALLY ARE GAPS: STATISTICAL PHYLOGEOGRAPHY OF HYDROTHERMAL VENT INVERTEBRATES

The invertebrate animals endemic to deep-sea hydrothermal vents are distributed intermittently along relatively linear oceanic ridge axes. A one-dimensional stepping-stone model, therefore, provides a reasonable starting hypothesis of population structure for these species. Nevertheless, population genetic studies of many species from eastern Pacific vents did not detect the expected signatures of isolation-by-distance (IBD). Instead, distinct patterns of geographical subdivision have been attributed to the unique dispersal modes of individual species, topographical discontinuities of the ridge axes, nonequilibrium metapopulation scenarios and cryptic species. Here, we reexamined these inferences in light of expectations generated by computer simulations of a one-dimensional stepping-stone model. We evaluated whether the previously inferred subdivisions are statistically robust to an alternative explanation that continuous stepping-stone migration has occurred along the ridge axes but discontinuities in the sampling design (gaps) have generated the apparent disjunctions. We found that previous inferences about barriers to gene flow (vicariance) were supported in many cases, but that failures to detect evidence for IBD could be explained by low statistical power associated with the sampling effort. The simulation approaches presented here might be useful for testing the significance of inferred phylogeographic gaps in other species.

Changes in Fire Severity across Gradients of Climate, Fire Size, and Topography: A Landscape Ecological Perspective

Fire events contribute to landscape pattern at multiple spatial and temporal scales, and a landscape perspective can provide relevant information for assessing how a changing fire regime will influence pattern-process dynamics. We investigated how the amount and spatial arrangement of high-severity patches varied among 20 fires that occurred in Arizona and New Mexico, USA, across gradients in fire size and climate. The climate gradient was based on the Multivariate El Niño Southern Oscillation Index. In the southwestern US, cool dry conditions are prevalent in La Niña at one extreme, and at the other end, warm wet conditions occur in El Niño. Fires were generally larger in La Niña climates; however, several fires deviated from this trend. Moreover, some spatial properties of severity did not correspond to fire size or to changes in climate. Within the nine largest fires, we analyzed the influence of landscape characteristics defined by topographic discontinuity on spatial patterns of fire severity. Characteristics of topography altered spatial patterns of severity at some places within all the nine fires, and we identified thresholds where small changes in terrain resulted in dramatic effects on severity. From a management perspective, consideration of the spatial configuration of severely burned areas can help qualify judgments based on fire size or severity composition alone by linking pattern to ecological processes of interest. Furthermore, examination of individual fire events could lead to identification of climate contexts in which fires can meet management objectives for sustainable forests.