Search

Precipitation channelled down tree stems (stemflow) or into drip points of 'throughfall' beneath trees results in spatially concentrated inputs of water and chemicals to the ground. Currently, these flows are poorly characterised due to uncertainties about which branches redirect rainfall to stemflow or throughfall drip points.We introduce a graph theoretic algorithm that 'prunes' quantitative structural models of trees (derived from terrestrial LiDAR) to identify branches contributing to stemflow and those contributing to throughfall drip points. To demonstrate the method's utility, we analysed two trees with similar canopy sizes but contrasting canopy architecture and rainfall partitioning behaviours.For both trees, the branch 'watershed' area contributing to stemflow (under conditions assumed to represent moderate precipitation intensity) was found to be only half of the total ground area covered by the canopy. The study also revealed significant variations between trees in the number and median contribution areas of modelled throughfall drip points (69 vs. 94 drip points tree−1, with contributing projected areas of 28.6 vs. 7.8 m2 tree−1, respectively). Branch diameter, surface area, volumes and woody area index of components contributing to stemflow and throughfall drip points may play a role in the trees' differing rainfall partitioning behaviours.Our pruning algorithm, enabled by the proliferation of LiDAR observations of canopy structure, promises to enhance studies of canopy hydrology. It offers a novel approach to refine our understanding of how trees interact with rainfall, thereby broadening the utility of existing LiDAR data in environmental research. [ABSTRACT FROM AUTHOR]

For one‐third of Earth's land surface, precipitation passes through tree canopies (as throughfall or stemflow) before entering watersheds. Over a century of research has described fluxes of water and solutes along these "hydrologic highways", yet little is known about their "traffic" – that is, the organisms and nonliving particulates frequently discarded from water samples after filtration in the lab. A comprehensive understanding of the composition of sub‐canopy precipitation is necessary to estimate the total nutrient and pollutant inputs to watersheds for redistribution downstream, as well as to systematically investigate precipitation effects on organismal exchanges along the atmosphere–plant–soil continuum. Here, we review current concepts and research showing that the hydrologic highways from tree canopies to soil carry ecologically relevant quantities of biologic (viruses, microbes, microfauna, and meiofauna) and abiotic particulates. Their fate may have important consequences for the biogeochemistry and biodiversity of terrestrial systems. [ABSTRACT FROM AUTHOR]

Reduced sediment supply and rising sea levels are driving land submergence on deltas worldwide, motivating engineering practices that divert water and sediment to sustain coastal landforms. However, lobe response following channel abandonment by diversions has not been constrained by field‐scale studies. Herein, avulsion and engineered diversion scenarios are explored for the Huanghe delta (China), where three lobes were abandoned in the last 40 yr. Two lobes were completely cut off by diversions, and one naturally by an avulsion. Shoreline retreat rates are strikingly different: ∼400 m/yr for diverted lobes and ∼90 m/yr for avulsed lobe. We hypothesize that this variability is linked to vegetal cover across lobes, and therefore the capacity to buffer hydrodynamic reworking of shoreface sediment. Furthermore, the vegetal cover is related to lobe salinity and elevation, which vary by abandonment style. We offer this as a case study to inform about the efficacy of future delta diversions. Plain Language Summary: Sediment and water diversions are an important tool to combat land loss of deltaic coastlines. However, nourishment of drowned coastal land typically requires diverting water and sediment away from another region of the coast, potentially resulting in additional land loss. This study finds that the stability of shorelines facing reduced sediment supply is dependent on elevation before sediment loss, freshwater supply, and vegetation coverage. These findings have implications for future sediment and water diversions, by informing strategies that optimize land preservation. Key Points: Coastline retreat following channel abandonment is assessed for engineered diversions and natural avulsions on the Huanghe delta (China)Antecedent channel topography, riverine water input, and vegetation vary depending on abandonment style, and impact delta lobe stabilityBy mimicking conditions for natural channel avulsions, lobes abandoned by engineering practices may be made resilient to shoreline retreat [ABSTRACT FROM AUTHOR]

Fifteen transects of sediment cores located off the central Texas coast between Matagorda Peninsula and North Padre Island were investigated to examine the offshore record of Holocene evolution of the central Texas coast. The transects extend from near the modern shoreline to beyond the toe of the shoreface. Lithology, grain size and fossil content were used to identify upper shoreface, lower shoreface, ebb‐tidal delta and marine mud lithofacies. Interpretations of these core transects show a general stratigraphic pattern across the study area that indicates three major episodes of shoreface displacement. First, there was an episode of shoreface progradation that extended up to 5 km seaward. Second, an episode of landward shoreline displacement is indicated by 3–4 km of marine mud onlap. Third, the marine muds are overlain by shoreface sands, which indicates another episode of shoreface progradation of up to 5 km seaward. Radiocarbon ages constrain the onset of the first episode of progradation to ca 6.5 ka, ending at ca 5.0 ka when the rate of sea‐level rise slowed from an average rate of 1.6–0.5 mm/yr. Results from sediment budget calculations and sediment transport modelling based on reasonable estimates of an ancient shoreline shape and wave climate indicate that the first progradation was a result of sand supplied from erosion of the offshore Colorado and Rio Grande deltas. The transgressive phase occurred between ca 4.9 ka and ca 1.6 ka and coincided with a major expansion of the Texas Mud Blanket, which resulted in burial of offshore sand sources and the shoreface being inundated with mud. The second, more recent episode of shoreface progradation began ca 500 years ago with a maximum rate of ca 6 m/yr. This most recent change signals a healing phase of coastal evolution from the late Holocene transgressive event. Currently, the shoreline along the central Texas coast is retreating landward at an average rate of 0.30 m/yr, indicating that the second progradation event has ended. [ABSTRACT FROM AUTHOR]

Many published interpretations of ancient fluvial systems have relied on observations of extensive outcrops of thick successions. This paper, in contrast, demonstrates that a regional understanding of palaeoriver kinematics, depositional setting and sedimentation rates can be interpreted from local sedimentological measurements of bedform and barform strata. Dune and bar strata, channel planform geometry and bed topography are measured within exhumed fluvial strata exposed as ridges in the Ruby Ranch Member of the Cretaceous Cedar Mountain Formation, Utah, USA. The ridges are composed of lithified stacked channel belts, representing at least five or six re‐occupations of a single‐strand channel. Lateral sections reveal well‐preserved barforms constructed of subaqueous dune cross‐sets. The topography of palaeobarforms is preserved along the top surface of the outcrops. Comparisons of the channel‐belt centreline to local palaeotransport directions indicate that channel planform geometry was preserved through the re‐occupations, rather than being obscured by lateral migration. Rapid avulsions preserved the state of the active channel bed and its individual bars at the time of abandonment. Inferred minimum sedimentation durations for the preserved elements, inferred from cross‐set thickness distributions and assumed bedform migration rates, vary within a belt from one to ten days. Using only these local sedimentological measurements, the depositional setting is interpreted as a fluvial megafan, given the similarity in river kinematics. This paper provides a systematic methodology for the future synthesis of vertical and planview data, including the drone‐equipped 2020 Mars Rover mission, to exhumed fluvial and deltaic strata. [ABSTRACT FROM AUTHOR]

The distribution of cross‐set thicknesses is important data for reconstructing ancient aeolian dune fields from the strata they accumulated, but most aeolian strata on Mars must be observed from satellite. We hypothesize that remote sensing resolution limits will affect cross‐set thickness measurements and the dune‐field reconstructions that follow. Here we test this hypothesis using a numerical experiment mimicking the effects of satellite image resolution limits performed on a distribution of aeolian cross‐set thicknesses measured in the field from the Jurassic Page Sandstone, Arizona, USA. Page set thicknesses are exponentially distributed, representing the accumulations of dry dune fields (no water table interactions with the dunes) in a state of net‐sediment bypass. When observed from satellite, set‐thickness measurements increase as adjacent sets become indistinguishable, based on the map‐view distance between their upper and lower bounding surfaces. This is termed the exposure distance of a cross set and is a function of (1) the set thickness, (2) the dip of the outcrop surface, and (3) the number of satellite image pixels required to detect a set (detection limit). By running experiments using outcrop dips from 1° to 60° and detection limits from 0.75 to 2.50 m (3 to 10 High‐Resolution Imaging Science Experiment pixels), we find that gently sloping surfaces (< 13°) at all detection limits are associated with the least blending of adjacent sets, conserving the net‐bypass interpretation made from the true set thicknesses. Although these results are specific to the Page, they can be used as a guide for future Mars work. Plain Language Summary: Cross sets are sedimentary deposits left by wind‐blown sand dunes. Cross sets can be preserved for long amounts of time as sedimentary rocks, where variability in their thickness can be analyzed to understand the motion of ancient dunes and the processes that helped the dunes move, including ancient winds, the presence or lack of near‐surface groundwater, ancient topography, and tectonics. On Earth, we can accurately measure cross‐set thicknesses in the field, but for Mars we are mostly limited to satellite images. Here we mimic the uncertainties present in a satellite image of Mars by altering field‐measured thicknesses from Earth. We perform this numerical experiment on field‐measured cross‐set thicknesses from the Page Sandstone, Arizona. The altered thicknesses are checked for how they might affect our understanding of the ancient Page dune fields. Based on these experiments, satellite images offer a risk of misinterpretation, but good measurements can be made at High Resolution Imaging Science Experiment image resolution if the slope of the rock outcrop is shallow, 13° from horizontal or less. At these shallow slopes, the thinnest cross sets are exposed over long distances, making them detectable from orbit. Though these results are specific to the Page, they can help guide future Mars work. Key Points: The perceived thicknesses of aeolian cross sets are increased (and their distributions altered) due to remote sensing resolution limitsImage resolution limits can severely alter interpretations of aeolian sandstones, even at HiRISE resolution (best available at 25 cm/pixel)Accurate measurements from HiRISE images require outcrop slopes less than 13°, such that thin sets are exposed over detectable distances [ABSTRACT FROM AUTHOR]

The formative conditions for bedform spurs and their roles in bedform dynamics and associated sediment transport are described herein. Bedform spurs are formed by helical vortices that trail from the lee surface of oblique segments of bedform crest lines. Trailing helical vortices quickly route sediment away from the lee surface of their parent bedform, scouring troughs and placing this bed material into the body of the spur. The geometric configuration of bedform spurs to their parent bedform crests is predicted by a cross-stream Strouhal number. When present, spur-bearing bedforms and their associated trailing helical wakes exert tremendous control on bedform morphology by routing enhanced sediment transport between adjacent bedforms. Field measurements collected at the North Loup River, Nebraska, and flume experiments described in previous studies demonstrate that this trailing helical vortex-mediated sediment transport is a mechanism for bedform deformation, interactions and transitions between two-dimensional and three-dimensional bedforms. [ABSTRACT FROM AUTHOR]

Aeolian dune motion is thought to be driven by an annual cycle of sediment-transporting wind events. Each wind event drives uneven motion of dune crestlines, yet dune crestlines align as a trend to an annual cycle of wind. Understanding the variability in dune motion over such a cycle aids the interpretation of aeolian cross-stratification, often available only in the limiting exposure of core and outcrop. Digital elevation models obtained by light detection and ranging are used to estimate dune brink motion and sediment flux along the sinuous crestlines of crescentic dunes at White Sands gypsum dune field (south-central New Mexico, USA) over an annual cycle of wind. In tandem, meteorological observations over the same annual cycle are used to drive a kinematic model of dune crestline motion. Wind-driven kinematic modelling does well to predict the mean and overall variation in sediment flux with compass direction. Digital elevation model-based estimations of brink motion and sediment flux reveal that dune motion and sediment flux very nearly follow a circular normal distribution. Dunes at White Sands were found to achieve steady mean values of lee surface dip direction, brink motion and sediment flux within a sample window the size of approximately six dunes of average crestline length. Due to the symmetrical distribution of dune motion about the average lee surface dip direction, uneven motion of dune crestlines averages to become motion of dune crestlines normal to a trend, as predicted by wind-driven kinematic models. [ABSTRACT FROM AUTHOR]

Hyporheic water is thought to infiltrate at the head of a riffle, which in turn is complemented by upwelling back to the stream at the tail of the riffle in a pool--riffle--pool (PRP) sequence. Heat tracing is a potentially useful method to characterize these hyporheic flow paths and quantify associated fluxes. Temperature was monitored within a PRP sequence for several days. Temperature in the hyporheic zone reflected the diel temperature change in the river but not uniformly. The observed thermal pattern exhibited deeper penetration of thermal oscillations below the head pool and shallower penetration below the tail pool. This pattern is consistent with the conceptual model of hyporheic exchange over a PRP sequence. One-dimensional analytical heat transport models were used at different points below the PRP sequence to estimate distributed vertical fluid fluxes. The calculated fluxes exhibit a trend that follows the expected distribution for a PRP sequence but modified for a losing stream (net fluid flux into the streambed). Deviation of both magnitude and distribution of fluxes from the conceptual "downwelling-to-upwelling" model is due partly to the dominantly losing conditions at the study site, but the trends are consistent with a losing stream undergoing hyporheic exchange. Violation of the assumptions in the analytical models most likely adds error to flux estimates. For this study, flux estimation methods using a temperature time-series amplitude analysis more closely matched field measurements than phase methods. [ABSTRACT FROM AUTHOR]