We present results from monitoring of hydrology and erosion at channel heads in a discontinuous e... more We present results from monitoring of hydrology and erosion at channel heads in a discontinuous ephemeral stream system in southeastern Arizona rangelands. At this field site, alluvial headwalls ~1 m high mark the transition from unchannelized valleys to actively incising arroyos. The headwalls are vertical to overhanging in cross-section and amphitheater-shaped in planform. The local landscape is made up of well- consolidated clay-rich soil and sediment. Possible processes involved in headwall retreat include groundwater sapping, plunge-pool erosion, and soil tension-fracturing. To evaluate the relative efficacy of such processes and to test and refine models of incision and headcut migration rate, we deployed 40+ sensors at and immediately upstream and downstream of channel headwalls. Wired sensors and a datalogger recorded precipitation, overland flow, channel discharge, and soil moisture at a one minute interval. Additionally, a custom wireless sensor network with cellular telemetry was developed and deployed to measure shallow overland flow. At our field site, overland flow can occur from less than one centimeter of precipitation. We tracked landscape erosion through a combination of time-lapse photography, repeat RTK GPS and terrestrial LiDAR surveys. Over the monitoring period, headwalls retreated 10s of cm during several flow events. When the sediment becomes saturated (e.g., from flow down headwall faces and standing water in plunge pools), it has an order-of-magnitude lower shear strength than when dry. Erosion and retreat of channel heads appear to occur by a combination of erosive flow down saturated channel headwalls, plunge-pool erosion at high flows, and mass wasting along vertical tension cracks between runoff events. The amphitheater form is largely due to plunge-pool undercutting and vertical differences in soil material properties, particularly clay content. Though shallow groundwater sapping is widely thought to lead to amphitheater-shaped channel heads, we found no evidence of moisture flux below 40 cm depth directly upstream of the ~1 m channel headwalls. Our results have implications for understanding runoff- erosion dynamics, arroyo cutting, rangeland soil erosion, and may also provide insight into the formation of larger amphitheater-headed channels and channel initiation processes in general.
Stream power-based models of bedrock landscape development are effective at producing synthetic t... more Stream power-based models of bedrock landscape development are effective at producing synthetic topography with realistic fl uvial-network topology and three-dimensional topography, but they are diffi cult to calibrate. This paper examines ways in which fi eld observations, geochronology, and digital elevation model (DEM) data can be used to calibrate a bedrock landscape development model for a specifi c study site. We fi rst show how uplift rate, bedrock erodibility, and landslide threshold slope are related to steady-state relief, hypsometry, and drainage density for a wide range of synthetic topographies produced by a stream power-based planform landscape development model. Our results indicate that low uplift rates and high erodibility result in low-relief, high drainage density, fl uvially dominated topography, and high uplift rates and low erodibility leads to high-relief, low drainage density, mass wasting-dominated topography. Topography made up of a combination of fl uvial channels and threshold slopes occurs for only a relatively narrow range of model parameters.
In landscapes where airborne laser swath mapping (ALSM) exists, terrestrial laser scanning (TLS) ... more In landscapes where airborne laser swath mapping (ALSM) exists, terrestrial laser scanning (TLS) can be used to update high-resolution topographic models for quantification of landscape change. At Mill Gulch in northern California, we scanned an active earthflow using TLS in 2011 that had also been imaged by ALSM in 2003 and 2007. In order to evaluate change at the sub-meter level between the ALSM and TLS data, we generated a custom, 30 cm resolution ALSM digital elevation model (DEM), employed geographic transformations to align the disparate datasets, and refined the vertical alignment using an unaltered road surface. We then conducted vegetation removal from the TLS data, gridded it to 30 cm, and produced detailed maps of topographic evolution. Previous work comparing the 2003 and 2007 ALSM data indicated that this earthflow translated blocks of material as much as 5 m/yr and that significant material was removed by the channel at the toe of the earthflow, leading to a net elevat...
Landscape evolution models are often calibrated using field data that is inferred from current la... more Landscape evolution models are often calibrated using field data that is inferred from current landscape configuration, from relict landforms such as terraces, or from spatially-averaged millennial or longer timescale landscape development rates. Recent approaches that include a combination of repeat terrestrial laser scanning (TLS) and deployment of environmental sensors can provide topographic and environmental data that can both inform many of the parameters of landscape evolution models and be used for comparison to model outputs. We present cm-scale models of landscape change and quantification of environmental forcing (precipitation, hydrology, etc.) from rapidly changing landscapes. These were generated from repeat TLS scans in badlands, severely burned first-order drainage basins, and ephemeral channels. From these maps, we can determine magnitudes and patterns of erosion and deposition, and directly link the observed changes to specific environmental forcing events. These o...
Airborne LiDAR has now been collected along many of the active fault zones in California. These d... more Airborne LiDAR has now been collected along many of the active fault zones in California. These data provide high resolution representation of fault zone topography that permits investigation into temporal and spatial fault zone evolution. They also provide baseline data for detailed post-earthquake (or other event) landscape change detection. Here we describe analysis of complex fault zone topography along Mustang Ridge where the San Andreas Fault transfers slip onto an array of subsidiary faults in a km-wide stepover zone. From the 2007 Earthscope LiDAR, relative rates of subsidiary fault slip can be constrained using scarp morphologic age determination (using a linear diffusion model and extracted profiles), channel profile extraction, two dimensional spectral analysis, and extraction of scarp morphologic ages using new wavelet-like methods. We also report on use of multitemporal LiDAR (2007 Earthscope NoCAL data collected by NCALM, and 2003 NASA/USGS NSAF data) for detailed chan...
Geodesists typically model fault zones as a single failure plane that undergoes either repeated r... more Geodesists typically model fault zones as a single failure plane that undergoes either repeated rupture or continuous slip, but in reality, these features are broad, complicated zones consisting of many active and ancient fault strands. Understanding how these zones develop over time may reveal information about the temporal evolution and physical properties of a fault zone. The topography of active fault zones records this recent history of fault development as abrupt, linear breaks in topographic slope, commonly referred to as fault scarps. Since the 1980s, paleoseismologists have used the degree of change in scarp profile form to determine the relative age of different fault scarps in a fault zone, under the supposition that those scarps that are more degraded are older than less degraded scarps. From these analyses, a morphologic age is determined. It conflates the true age of the formation of a scarp with the rates of mass transport by erosive processes. Morphologic measurement...
The study of mass and energy transfer across landscapes has recently evolved to comprehensive con... more The study of mass and energy transfer across landscapes has recently evolved to comprehensive considerations acknowledging the role of biota and humans as geomorphic agents, as well as the importance of small-scale landscape features. A contributing and supporting factor to this evolution is the emergence over the last two decades of technologies able to acquire high resolution topography (HRT) (meter and sub-meter resolution) data. Landscape features can now be captured at an appropriately fine spatial resolution at which surface processes operate; this has revolutionized the way we study Earth-surface processes. The wealth of information contained in HRT also presents considerable challenges. For example, selection of the most appropriate type of HRT data for a given application is not trivial. No definitive approach exists for identifying and filtering erroneous or unwanted data, yet inappropriate filtering can create artifacts or eliminate/distort critical features. Estimates of errors and uncertainty are often poorly defined and typically fail to represent the spatial heterogeneity of the dataset, which may introduce bias or error for many analyses. For ease of use, gridded products are typically preferred rather than the more information-rich point cloud representations. Thus many users take advantage of only a fraction of the available data, which has furthermore been subjected to a series of operations often not known or investigated by the user. Lastly, standard HRT analysis work-flows are yet to be established for many popular HRT operations, which has contributed to the limited use of point cloud data. In this review, we identify key research questions relevant to the Earth-surface processes community within the theme of mass and energy transfer across landscapes and offer guidance on how to identify the most appropriate topographic data type for the analysis of interest. We describe the operations commonly performed from raw data to raster products and we identify key considerations and suggest appropriate work-flows for each, pointing to useful resources and available tools. Future research directions should stimulate further development of tools that take advantage of the wealth of information contained in the HRT data and address the present and upcoming research needs such as the ability to filter out unwanted data, compute spatially variable estimates of uncertainty and perform multi-scale analyses. While we focus primarily on HRT applications for mass and energy transfer, we envision this review to be relevant beyond the Earth-surface processes community for a much broader range of applications involving the analysis of HRT.
Variations in erosion and deposition for the last fifty years (based on estimates from 137Cs prof... more Variations in erosion and deposition for the last fifty years (based on estimates from 137Cs profiles) on surfaces (Late Pleistocene to Late Holocene in age) making up the Fortymile Wash alluvial fan south of Yucca Mountain, is a function of surface age and of desert pavement ...
We study whether arroyo channel head retreat in dryland discontinuous ephemeral streams is driven... more We study whether arroyo channel head retreat in dryland discontinuous ephemeral streams is driven by surface runoff, seepage erosion, mass wasting, or some combination of these hydrogeomorphic processes. We monitored precipitation, overland fl ow, soil moisture, and headcut migration over several seasonal cycles at two adjacent rangeland channel heads in southern Arizona. Erosion occurred by headward retreat of vertical to overhanging faces, driven dominantly by surface runoff. No evidence exists for erosion caused by shallow-groundwater-related processes, even though similar theater-headed morphologies are sometimes attributed to seepage erosion by emerging groundwater. At our fi eld site, vertical variation in soil shear strength infl uenced the persistence of the characteristic theater-head form. The dominant processes of erosion included removal of grains and soil aggregates during even very shallow (1-3 cm) overland fl ow events by runoff on vertical to overhanging channel headwalls, plunge-pool erosion during higher-discharge runoff events, immediate postrunoff wet mass wasting, and minor intra-event dry mass wasting on soil tension fractures developing subparallel to the headwall. Multiple stepwise linear regression indicates that the migration rate is most strongly correlated with fl ow duration and total precipitation and is poorly correlated with peak fl ow depth or time-integrated fl ow depth. The studied channel heads migrated upslope with a self-similar morphologic form under a wide range of hydrological conditions, and the most powerful fl ash fl oods were not always responsible for the largest changes in landscape form in this environment.
Evolution of landscape heterogeneity is controlled by coupled Earth system dynamics, and the resu... more Evolution of landscape heterogeneity is controlled by coupled Earth system dynamics, and the resulting process complexity is a major hurdle to cross towards a unified theory of catchment hydrology. The Biosphere 2 Landscape Evolution Observatory (LEO), a 334.5 m 2 artificial hillslope built with homogeneous soil, may have evolved into heterogeneous soil during the first experiment driven by an intense rainfall event. The experiment produced predominantly seepage face water outflow, but also generated overland flow, causing superficial erosion and the formation of a small channel. In this paper, we explore the hypothesis of incipient heterogeneity development in LEO and its effect on overland flow generation by comparing the modeling results from a three-dimensional physically based hydrological model with measurements of total mass change and seepage face flow. Our null hypothesis is that the soil is hydraulically homogeneous, while the alternative hypothesis is that LEO developed downstream heterogeneity from transport of fine sediments driven by saturated subsurface flow. The heterogeneous case is modeled by assigning saturated hydraulic conductivity at the LEO seepage face (K sat,sf ) different from that of the rest (K sat ). A range of values for K sat , K sat,sf , soil porosity, and pore size distribution is used to account for uncertainties in estimating these parameters, resulting in more than 20 000 simulations. It is found that the best runs under the heterogeneous soil hypothesis produce smaller errors than those under the null hypothesis, and that the heterogeneous runs yield a higher probability of best model performance than the homogeneous runs. These results support the alternative hypothesis of localized incipient heterogeneity of the LEO soil, which facilitated generation of overland flow. This modeling study of the first LEO experiment suggests an important role of coupled water and sediment transport processes in the evolution of subsurface heterogeneity and on overland flow generation, highlighting the need of a coupled modeling system that integrates across disciplinary processes.
Subsurface flow and storage dynamics at hillslope scale are difficult to ascertain, often in part... more Subsurface flow and storage dynamics at hillslope scale are difficult to ascertain, often in part due to a lack of sufficient high-resolution measurements and an incomplete understanding of boundary conditions, soil properties, and other environmental aspects. A continuous and extreme rainfall experiment on an artificial hillslope at Biosphere 2's Landscape Evolution Observatory (LEO) resulted in saturation excess overland flow and gully erosion in the convergent hillslope area. An array of 496 soil moisture sensors revealed a two-step saturation process. First, the downward movement of the wetting front brought soils to a relatively constant but still unsaturated moisture content. Second, soils were brought to saturated conditions from below in response to rising water tables. Convergent areas responded faster than upslope areas, due to contributions from lateral subsurface flow driven by the topography of the bottom boundary, which is comparable to impermeable bedrock in natural environments. This led to the formation of a groundwater ridge in the convergent area, triggering saturation excess runoff generation. This unique experiment demonstrates, at very high spatial and temporal resolution, the role of convergence on subsurface storage and flow dynamics. The results bring into question the representation of saturation excess overland flow in conceptual rainfall-runoff models and land-surface models, since flow is gravity-driven in many of these models and upper layers cannot become saturated from below. The results also provide a baseline to study the role of the coevolution of ecological and hydrological processes in determining landscape water dynamics during future experiments in LEO.
The presence of genuinely zero-age or near-zero-age grains in modern-age and very young samples p... more The presence of genuinely zero-age or near-zero-age grains in modern-age and very young samples poses a problem for many existing burial dose estimation procedures used in optical (optically stimulated luminescence, OSL) dating. This difficulty currently necessitates consideration of relatively simplistic and statistically inferior age models. In this study, we investigate the potential for using modified versions of the statistical age models of Galbraith et al. [Galbraith, R.F., Roberts, R.G., Laslett, G.M., Yoshida, H., Olley, J.M., 1999. Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia: Part I, experimental design and statistical models. Archaeometry 41, 339-364.] to provide reliable equivalent dose (D e ) estimates for young and modern-age samples that display negative, zero or near-zero D e estimates. For this purpose, we have revised the original versions of the central and minimum age models, which are based on log-transformed D e values, so that they can be applied to unlogged D e estimates and their associated absolute standard errors. The suitability of these 'un-logged' age models is tested using a series of known-age fluvial samples deposited within two arroyo systems from the American Southwest. The un-logged age models provide accurate burial doses and final OSL ages for roughly three-quarters of the total number of samples considered in this study. Sensitivity tests reveal that the un-logged versions of the central and minimum age models are capable of producing accurate burial dose estimates for modern-age and very young (<350 yr) fluvial samples that contain (i) more than 20% of well-bleached grains in their D e distributions, or (ii) smaller sub-populations of wellbleached grains for which the D e values are known with high precision. Our results indicate that the original (log-transformed) versions of the central and minimum age models are still preferable for most routine dating applications, since these age models are better suited to the statistical properties of typical single-grain and multi-grain single-aliquot D e datasets. However, the unique error properties of modernage samples, combined with the problems of calculating natural logarithms of negative or zero-Gy D e values, mean that the un-logged versions of the central and minimum age models currently offer the most suitable means of deriving accurate burial dose estimates for very young and modern-age samples.
ABSTRACT Rainsplash can be an important factor in the evolution of landscapes, particularly those... more ABSTRACT Rainsplash can be an important factor in the evolution of landscapes, particularly those with low vegetation density and weak lithology or soil mantle. Much of the previous work on the process of rainsplash has been theoretical. However, we used a laser precipitation monitor (LPM) and a terrestrial laser scanner (TLS) at Biosphere 2 to test the applicability of a diffusion model to rainsplash erosion. Topographic smoothing resulting from rainsplash has been modeled by applying the diffusion equation to the topographic evolution of landscapes. Various theoretical models suggest that the diffusivity coefficient, D, in the diffusion equation will increase linearly with rainfall intensity and nonlinearly with the ratio of raindrop diameter to particle size diameter. This study tested these theoretical relationships experimentally by raining on model landscapes made of unconsolidated sediment having a range of particle size distributions ranging from sand to loamy sand. Using a laser precipitation monitor, we characterized raindrop size distribution and intensity from a variety of sprinkler configurations. The raindrops ranged from a mean drop diameter of 0.39 mm to 0.55 mm. A terrestrial laser scanner was used to create topographic models of the landscapes during experimentation. Inverse modeling was then used to test a diffusion model&#39;s ability to predict landscape change due to rainsplash and to determine the diffusivity values corresponding to each set of conditions. The main objective was to quantify how the inferred diffusivity values vary with rainfall intensity, raindrop diameter, and particle size diameter. With the results obtained from this experiment, we hope to provide further data that could be used to refine theoretical models of rainsplash.
Zero-order drainage basins, and their constituent hillslopes, are the fundamental geomorphic unit... more Zero-order drainage basins, and their constituent hillslopes, are the fundamental geomorphic unit comprising much of Earth's uplands. The convergent topography of these landscapes generates spatially variable substrate and moisture content, facilitating biological diversity and influencing how the landscape filters precipitation and sequesters atmospheric carbon dioxide. In light of these significant ecosystem services, refining our understanding of how these functions are affected by landscape evolution, weather variability, and long-term climate change is imperative. In this paper we introduce the Landscape Evolution Observatory (LEO): a largescale controllable infrastructure consisting of three replicated artificial landscapes (each 330 m 2 surface area) within the climate-controlled Biosphere 2 facility in Arizona, USA. At LEO, experimental manipulation of rainfall, air temperature, relative humidity, and wind speed are possible at unprecedented scale. The Landscape Evolution Observatory was designed as a community resource to advance understanding of how topography, physical and chemical properties of soil, and biological communities coevolve, and how this coevolution affects water, carbon, and energy cycles at multiple spatial scales. With well-defined boundary conditions and an extensive network of sensors and samplers, LEO enables an iterative scientific approach that includes numerical model development and virtual experimentation, physical experimentation, data analysis, and model refinement. We plan to engage the broader scientific community through public dissemination of data from LEO, collaborative experimental design, and community-based model development.
1] Vegetation bands are periodic bands of vegetation, separated by interband spaces devoid of veg... more 1] Vegetation bands are periodic bands of vegetation, separated by interband spaces devoid of vegetation, oriented parallel to the topographic contour in some gently sloping arid to semiarid environments. Models of vegetation band formation attribute their formation to positive feedbacks among vegetation density, soil porosity/permeability, and infiltration rates. Here we present an alternative model based on field measurements at our study sites in southern Nevada. In this model, interband spaces between vegetation bands form because topographic mounds beneath vegetation bands detain water upslope from vegetation bands, leading to hydrologic and sedimentologic conditions that inhibit the survival of plants in interband spaces. We used terrestrial laser scanning (TLS) to create high-resolution ($10 cm 2 /pixel) raster data sets of bare-earth topography and canopy height for four study sites. Analyses of the TLS data, in addition to measurements of soil shear strength and particle size, document the potential for detention in interband spaces and a near-inverse proportionality between band spacing and regional slope. We describe a cellular automaton model (herein called model 1) for vegetation band formation that includes just two user-defined parameters and that generates vegetation bands similar to those at our field sites, including the inverse proportionality between spacing and regional slope. A second model (model 2) accurately predicts the width of vegetation bands in terms of the number and spacing of plants and the geometry of individual plant mounds. We also present a GIS-based analysis that predicts where bands occur within a region based on topographic and hydroclimatic controls.
We present results from monitoring of hydrology and erosion at channel heads in a discontinuous e... more We present results from monitoring of hydrology and erosion at channel heads in a discontinuous ephemeral stream system in southeastern Arizona rangelands. At this field site, alluvial headwalls ~1 m high mark the transition from unchannelized valleys to actively incising arroyos. The headwalls are vertical to overhanging in cross-section and amphitheater-shaped in planform. The local landscape is made up of well- consolidated clay-rich soil and sediment. Possible processes involved in headwall retreat include groundwater sapping, plunge-pool erosion, and soil tension-fracturing. To evaluate the relative efficacy of such processes and to test and refine models of incision and headcut migration rate, we deployed 40+ sensors at and immediately upstream and downstream of channel headwalls. Wired sensors and a datalogger recorded precipitation, overland flow, channel discharge, and soil moisture at a one minute interval. Additionally, a custom wireless sensor network with cellular telemetry was developed and deployed to measure shallow overland flow. At our field site, overland flow can occur from less than one centimeter of precipitation. We tracked landscape erosion through a combination of time-lapse photography, repeat RTK GPS and terrestrial LiDAR surveys. Over the monitoring period, headwalls retreated 10s of cm during several flow events. When the sediment becomes saturated (e.g., from flow down headwall faces and standing water in plunge pools), it has an order-of-magnitude lower shear strength than when dry. Erosion and retreat of channel heads appear to occur by a combination of erosive flow down saturated channel headwalls, plunge-pool erosion at high flows, and mass wasting along vertical tension cracks between runoff events. The amphitheater form is largely due to plunge-pool undercutting and vertical differences in soil material properties, particularly clay content. Though shallow groundwater sapping is widely thought to lead to amphitheater-shaped channel heads, we found no evidence of moisture flux below 40 cm depth directly upstream of the ~1 m channel headwalls. Our results have implications for understanding runoff- erosion dynamics, arroyo cutting, rangeland soil erosion, and may also provide insight into the formation of larger amphitheater-headed channels and channel initiation processes in general.
Stream power-based models of bedrock landscape development are effective at producing synthetic t... more Stream power-based models of bedrock landscape development are effective at producing synthetic topography with realistic fl uvial-network topology and three-dimensional topography, but they are diffi cult to calibrate. This paper examines ways in which fi eld observations, geochronology, and digital elevation model (DEM) data can be used to calibrate a bedrock landscape development model for a specifi c study site. We fi rst show how uplift rate, bedrock erodibility, and landslide threshold slope are related to steady-state relief, hypsometry, and drainage density for a wide range of synthetic topographies produced by a stream power-based planform landscape development model. Our results indicate that low uplift rates and high erodibility result in low-relief, high drainage density, fl uvially dominated topography, and high uplift rates and low erodibility leads to high-relief, low drainage density, mass wasting-dominated topography. Topography made up of a combination of fl uvial channels and threshold slopes occurs for only a relatively narrow range of model parameters.
In landscapes where airborne laser swath mapping (ALSM) exists, terrestrial laser scanning (TLS) ... more In landscapes where airborne laser swath mapping (ALSM) exists, terrestrial laser scanning (TLS) can be used to update high-resolution topographic models for quantification of landscape change. At Mill Gulch in northern California, we scanned an active earthflow using TLS in 2011 that had also been imaged by ALSM in 2003 and 2007. In order to evaluate change at the sub-meter level between the ALSM and TLS data, we generated a custom, 30 cm resolution ALSM digital elevation model (DEM), employed geographic transformations to align the disparate datasets, and refined the vertical alignment using an unaltered road surface. We then conducted vegetation removal from the TLS data, gridded it to 30 cm, and produced detailed maps of topographic evolution. Previous work comparing the 2003 and 2007 ALSM data indicated that this earthflow translated blocks of material as much as 5 m/yr and that significant material was removed by the channel at the toe of the earthflow, leading to a net elevat...
Landscape evolution models are often calibrated using field data that is inferred from current la... more Landscape evolution models are often calibrated using field data that is inferred from current landscape configuration, from relict landforms such as terraces, or from spatially-averaged millennial or longer timescale landscape development rates. Recent approaches that include a combination of repeat terrestrial laser scanning (TLS) and deployment of environmental sensors can provide topographic and environmental data that can both inform many of the parameters of landscape evolution models and be used for comparison to model outputs. We present cm-scale models of landscape change and quantification of environmental forcing (precipitation, hydrology, etc.) from rapidly changing landscapes. These were generated from repeat TLS scans in badlands, severely burned first-order drainage basins, and ephemeral channels. From these maps, we can determine magnitudes and patterns of erosion and deposition, and directly link the observed changes to specific environmental forcing events. These o...
Airborne LiDAR has now been collected along many of the active fault zones in California. These d... more Airborne LiDAR has now been collected along many of the active fault zones in California. These data provide high resolution representation of fault zone topography that permits investigation into temporal and spatial fault zone evolution. They also provide baseline data for detailed post-earthquake (or other event) landscape change detection. Here we describe analysis of complex fault zone topography along Mustang Ridge where the San Andreas Fault transfers slip onto an array of subsidiary faults in a km-wide stepover zone. From the 2007 Earthscope LiDAR, relative rates of subsidiary fault slip can be constrained using scarp morphologic age determination (using a linear diffusion model and extracted profiles), channel profile extraction, two dimensional spectral analysis, and extraction of scarp morphologic ages using new wavelet-like methods. We also report on use of multitemporal LiDAR (2007 Earthscope NoCAL data collected by NCALM, and 2003 NASA/USGS NSAF data) for detailed chan...
Geodesists typically model fault zones as a single failure plane that undergoes either repeated r... more Geodesists typically model fault zones as a single failure plane that undergoes either repeated rupture or continuous slip, but in reality, these features are broad, complicated zones consisting of many active and ancient fault strands. Understanding how these zones develop over time may reveal information about the temporal evolution and physical properties of a fault zone. The topography of active fault zones records this recent history of fault development as abrupt, linear breaks in topographic slope, commonly referred to as fault scarps. Since the 1980s, paleoseismologists have used the degree of change in scarp profile form to determine the relative age of different fault scarps in a fault zone, under the supposition that those scarps that are more degraded are older than less degraded scarps. From these analyses, a morphologic age is determined. It conflates the true age of the formation of a scarp with the rates of mass transport by erosive processes. Morphologic measurement...
The study of mass and energy transfer across landscapes has recently evolved to comprehensive con... more The study of mass and energy transfer across landscapes has recently evolved to comprehensive considerations acknowledging the role of biota and humans as geomorphic agents, as well as the importance of small-scale landscape features. A contributing and supporting factor to this evolution is the emergence over the last two decades of technologies able to acquire high resolution topography (HRT) (meter and sub-meter resolution) data. Landscape features can now be captured at an appropriately fine spatial resolution at which surface processes operate; this has revolutionized the way we study Earth-surface processes. The wealth of information contained in HRT also presents considerable challenges. For example, selection of the most appropriate type of HRT data for a given application is not trivial. No definitive approach exists for identifying and filtering erroneous or unwanted data, yet inappropriate filtering can create artifacts or eliminate/distort critical features. Estimates of errors and uncertainty are often poorly defined and typically fail to represent the spatial heterogeneity of the dataset, which may introduce bias or error for many analyses. For ease of use, gridded products are typically preferred rather than the more information-rich point cloud representations. Thus many users take advantage of only a fraction of the available data, which has furthermore been subjected to a series of operations often not known or investigated by the user. Lastly, standard HRT analysis work-flows are yet to be established for many popular HRT operations, which has contributed to the limited use of point cloud data. In this review, we identify key research questions relevant to the Earth-surface processes community within the theme of mass and energy transfer across landscapes and offer guidance on how to identify the most appropriate topographic data type for the analysis of interest. We describe the operations commonly performed from raw data to raster products and we identify key considerations and suggest appropriate work-flows for each, pointing to useful resources and available tools. Future research directions should stimulate further development of tools that take advantage of the wealth of information contained in the HRT data and address the present and upcoming research needs such as the ability to filter out unwanted data, compute spatially variable estimates of uncertainty and perform multi-scale analyses. While we focus primarily on HRT applications for mass and energy transfer, we envision this review to be relevant beyond the Earth-surface processes community for a much broader range of applications involving the analysis of HRT.
Variations in erosion and deposition for the last fifty years (based on estimates from 137Cs prof... more Variations in erosion and deposition for the last fifty years (based on estimates from 137Cs profiles) on surfaces (Late Pleistocene to Late Holocene in age) making up the Fortymile Wash alluvial fan south of Yucca Mountain, is a function of surface age and of desert pavement ...
We study whether arroyo channel head retreat in dryland discontinuous ephemeral streams is driven... more We study whether arroyo channel head retreat in dryland discontinuous ephemeral streams is driven by surface runoff, seepage erosion, mass wasting, or some combination of these hydrogeomorphic processes. We monitored precipitation, overland fl ow, soil moisture, and headcut migration over several seasonal cycles at two adjacent rangeland channel heads in southern Arizona. Erosion occurred by headward retreat of vertical to overhanging faces, driven dominantly by surface runoff. No evidence exists for erosion caused by shallow-groundwater-related processes, even though similar theater-headed morphologies are sometimes attributed to seepage erosion by emerging groundwater. At our fi eld site, vertical variation in soil shear strength infl uenced the persistence of the characteristic theater-head form. The dominant processes of erosion included removal of grains and soil aggregates during even very shallow (1-3 cm) overland fl ow events by runoff on vertical to overhanging channel headwalls, plunge-pool erosion during higher-discharge runoff events, immediate postrunoff wet mass wasting, and minor intra-event dry mass wasting on soil tension fractures developing subparallel to the headwall. Multiple stepwise linear regression indicates that the migration rate is most strongly correlated with fl ow duration and total precipitation and is poorly correlated with peak fl ow depth or time-integrated fl ow depth. The studied channel heads migrated upslope with a self-similar morphologic form under a wide range of hydrological conditions, and the most powerful fl ash fl oods were not always responsible for the largest changes in landscape form in this environment.
Evolution of landscape heterogeneity is controlled by coupled Earth system dynamics, and the resu... more Evolution of landscape heterogeneity is controlled by coupled Earth system dynamics, and the resulting process complexity is a major hurdle to cross towards a unified theory of catchment hydrology. The Biosphere 2 Landscape Evolution Observatory (LEO), a 334.5 m 2 artificial hillslope built with homogeneous soil, may have evolved into heterogeneous soil during the first experiment driven by an intense rainfall event. The experiment produced predominantly seepage face water outflow, but also generated overland flow, causing superficial erosion and the formation of a small channel. In this paper, we explore the hypothesis of incipient heterogeneity development in LEO and its effect on overland flow generation by comparing the modeling results from a three-dimensional physically based hydrological model with measurements of total mass change and seepage face flow. Our null hypothesis is that the soil is hydraulically homogeneous, while the alternative hypothesis is that LEO developed downstream heterogeneity from transport of fine sediments driven by saturated subsurface flow. The heterogeneous case is modeled by assigning saturated hydraulic conductivity at the LEO seepage face (K sat,sf ) different from that of the rest (K sat ). A range of values for K sat , K sat,sf , soil porosity, and pore size distribution is used to account for uncertainties in estimating these parameters, resulting in more than 20 000 simulations. It is found that the best runs under the heterogeneous soil hypothesis produce smaller errors than those under the null hypothesis, and that the heterogeneous runs yield a higher probability of best model performance than the homogeneous runs. These results support the alternative hypothesis of localized incipient heterogeneity of the LEO soil, which facilitated generation of overland flow. This modeling study of the first LEO experiment suggests an important role of coupled water and sediment transport processes in the evolution of subsurface heterogeneity and on overland flow generation, highlighting the need of a coupled modeling system that integrates across disciplinary processes.
Subsurface flow and storage dynamics at hillslope scale are difficult to ascertain, often in part... more Subsurface flow and storage dynamics at hillslope scale are difficult to ascertain, often in part due to a lack of sufficient high-resolution measurements and an incomplete understanding of boundary conditions, soil properties, and other environmental aspects. A continuous and extreme rainfall experiment on an artificial hillslope at Biosphere 2's Landscape Evolution Observatory (LEO) resulted in saturation excess overland flow and gully erosion in the convergent hillslope area. An array of 496 soil moisture sensors revealed a two-step saturation process. First, the downward movement of the wetting front brought soils to a relatively constant but still unsaturated moisture content. Second, soils were brought to saturated conditions from below in response to rising water tables. Convergent areas responded faster than upslope areas, due to contributions from lateral subsurface flow driven by the topography of the bottom boundary, which is comparable to impermeable bedrock in natural environments. This led to the formation of a groundwater ridge in the convergent area, triggering saturation excess runoff generation. This unique experiment demonstrates, at very high spatial and temporal resolution, the role of convergence on subsurface storage and flow dynamics. The results bring into question the representation of saturation excess overland flow in conceptual rainfall-runoff models and land-surface models, since flow is gravity-driven in many of these models and upper layers cannot become saturated from below. The results also provide a baseline to study the role of the coevolution of ecological and hydrological processes in determining landscape water dynamics during future experiments in LEO.
The presence of genuinely zero-age or near-zero-age grains in modern-age and very young samples p... more The presence of genuinely zero-age or near-zero-age grains in modern-age and very young samples poses a problem for many existing burial dose estimation procedures used in optical (optically stimulated luminescence, OSL) dating. This difficulty currently necessitates consideration of relatively simplistic and statistically inferior age models. In this study, we investigate the potential for using modified versions of the statistical age models of Galbraith et al. [Galbraith, R.F., Roberts, R.G., Laslett, G.M., Yoshida, H., Olley, J.M., 1999. Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia: Part I, experimental design and statistical models. Archaeometry 41, 339-364.] to provide reliable equivalent dose (D e ) estimates for young and modern-age samples that display negative, zero or near-zero D e estimates. For this purpose, we have revised the original versions of the central and minimum age models, which are based on log-transformed D e values, so that they can be applied to unlogged D e estimates and their associated absolute standard errors. The suitability of these 'un-logged' age models is tested using a series of known-age fluvial samples deposited within two arroyo systems from the American Southwest. The un-logged age models provide accurate burial doses and final OSL ages for roughly three-quarters of the total number of samples considered in this study. Sensitivity tests reveal that the un-logged versions of the central and minimum age models are capable of producing accurate burial dose estimates for modern-age and very young (<350 yr) fluvial samples that contain (i) more than 20% of well-bleached grains in their D e distributions, or (ii) smaller sub-populations of wellbleached grains for which the D e values are known with high precision. Our results indicate that the original (log-transformed) versions of the central and minimum age models are still preferable for most routine dating applications, since these age models are better suited to the statistical properties of typical single-grain and multi-grain single-aliquot D e datasets. However, the unique error properties of modernage samples, combined with the problems of calculating natural logarithms of negative or zero-Gy D e values, mean that the un-logged versions of the central and minimum age models currently offer the most suitable means of deriving accurate burial dose estimates for very young and modern-age samples.
ABSTRACT Rainsplash can be an important factor in the evolution of landscapes, particularly those... more ABSTRACT Rainsplash can be an important factor in the evolution of landscapes, particularly those with low vegetation density and weak lithology or soil mantle. Much of the previous work on the process of rainsplash has been theoretical. However, we used a laser precipitation monitor (LPM) and a terrestrial laser scanner (TLS) at Biosphere 2 to test the applicability of a diffusion model to rainsplash erosion. Topographic smoothing resulting from rainsplash has been modeled by applying the diffusion equation to the topographic evolution of landscapes. Various theoretical models suggest that the diffusivity coefficient, D, in the diffusion equation will increase linearly with rainfall intensity and nonlinearly with the ratio of raindrop diameter to particle size diameter. This study tested these theoretical relationships experimentally by raining on model landscapes made of unconsolidated sediment having a range of particle size distributions ranging from sand to loamy sand. Using a laser precipitation monitor, we characterized raindrop size distribution and intensity from a variety of sprinkler configurations. The raindrops ranged from a mean drop diameter of 0.39 mm to 0.55 mm. A terrestrial laser scanner was used to create topographic models of the landscapes during experimentation. Inverse modeling was then used to test a diffusion model&#39;s ability to predict landscape change due to rainsplash and to determine the diffusivity values corresponding to each set of conditions. The main objective was to quantify how the inferred diffusivity values vary with rainfall intensity, raindrop diameter, and particle size diameter. With the results obtained from this experiment, we hope to provide further data that could be used to refine theoretical models of rainsplash.
Zero-order drainage basins, and their constituent hillslopes, are the fundamental geomorphic unit... more Zero-order drainage basins, and their constituent hillslopes, are the fundamental geomorphic unit comprising much of Earth's uplands. The convergent topography of these landscapes generates spatially variable substrate and moisture content, facilitating biological diversity and influencing how the landscape filters precipitation and sequesters atmospheric carbon dioxide. In light of these significant ecosystem services, refining our understanding of how these functions are affected by landscape evolution, weather variability, and long-term climate change is imperative. In this paper we introduce the Landscape Evolution Observatory (LEO): a largescale controllable infrastructure consisting of three replicated artificial landscapes (each 330 m 2 surface area) within the climate-controlled Biosphere 2 facility in Arizona, USA. At LEO, experimental manipulation of rainfall, air temperature, relative humidity, and wind speed are possible at unprecedented scale. The Landscape Evolution Observatory was designed as a community resource to advance understanding of how topography, physical and chemical properties of soil, and biological communities coevolve, and how this coevolution affects water, carbon, and energy cycles at multiple spatial scales. With well-defined boundary conditions and an extensive network of sensors and samplers, LEO enables an iterative scientific approach that includes numerical model development and virtual experimentation, physical experimentation, data analysis, and model refinement. We plan to engage the broader scientific community through public dissemination of data from LEO, collaborative experimental design, and community-based model development.
1] Vegetation bands are periodic bands of vegetation, separated by interband spaces devoid of veg... more 1] Vegetation bands are periodic bands of vegetation, separated by interband spaces devoid of vegetation, oriented parallel to the topographic contour in some gently sloping arid to semiarid environments. Models of vegetation band formation attribute their formation to positive feedbacks among vegetation density, soil porosity/permeability, and infiltration rates. Here we present an alternative model based on field measurements at our study sites in southern Nevada. In this model, interband spaces between vegetation bands form because topographic mounds beneath vegetation bands detain water upslope from vegetation bands, leading to hydrologic and sedimentologic conditions that inhibit the survival of plants in interband spaces. We used terrestrial laser scanning (TLS) to create high-resolution ($10 cm 2 /pixel) raster data sets of bare-earth topography and canopy height for four study sites. Analyses of the TLS data, in addition to measurements of soil shear strength and particle size, document the potential for detention in interband spaces and a near-inverse proportionality between band spacing and regional slope. We describe a cellular automaton model (herein called model 1) for vegetation band formation that includes just two user-defined parameters and that generates vegetation bands similar to those at our field sites, including the inverse proportionality between spacing and regional slope. A second model (model 2) accurately predicts the width of vegetation bands in terms of the number and spacing of plants and the geometry of individual plant mounds. We also present a GIS-based analysis that predicts where bands occur within a region based on topographic and hydroclimatic controls.
Uploads
Papers by Stephen Delong