Which way do you lean? Using slope aspect variations to understand Critical Zone processes and feedbacks

Soil‐mantled pole‐facing hillslopes on Earth tend to be steeper, wetter, and have more vegetation cover compared with adjacent equator‐facing hillslopes. These and other slope aspect controls are often the consequence of feedbacks among hydrologic, ecologic, pedogenic, and geomorphic processes triggered by spatial variations in mean annual insolation. In this paper we review the state of knowledge on slope aspect controls of Critical Zone (CZ) processes using the latitudinal and elevational dependence of topographic asymmetry as a motivating observation. At relatively low latitudes and elevations, pole‐facing hillslopes tend to be steeper. At higher latitudes and elevations this pattern reverses. We reproduce this pattern using an empirical model based on parsimonious functions of latitude, an aridity index, mean‐annual temperature, and slope gradient. Using this empirical model and the literature as guides, we present a conceptual model for the slope‐aspect‐driven CZ feedbacks that generate asymmetry in water‐limited and temperature‐limited end‐member cases. In this conceptual model the dominant factor driving slope aspect differences at relatively low latitudes and elevations is the difference in mean‐annual soil moisture. The dominant factor at higher latitudes and elevations is temperature limitation on vegetation growth. In water‐limited cases, we propose that higher mean‐annual soil moisture on pole‐facing hillslopes drives higher soil production rates, higher water storage potential, more vegetation cover, faster dust deposition, and lower erosional efficiency in a positive feedback. At higher latitudes and elevations, pole‐facing hillslopes tend to have less vegetation cover, greater erosional efficiency, and gentler slopes, thus reversing the pattern of asymmetry found at lower latitudes and elevations. Our conceptual model emphasizes the linkages among short‐ and long‐timescale processes and across CZ sub‐disciplines; it also points to opportunities to further understand how CZ processes interact. We also demonstrate the importance of paleoclimatic conditions and non‐climatic factors in influencing slope aspect variations. Copyright © 2017 John Wiley & Sons, Ltd.     

Relationships between physical‐geochemical soil properties and erodibility of streambanks among different physiographic provinces of Tennessee,USA

Erosion of cohesive soils in fluvial environments is dependent on physical, geochemical and biological properties, which govern inter‐particle attraction forces and control detachment rates from stream beds and banks. Most erosion rate models are based on the excess shear stress equation where the soil erodibility coefficient (k_d) is multiplied by the difference between the boundary hydraulic shear stress (τ_b) and the soil critical shear stress (τ_c). Both k_d and τ_c are a function of soil properties and must be obtained through in situ field or laboratory testing. Many studies have generated predictive relationships for k_d and τ_c derived from various soil properties. These studies typically were conducted in watersheds within a single physiographic region with a common surficial geology and/or investigated a limited number of soil properties, particularly geochemical properties. With widely reported differences in relationships between τ_c and soil properties, this study investigated differences in predictive relationships for τ_c among different physiographic provinces in Tennessee, USA. Erodibility parameters were determined in the field using a mini‐jet test device. Among these provinces, statistically four unique clusters were identified from a dataset of 128 observations and these data clusters were used to develop predictive models for τ_c to identify dominant properties governing erosion. In these clusters, 16 significant physical and geochemical soil properties were identified for τ_c prediction. Among these soil properties, water content and passing #200 sieve (percentage soil less than 75 μm) were the dominant controlling parameters to predict τ_c in addition to clay percentage (< 2 μm), bulk density, and soil pore water chemistry. This study suggests that unique relationships exist for physiographic provinces that are likely due to soil physical‐geochemical processes associated with surficial geology that determine minerology of the cohesive soil. Copyright © 2017 John Wiley & Sons, Ltd.     

Asymmetry of weathering‐limited hillslopes: the importance of diurnal covariation in solar insolation and temperature

Hillslope asymmetry, i.e. variation in hillslope form as a function of slope aspect and/or mean solar insolation, has been documented in many climates and geologic contexts. Such patterns have the potential to help us better understand the hydrologic, ecologic, and geomorphologic processes and feedbacks operating on hillslopes. Here we document asymmetry in the fraction of hillslope relief accommodated by cliffs in weathering‐limited hillslopes of drainage basins incised into the East Kaibab Monocline (northern Arizona) and Raplee Ridge Monocline (southern Utah) of the southern Colorado Plateau. We document that south‐ and west‐facing hillslopes have a larger proportion of hillslope relief accommodated by cliffs compared with north‐ and east‐facing hillslopes. Cliff abundance correlates positively with mean solar insolation and, by inference, negatively with soil/rock moisture. Solar insolation control of hillslope asymmetry is an incomplete explanation, however, because it cannot account for the fact that the greatest asymmetry occurs between southwest‐ and northeast‐facing hillslopes rather than between south‐ and north‐facing hillslopes in the study sites. Modeling results suggest that southwest‐facing hillslopes are more cliff‐dominated than southeast‐facing hillslopes of the same mean solar insolation in part because potential evapotranspiration rates, which control the soil/rock moisture that drives weathering, are controlled by the product of solar insolation and a nonlinear function of surface temperature, together with the fact that southwest‐facing hillslopes receive peak solar insolation during warmer times of day compared with southeast‐facing hillslopes. The dependence of water availability on both solar insolation and surface temperature highlights the importance of the diurnal cycle in controlling water availability, and it provides a general explanation for the fact that vegetation cover tends to exhibit the greatest difference between northeast‐ and southwest‐facing hillslopes in the Northern Hemisphere and between southeast‐ and northwest‐facing hillslopes in the Southern Hemisphere. Copyright © 2017 John Wiley & Sons, Ltd.     

Evaluating the potential of multi-view data extraction from small Unmanned Aerial Systems (UASs) for object-based classification for Wetland land covers

Unmanned Aerial Systems (UASs) have the potential to provide multi-view data, but the approaches used to extract the multi-view data from UAS and investigation of their use in image classification are currently unavailable in publications to our best knowledge. This study presents a method that combines collinearity equations and a two-phase optimization procedure to automatically project a point from real world coordinate system of an orthoimage to UAS image coordinate system (row and column numbers) to be used in multi-view data extraction. The results show average errors for the computed UAS column and row numbers were 1.6 and 1.8 pixels respectively evaluated with leave-one-out method. Based on this algorithm, it’s also for the first time that object-based multi-view data were extracted and presented, and the potential of using the multi-view data to aid Geographic Object-Based Image Analysis(GEOBIA) through bidirectional reflectance distribution function (BRDF) modelling was evaluated with two representatives of BRDFs, the Rahman-Pinty-Verstraete(RPV) and Ross-Thick-LiSparse (RTLS). Our results indicate the RPV model tends to overestimate the bidirectional reflectance for land cover types with high reflectance, while perform well for those with relatively low reflectance in our study area. To test the impact of using multi-view data on image classification, we extracted parameters from BRDF models and used these parameters as object features for object-based classification. The 10-fold cross validation results show that the 3-parameter RTLS significantly improved overall accuracy compared to the classifications relying only on the orthoimage features, while other BRDF models did not show significant improvements, raising the needs to develop new methods to better utilize the multi-view information in GEOIBA in the future.     

Exploring LiDAR data for mapping the micro-topography and tidal hydro-dynamics of mangrove systems: An example from southeast Queensland,Australia

The aim was to explore the use of Light Detection and Ranging (LiDAR) data to map the micro-topography of an intertidal wetland in southeast Queensland Australia. The driver for this was the need to identify and map the habitats of the immature stages of an aedine disease vector mosquito (Aedes vigilax (Skuse)). We derived a high resolution digital elevation model (DEM) data set at a vertical resolution of 0.05 m from LiDAR data. The relative accuracy of the DEM across the site was tested by comparing water depth predictions derived from the DEM against in-situ water depth readings from pressure sensors over a 10-day tidal cycle, which included high spring tides. We found that the field observations of micro-topographic units important for mosquito management matched those delineated from the DEM. The micro-topography included a low berm or central ridge that was more or less continuous across the site, a shallow back basin and fringing mangroves. The fringing mangroves had unimpeded connection to the tidal source, however the central ridge blocked tidal water from the back basin for all but the highest tides. Eggshell survey indicated that the back basin was the area suitable for immature mosquitoes. We conclude that LiDAR data has application for understanding and mapping the structure of mangrove wetlands. We have also demonstrated (in a small area) that LiDAR is useful for modelling the effect of sea level changes on the coastal fringe. LiDAR may be the only method to inform research on changes to land use and ecosystems caused by sea level change.     

Concept analysis and laboratory observations on a water piercing missile launcher

The water piercing missile launcher (WPML) is a new concept for launching missiles from submerged platforms. The WPML employs a high speed gas jet, using rocket exhaust as the gas source, to create a dry path underwater through which a missile may pass without contacting water. The gas jet is deflected due to cross flow and the gas jet trajectory is computed through a semi-empirical relationship commonly used to describe single phase jet deflection. This relationship, which requires an experimentally determined constant to predict jet deflection, is computed using experimental data. Uncoupled simulations of rocket exhaust and missile dynamics are shown to demonstrate how such a launcher could be utilized in launching a generic artillery missile. Although the results indicate the optimal launch depth, in terms of maximizing the launch depth while minimizing missile restraint time, is 14 m given an 8 m/s submarine speed it may be possible to launch a missile from a moving submarine at a speed of 5 m/s at a maximum depth of 20 m.     

A review of human and natural changes in Maya Lowland wetlands over the Holocene

In the Maya Lowlands of Mexico, Belize, and Guatemala two main types of wetlands have played important roles in human history: bajos or intermittently wet environments of the upland, interior Yucatán and perennial wetlands of the coastal plains. Many of the most important Maya sites encircle the bajos, though our growing evidence for human–wetland interactions is still sparse. The deposits of these wetlands record two main eras of slope instability and wetland aggradation: the Pleistocene–Holocene transition as rainfall increased and forests eclipsed savannas and the Maya Preclassic to Classic as deforestation, land-use intensity, and drying increased. The ancient Maya adapted with terraces around these bajo margins but retracted in the Late Preclassic in some areas. The perennial wetlands of the coastal plains have different histories, and the first conceptual model of human–wetland interaction described intensive wetland agriculture in the Preclassic through Classic based on raised fields and canals. But a second model arose that interpreted the wetland stratigraphy and canals as more indicative of natural aggradation by accelerated erosion and gypsum precipitation that buried Archaic and Preclassic fields and there was little Classic era use. We present new data on a third and fourth model in this study. The third is a hybrid of the models one and two, including the Archaic to Preclassic aggradation of the second model, and the first model's Classic period fields and canals as piecemeal attempts by the Maya to adapt to these and other environmental changes. The fourth conceptual model describes a very Late/Terminal Classic, preplanned project on a floodplain. These wetland fields were short-lived, aggraded rapidly but with some reoccupation in the Postclassic. All of these new models display the burgeoning evidence for intricate Maya interactions with wetlands, and the diversity of evidence from the relatively few studies underscores the infancy of our understanding of Maya interaction with tropical wetlands.