28 research outputs found

    A topographic mechanism for arcing of dryland vegetation bands

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    Banded patterns consisting of alternating bare soil and dense vegetation have been observed in water-limited ecosystems across the globe, often appearing along gently sloped terrain with the stripes aligned transverse to the elevation gradient. In many cases these vegetation bands are arced, with field observations suggesting a link between the orientation of arcing relative to the grade and the curvature of the underlying terrain. We modify the water transport in the Klausmeier model of water-biomass interactions, originally posed on a uniform hillslope, to qualitatively capture the influence of terrain curvature on the vegetation patterns. Numerical simulations of this modified model indicate that the vegetation bands change arcing-direction from convex-downslope when growing on top of a ridge to convex-upslope when growing in a valley. This behavior is consistent with observations from remote sensing data that we present here. Model simulations show further that whether bands grow on ridges, valleys, or both depends on the precipitation level. A survey of three banded vegetation sites, each with a different aridity level, indicates qualitatively similar behavior.Comment: 26 pages, 13 figures, 2 table

    Characterizing Mosquito Flight Using Measurement And Simulation

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    The mosquito Aedes aegypti is a dengue fever vector. Via its flight, it spreads potentially fatal disease to millions of people each year. In this dissertation I describe my work recording Aedes males using high speed imaging, quantifying and analyzing their motion, and simulating their flight. I describe image processing techniques that have allowed us to characterize their body position and orientation as well as their wing motion. We find that mosquitoes fly with a sideways component to their flight more often than other recorded Dipterans, and rely on sideslipping turns to change their flight direction. We show quantitatively that they use their stroke plane roll angle to generate sideways accelerations. We also show that, unlike many Dipterans, they do not use their pitch angle to control forward acceleration. Their body roll angle is thus central to the control of their motion. Using computer simulation to probe the stability characteristics of their flight we find that, like other Dipterans, the motion of these mosquitoes lies near the boundary between asymptotic stability and instability. However, the linearized map describing the motion of the body from one wingbeat to the next is not self-adjoint, resulting in potentially large growth of perturbations on the shorter timescales relevant to mosquito motion. These perturbations are rotated as they grow, potentially leading to a reduction in the dimension of the controller

    Signatures of human impact on self-organized vegetation in the Horn of Africa

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    In many dryland environments, vegetation self-organizes into bands that can be clearly identified in remotely-sensed imagery. The status of individual bands can be tracked over time, allowing for a detailed remote analysis of how human populations affect the vital balance of dryland ecosystems. In this study, we characterize vegetation change in areas of the Horn of Africa where imagery taken in the early 1950s is available. We find that substantial change is associated with steep increases in human activity, which we infer primarily through the extent of road and dirt track development. A seemingly paradoxical signature of human impact appears as an increase in the widths of the vegetation bands, which effectively increases the extent of vegetation cover in many areas. We show that this widening occurs due to altered rates of vegetation colonization and mortality at the edges of the bands, and conjecture that such changes are driven by human-induced shifts in plant species composition. Our findings suggest signatures of human impact that may aid in identifying and monitoring vulnerable drylands in the Horn of Africa
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