1,721,102 research outputs found
The effects of gentle topographic variation on dispersal kernels of inertial particles
No full textSeed dispersal kernels of wind-dispersed species imprint the initial spatial template over which later demographic processes such as establishment or re-colonization operate from. A major knowledge gap in seed dispersal modeling by wind is the role of complex topography in modifying the kernel shape when referenced to the flat-world case. How complex topography modifies the dispersal kernels of inertial particles such as seeds is explored here via novel flume experiments. A train of gentle cosine hills and a canopy composed of densely arrayed rods with a roughness density resembling the leaf area density profile of hardwood species at maximum leaf area were used to represent a canopy-hill system. Spherical inertial particles having a coefficient of variation in terminal velocity commensurate with values reported from field studies were employed as model seeds. It was demonstrated that the seed dispersal kernel maintained its canonical ‘Wald' form as derived from simplified turbulent dispersion theories for flat terrain and vertically uniform flow field. Seeds released near the canopy top and from the top of the hill have a dispersal kernel mean distance that was about 35% times larger than its counterpart for releases from the bottom of the hill. Moreover, the probability of occurrence of long-distance dispersal (LDD) events, defined here as seeds traversing a longitudinal distance exceeding 10 times the canopy height, were one order of magnitude larger for seeds released near the canopy top and from the hill top when compared to their counterpart released from the hill bottom. Citation: Katul, G. G., and D. Poggi (2012), The effects of gentle topographic variation on dispersal kernels of inertial particle
The effect of canopy roughness density on the constitutive components of the dispersive stresses
No full textMODELLI DI PRIMO ORDINE E RESISTENZE INDOTTE DALLA VEGETAZIONE: UN APPROCCIO TEORICO-SPERIMENTALE
No full textThe root-zone soil moisture spectrum in a mediterranean ecosystem
Full text linkStorage of water within soil pores of the root zone introduce memory effects in the dynamics of soil moisture that are considerably longer than the integral timescale of many atmospheric processes. Thus, hydro-climatic states can be “sustained” through land-surface heat and water vapor fluxes primarily because they can “feed off” on this long-term soil moisture memory. Root-zone soil moisture memory is only but one feature characterizing the spectrum of soil moisture dynamics, which is analyzed here using a combination of long-term measurements and models. In particular, the spectrum of root-zone soil moisture content in a Mediterranean ecosystem is examined using 14-years of half-hourly measurements. A distinguishing hydro-climatic feature in such ecosystems is that sources (mainly rainfall) and sinks (mainly evapotranspiration) of soil moisture are roughly out of phase with each other. For over 4 decades of time scales and 7 decades of energy, the canonical shape of the measured soil moisture spectrum is shown to be approximately Lorentzian determined by the soil moisture variance and its memory but with two exceptions: the occurrences of a peak at diurnal-to-daily time scales and a weaker peak at near annual time scales. Model calculations and spectral analysis demonstrate that diurnal and seasonal variations in hydroclimate forcing responsible for variability in evapotranspiration had minor impact on the normalized shape of the soil moisture spectrum. However, their impact was captured by adjustments in the temporal variance. These findings indicate that precipitation and not evapotranspiration variability dominates the multi-scaling properties of soil moisture variability consistent with prior climate model simulations. Furthermore, the soil moisture memory inferred by the annual peak of soil moisture (340 d) is consistent with climate model simulations, while the memory evaluated from the loss function of a linearized mass balance approach leads to a smaller value (50 d), highlighting the effect of weak non-stationarity on soil moisture variability
A note on aerosol sized particle deposition onto dense and tall canopies situated on gentle cosine hills
No full textInverse Cascade Evidenced by Information Entropy of Passive Scalars in Submerged Canopy Flows
Full text linkTurbulent mixing of scalars within canopies is investigated using a flume experiment with canopy-like rods of height h mounted to the channel bed. The data comprised a time sequence of high-resolution images of a dye recorded in a plane parallel to the bed at z/h= 0.2. Image processing shows that von Kármán wakes shed by canopy drag and downward turbulent transport from upper canopy layers impose distinct scaling regimes on the scalar spectrum. Measures from information theory are then used to explore the dominant directionality of the interaction between small and large scales underlying these two spectral regimes, showing that the arrival of sweeps from aloft establishes an inertial-range spectrum with forward “information” cascade. In contrast, wake growth with downstream distance leads to persistent upscale transfer (inverse cascade) of scalar variance, which hints at their nondiffusive character and the significance of the stem diameter as an active length scale in canopy turbulence
Relation between the spectral properties of wall turbulence and the scaling of the Darcy-Weisbach friction factor
Full text linkEmpirical formulas describing the Darcy-Weisbach friction factor remain indispensable for applications in sciences and engineering dealing with turbulent flows. Despite their practical significance, these formulas have remained without theoretical interpretation for many decades. To close this knowledge gap, much research has been devoted to the development of the so-called "spectral link"introduced in the early 2000s. Such a theory is entirely based on elegant phenomenological arguments that make no contact with equations describing turbulent wall flows. The spectral link spawned alternative approaches, now labeled "cospectral budget"(or CSB) models, that describe how turbulent eddies contribute to wall stresses. The CSB overcomes some of the shortcomings of the phenomenological approach and is here employed to provide a thorough clarification of the link between spectral properties of velocity fluctuations and the scaling of friction factors in turbulent pipe flows in the hydraulically smooth and fully rough regimes
Unsteady overland flow on flat surfaces induced by spatial permability contrasts
No full textLateral redistribution of surface water in patchy arid ecosystems has been hypothesized to contribute to the maintenance of vegetation patches through the provision of a water subsidy from bare sites to vegetated sites. Such runon-runoff processes occur during Hortonian runoff events on topographically sloping ground. Surface flow redistribution may also occur on topographically flat ground if the presence of the vegetation patch creates a contrast in infiltration rate, leading to a free-surface gradient in ponded water. The precise dynamics and the eco-hydrologic role of this process has resisted complete theoretical treatment to date. Here the overland flow equations are modified to account for the presence of vegetation situated over a flat surface. The resulting model is solved numerically to determine whether this mechanism could influence the spatial partitioning of water in patchy arid ecosystems. Assumptions made about infiltration processes and overland flow in existing eco-hydrologic models of patchy and patterned arid ecosystems are evaluated in comparison to the solution of the ‘full' coupled Saint-Venant equations with various infiltration models. The results indicate that the optimization of vegetation spatial patch scales with respect to water redistribution may be determined by the size of the infiltration redistribution length L over which the presence of an infiltration contrast perturbs baseline infiltration behavio
A flow resistance model for assessing the impact of vegetation on flood routing mechanics
No full textThe specification of a flow resistance factor to account for vegetative effects in the Saint-Venant equation (SVE) remains uncertain and is a subject of active research in flood routing mechanics. Here, an nalytical model for the flow resistance factor is proposed for submerged vegetation, where the water depth is commensurate with the canopy height and the roughness Reynolds number is sufficiently large so as to ignore viscous effects. The analytical model predicts that the resistance factor varies with three canonical length scales:
the adjustment length scale that depends on the foliage drag and leaf area density, the canopy height, and the water level. These length scales can reasonably be inferred from a range of remote sensing products making the proposed flow resistance model eminently
suitable for operational flood routing. Despite the numerous simplifications, agreement between measured and modeled resistance factors and bulk velocities is reasonable across a range of experimental and field studies. The proposed model asymptotically recovers the flow resistance formulation when the water depth greatly exceeds the canopy height. This analytical treatment provides a unifying framework that links the resistance factor to a
number of concepts and length scales already in use to describe canopy turbulence. The implications of the coupling between the resistance factor and the water depth on solutions to the SVE are explored via a case study, which shows a reasonable match between
empirical design standard and theoretical predictions
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