2,937 research outputs found
Il ruolo di Elia Volpi nella vendita delle sculture di Alceo Dossena. Fotografie e lettere inedite
No full textIl contributo analizza il fondo fotografico dell'antiquario fiorentino Elia Volpi focalizzando su di un aspetto particolare: il commercio dei falsi, in particolar modo delle sculture realizzate attorno agli anni Venti del Novecento da Alceo Dossena. Le fotografie rinvenute nel fondo fotografico con sculture di Dossena, assieme alle lettere inviate da Volpi a Wilhelm von Bode, invitano a ripensare il ruolo avuto da Volpi nella vendita delle opere a importanti musei americani
Effects of hillslope dynamics and network geometry on the scaling properties of the hydrologic response
No full text"\"\\\"\\\\\\\"This paper investigates the specific contributions of river network geomorphology, hillslope flow dynamics and channel routing to the scaling behavior of the hydrologic response as function of drainage area. Scaling relationships emerged from the observations of geomorphological and hydrological data and were reproduced in previous works through mathematical models, for both idealized self-similar networks and natural basins. Recent literature highlighted that scale invariance of hydrological quantities depends not only on the metrics of the drainage catchment but also on effective flow routing. In this study we employ a geomorphological width function scheme to test the simple scaling hypothesis adopting more realistic dynamic conditions than in previous approaches, specifically taking into account the role of hillslopes. The analysis is based on the derivation of the characteristic distributions of path lengths and travel times, inferred from DEM processing and measurements of rainfall and runoff data. The study area is located in the Tiber River region (central Italy). Results indicate that, while scaling properties clearly emerge when the hydrologic response is defined on the basis of the sole geomorphology, scale invariance is broken when less idealized flow dynamics are taken into account. Lack of scaling appears in particular as a consequence of the catchment to catchment variability of hillslope velocities. (C) 2011 Elsevier Ltd. All rights reserved.\\\\\\\"\\\"\"
Effetto delle dinamiche di versante sulle proprietà di invarianza di scala nei bacini naturali
No full textViene indagato su un campione di bacini di estensione variabile tra 200 ed oltre 4000 km2 il comportamento di scala di due gruppi di grandezze caratteristiche dei bacini idrografici. Al primo gruppo appartengono variabili che dipendono unicamente dalla geomorfologia e riflettono la distribuzione delle lunghezze dei percorsi; nel secondo invece vengono incluse grandezze che dipendono dalle velocità di trasferimento su versante e canale e sono rappresentative della distribuzione dei tempi di arrivo. Si mostra come, a causa della variabilità introdotta dalla componente dinamica dei versanti, il comportamento scala-invariante caratteristico delle variabili geomorfologiche non si estende alle variabili geomorfologico-cinematiche. Particolare rilevanza viene attribuita, nella memoria, al comportamento di scala dei diversi meccanismi di dispersione. Il caso studio è rappresentato da tredici bacini localizzati nell’Italia centrale
Getting pixels and regions to agree with conditional random fields
No full textLand cover / land use classification of remotely sensed images is inherently geographical. The use of spatial information, accounting for neighborhood relationship and spatial smoothness of geographical objects, made its proofs in countless occasions and, especially when considering very high resolution images, methods ignoring spatial context do not perform well. In this paper, we propose a hybrid dual-layer conditional random field model that enforces spatial smoothness and consistency between the pixel and region-based maps. We formulate these intuitions as a standard energy minimization problem, and we show that finding a joint solution over both output spaces leads to strong improvements in the numerical and visual senses
Understanding the relative role of dispersion mechanisms across basin scales
No full textDifferent mechanisms are understood to represent the primary sources of the variance of travel time distribution in natural catchments. To quantify the fraction of variance introduced by each component, dispersion coefficients have been earlier defined in the framework of geomorphology-based rainfall-runoff models. In this paper we compare over a wide range of basin sizes and for a variety of runoff conditions the relative role of geomorphological dispersion, related to the heterogeneity of path lengths, and hillslope kinematic dispersion, generated by flow processes within the hillslopes. Unlike previous works, our approach does not focus on a specific study case; instead, we try to generalize results already obtained in previous literature stemming from the definition of a few significant parameters related to the metrics of the catchment and flow dynamics. We further extend this conceptual framework considering the effects of two additional variance-producing processes: the first covers the random variability of hillslope velocities (i.e. of travel times over hillslopes); the second deals with non-uniform production of runoff over the basin (specifically related to drainage density). Results are useful to clarify the role of hillslope kinematic dispersion and define under which conditions it counteracts or reinforces geomorphological dispersion. We show how its sign is ruled by the specific spatial distribution of hillslope lengths within the basin, as well as by flow conditions. Interestingly, while negative in a wide range of cases, kinematic dispersion is expected to become invariantly positive when the variability of hillslope velocity is large
A simple lumped rainfall runoff model accounting for the spatial variability of drainage density
No full textExploring the variability of the hydrologic response due to rainfall spatial heterogeneity: Analytical derivations and numerical simulations
No full text"The spatial distribution of rainfall can play a key role in determining the hydrological response of river basins. Circumstances under which this role is dampened or enhanced, while extensively discussed in previous literature works, have not yet received a clear assessment.. We propose an analytical approach to investigate the dependence of basin hydrograph variability on the spatial heterogeneity of rainfall, specifically considering the response of a catchment to an instantaneous unit pulse of spatially-variable excess rainfall. Catchment response to rainfall input is assumed to be completely described by the time-invariant distribution of travel times. The excess-rainfall field accounts for the variability of both precipitation and soil characteristics and is assumed to be represented as a stationary and isotropic process, defined by its variance and integral scale. The sensitivity of the hydrologic response to the rainfall spatial variability is evaluated describing the variance of the generated hydrograph as function of basin characteristics and the stochastic properties of excess-rainfall field. This analytical approach allow us to infer results that do not depend on a specific study case.. Results suggest that the variability of the hydrologic response due to spatial rainfall heterogeneity mainly depends on the ratio between the integral scale of the excess-rainfall field and basin drainage area. For basins which are small compared to the integral scale of excess rainfall, rainfall spatial heterogeneity has a minor relevance on the hydrologic response, i.e. basin sees the precipitation as uniformly distributed. In the opposite case, for relatively large basins the spatial variability of excess rainfall is averaged over the basin area and the hydrologic response is approximately equal to the travel time distribution; under such circumstances the system is ergodic. However, at the early and late branch of the travel time distribution, i.e. in areas close to the basin outlet and those far away from it, the coefficient of variation of the hydrographs may be still high even for large basins. This happens where the isochrones are small, leading to a poor averaging of the rainfall field (non-ergodic behavior).. These asymptotic limits have been confirmed by the performance of numerical simulations, based on a Monte Carlo approach and applied to a specific study basin. Numerical experiments use a simplified width-function geomorphological model. They helped us in understanding the behavior of the response function for intermediate conditions (medium size basins), when the square of the integral scale and the drainage are same order of magnitude..
Effetti della variabilità spaziale del campo di precipitazione sulla risposta idrologica: un’indagine attraverso modelli analitici e numerici semplificati
No full text"Vengono indagati gli effetti della variabilità spaziale dei campi di precipitazione netta sulla risposta idrologica, applicando un modello di trasformazione afflussi-deflussi geomorfologico basato sull’approccio per tempi di residenza. Sono analizzati, in particolare, diversi rapporti tra le scale spaziali caratteristiche del bacino e del campo di pioggia netta e diverse configurazioni di velocità di trasferimento. Obiettivo del lavoro è quello di determinare sotto quali condizioni la variabilità spaziale della pioggia appare in grado di produrre effetti rilevanti. Si suppone che la precipitazione netta sia rappresentata da un campo stazionario ed isotropo, di cui sono noti i momenti del primo e secondo ordine e la scala integrale. Gli sviluppi analitici presentati permettono la quantificazione degli effetti della variabilità del campo di pioggia netta per alcune condizioni asintotiche; per le condizioni intermedie (non asintotiche) si è fatto ricorso a simulazioni numeriche Monte Carlo.
Analytical modeling of the hydrologic response under moving rainstorms: Storm-catchment interaction and resonance
No full text"In this study, we propose a simple yet broad analytical framework for analyzing the hydrological. response of a catchment under moving rainstorms; the method can be used as a tool to explore the main. relevant features of the storm-catchment interactions. We analyze the response of the basin to an excess. rain field (rain contributing to direct runoff), which is assumed to be variable in space and time; catchment. response is supposed to be characterized by the time-invariant distribution of the travel time of. water particles within the basin. We use the framework developed herein to investigate the conditions. that enhance peak flow, leading to the so-called resonance effect, in terms of storm size, direction and. velocity. Our results show how resonance conditions depend on the relative size of the rainstorm with. respect to basin size. In particular, for storm sizes much smaller than the dimension of the catchment,. a complete resonance effect occurs for infinite combinations of the direction and speed of the moving rainstorm.. On the contrary, when storm size is much larger than the basin size, the flood peak tends to be. independent on rainstorm movement. In the intermediate conditions a partial resonance effect emerges. as a consequence of both the superposition of rainfall contributes in time and the increased flow response. of the basin; the latter is a result of the larger rainfall volume injected over time. For illustration purposes,. we present and discuss a case study based on the open-book idealized catchment.
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