1,721,030 research outputs found
Two-layer model of near-surface soil drying for time-continuous hydrologic simulations
No full textA two-layer soil water balance model is developed to provide an efficient and robust descriptionof land surface dynamics in response to atmospheric evaporative events. Soil, vegetation, and atmosphere arecoupled dynamically under the assumption that soil moisture profiles approximately preserve similarity duringsimultaneous atmospheric drying and gravity drainage. The exfiltration flux at the land surface in response tothe atmospheric evaporative demand is limited by relating the surface resistance to water vapor transfer in thePenman-Monteith equation to the near-surface soil status. In addition, the control of deeper soils on both exfiltrationand drainage is expressed by performing a time compression approximation water balance over the entiredrying profile and by scaling the obtained exfiltration and drainage fluxes to the near-surface soil layers. Thereliability and robustness of the proposed formulation is evaluated with rates of evaporation calculated frommeasurements of the Bowen ratio and soil moisture data obtained from time domain reflectometry measurementsfor a bare soil field in the Zwalmbeek catchment (Belgium)
Il telerilevamento nel controllo dei fenomeni idrologici
No full textLo sviluppo di modelli idrologici alle diverse scale spazio-temporali risulta notevolmente complicato dalla forte eterogeneità dei sistemi naturali. Data l'onerosità delle campagne di misura a terra, il telerilevamento può produrre un notevole impatto sulla descrizione dei processi idrologici. La minore precisione delle misure telerilevate rispetto a quelle tradizionali può infatti essere compensata dalla possibilità del terilevamento di fornire informazioni su estesi scenari in modo relativamente veloce ed economico. Tuttavia, sia i modelli idrologici distribuiti sia le tecniche di telerilevamento presentano aspetti non completamente risolti e, pertanto, lo sviluppo di sistemi integrati deve essere condotto con molta cautela. L'introduzione acritica di notevoli quantità di dati telerilevati nei modelli idrologici distribuiti può risultare non solo inefficace ma addirittura controproducente. Solo sulla base della conoscenza esatta dei limiti dei modelli idrologici, da un lato, e delle tecniche di rilevamento, dall'altro, può risultare possibile un efficace utilizzo dei dati telerilevati per il controllo dei fenomeni idrologici sul territorio.Per il corretto sviluppo della modellistica idrologica e del telerilevamento è necessario condurre validazioni separate, per confronto con poche misure affidabili piuttosto che con enormi masse di dati di incertezza non ben definita. Questi concetti vengono illustrati nel presente lavoro attraverso la descrizione delle attuali tendenze di sviluppo dei modelli idrologici distribuiti e delle possibili applicazioni del telerilevamento all'idrologia. Vengono inoltre riportati i principali risultati di un'applicazione del radar ad apertura sintetica per il telerilevamento dell'umidità del suolo
Role of hydraulic geometry in flood wave propagation
No full textThe role of hydraulic geometry in flood wave propagation is investigated by using a diffusion wave model with inertial effects. Power function relationships W = a’ Q^b’ and kS = r’ Q^y’ are used to reproduce the at-a-station variations of water-surface width W and Gauckler-Strickler conductance coefficient kS (the inverse of Manning resistance coefficient) with flow discharge Q. Downstream variations of coefficients a’ and r’ are not considered in this study. The considered hydraulic geometry relationships are incorporated into a diffusion wave model in which the term (1 − Ve^2), Ve being the Vedernikov number, multiplies the Hayami’s diffusivity Q/(2 W S0), S0 being the channel bed slope. This mathematical model is solved numerically by using a matched artificial diffusivity method. Numerical experiments are carried out by evaluating peak attenuation and mean peak celerity of flood waves propagating along channel reaches characterized by coefficients a’ and r’ equal to the average values observed in natural rivers, by all the combinations of exponents b’ and y’ laying in the range 0–0.5, and by values of S0 laying in the range 0.000125–0.032. It is found that: (1) peak attenuation and mean peak celerity display the minimum values for b’ = 0.5 and y’ = 0, (2) for high values of y’, Ve displays values greater than 1 indicating physical instability of flood waves, and (3) around the condition b’ = 0 and y’ = 0, for high values of Q/W and low values of S0, the Peclet number Pe (evaluated over the channel reach length) displays values less than 2 indicating unrealistic hydraulic diffusion (more storage effects than those produced by a reservoir). The region of the plane b’y’ representing relevant flood waves lays therefore between the instability region Ve > 1, where unstable flood waves are physically possible but rarely observed in natural channels and not reproducible with the considered model, and the region of unrealistic diffusion Pe < 2, where numerical solutions are possible but physically questionable. In this region, peak attenuation and mean peak celerity are found to be more sensitive to variations in y’ than in b’. The developed diffusion wave model provides a reliable description of the advection and diffusion processes determining travel times and storage variations at the channel reach scale, with clear implications for reproducing surface flows and their interaction with the subsurface. The obtained results indicate that stream channel geometry plays a critical role in runoff propagation, and thus caution must be exercised in river engineering when altering shape and resistance to flow of channels. In addition, they suggest that the developed model can be usefully combined with field data to understand how kinematic wave celerity and hydraulic diffusivity scale when mountain streams and hillslope rivulets are considered, a challenge for the definition of a new generation of distributed models that are really based on physics at all the spatial and temporal scales characterizing the different processes that occur within the drainage basin
On the control volume modelling of near-surface soil drying
No full textThe problem of simulating the topsoil dynamics in response to atmospheric evaporative events is considered in the present paper. It is emphasised how the assumption that soil moisture profiles approximately preserve similarity during simultaneous atmospheric drying and gravity drainage may be required in order to incorporate the effects ofdeeper soils in the near-surface soil control volume hydrologic modelling. The reliability of the proposed formulation is evaluated with rates of evaporation calculated from measurements of the Bowen ratio and soil moisture data obtained from time domain reflectometry measurements for a bare soil field in the Zwalmbeek catchment (Belgium)
On the spatial variation of resistance to flow in upland channel networks
No full textDetailed field measurements of channel properties and flow characteristics collectedin the Ashley and Cropp catchments (New Zealand) are used to investigate the spatialvariation of resistance to flow across upland channel networks. The application of theDarcy-Weisbach equation and semilogarithmic flow resistance relationships reveals thatmean flow velocities calculated from local measurements of bed material particle size,hydraulic depth, and channel bed slope may be inaccurate. The Manning-Gauckler-Strickler equation with resistance coefficient independent of bed material particle size isfound to be relatively more reliable but not sufficiently general to reproduce the spatialvariation of resistance to flow across a complex channel network. A new methodology isdeveloped by combining a hydraulic equation of the Manning-Gauckler-Strickler type, aflow discharge-upstream drainage area relationship, and geomorphological fluvialrelationships for mean flow velocity, Gauckler-Strickler resistance coefficient, hydraulicdepth, and friction slope. This methodology is found to improve the reproduction of thespatial variation of mean flow velocity across the Ashley catchment and appears of generalapplicability for the parameterization of resistance to flow in distributed catchmentmodels
Closure to discussion to "Diffusion wave modelling of distributed catchment dynamics" by V. M. Ponce
No full textThere is no abstract in this kind of contribution
Multivariate analysis of radar images for environmental monitoring
No full textThe identification of the relationship between radar reflectivity factor Z, expressed in mm6 m-3, and rainfall intensity R, expressed in mm h-1, is crucial for both the calibration and operational phases. The Marshall and Palmer relationship, which links together Z at the lowest constant altitude plan position indicator (CAPPI) level and R, is commonly used in operational hydrology. This relation is of the form Z = aR^(b). Coefficients a and b reflect the dependence of Z from the number and size distribution of meteors present in the volumes scanned by radar beam. However, as a and b (which depend on the type of precipitation) are affected by great variability and both Z and R are affected by errors, detailed statistical analyses of the Z-R relationship may help improving the operational capabilities of weather radar. In this framework, the aim of the paper is twofold: (1) to develop a non-parametric approach which is more flexible and offers more generalisation capabilities than the MP relationship;(2) to use a vector of all 11 reflectivity factors at different CAPPI levels. For this purposes, three kinds of neural networks are developed: the multi-layer perceptron, radial basis function networks and Bayesian networks. Models are trained and tested using a real data set of reflectivity observed by the Monte Grande weather radar (Teolo, Italy) and rainfall intensity measured at five rain gauges in the Cortina d’Ampezzo area (Northern Italian Dolomites), during the June 12, 1997 storm event (from 11.15am to 12.00pm). Reflectivity data are given at 11 CAPPI levels with 15-minute time resolution. Rainfall intensity data are measured at 5-minute time resolution and are averaged over the 15-minute time intervals of radar data to constitute integrated measurements
On the determination of surface flow paths from gridded elevation data
No full textMultiple flow direction algorithms are commonly thought to be a useful means for determining drainage areas from gridded elevation data. It remains however unclear whether these algorithms can be used to describe surface flow paths and gravity-driven processes across a terrain without causing unrealistic artificial dispersion of flow. To explore this issue, a unified algorithm for the determination of flow directions is developed and new methods for the validation of surface flow paths are introduced. The unified algorithm allows, by setting appropriate parameters, to perform local or path-based analyses, and to experiment different combinations of single and multiple flow directions in a morphologically significant manner. The new validation methods use drainage systems delineated from contour elevation data as a reference and consider the overlapping between these drainage systems and the corresponding drainage systems obtained from gridded elevation data. A purely morphologic analysis is carried out. The obtained results suggest that dispersive methods may be preferred over nondispersive methods if the computation of the spatial pattern of drainage area, especially along divergent terrains, is the main focus. On the other hand, the results reveal that path-based nondispersive methods should be preferred over dispersive methods if the delineation of drainage systems and surface flow paths is an important focus. Path-based nondispersive methods are found to be a reliable means for the determination of surface flow paths from gridded elevation data, and to provide therefore a sound basis for the distributed description of gravity-driven processes. Future work is needed to formulate models of physical dispersion for water, sediments and solutes upon this purely morphologic basis
Effect of wind on precipitation intercepted by steep mountain slopes
No full textA simple and efficient formulation is developed to describe the effects of wind speed and directionon ground level precipitation intercepted by steep mountain slopes. A 3D rainfall field is reproduced frommeasurements of (vertical) precipitation intensity, wind speed, and wind direction. The Marshall-Palmer dropsize distribution is used to express the volumetric precipitation water content as a function of measured precipitationintensity, and raindrops are assumed to move horizontally with the measured wind speed and direction.Land topography is described using digital elevation model data, and local contributions to ground level precipitationare calculated as the interception of the obtained 3D rainfall field by horizontal and vertical surfacesthat constitute the elemental land surface systems. The developed formulation is tested at the Acquabona andFiames mountain slopes, located in the Northern Italian Dolomites, near the town of Cortina d’Ampezzo, wheredebris flow phenomena often occur. Simulation results are corroborated, although in an indirect and approximatemanner, by field estimates of debris flow volumes delivered by the considered mountain slopes in response tomonitored storm events. Although more accurate and comprehensive validation is needed, the developed formulationappears to constitute a useful diagnostic tool for providing interpretation of storm-flow hydrographsdelivered by steep mountain slopes in response to storm precipitations affected by wind
Diffusion wave modeling of distributed catchment dynamics
No full textA diffusion wave model of distributed catchment dynamics is presented. The effects of catchment topography and river network structure on storm-flow response are incorporated by routing surface runoff in cascade throughout a digital elevation model (DEM) based conceptual transport network, where the Muskingum-Cunge scheme with variable parameters is used to describe surface runoff dynamics. Dynamic scaling of hydraulic geometry is also incorporated in the model by using the 1953 “at-a-station” and “downstream” relationships by Leopold and Maddock. Numerical experiments indicate that the model is more than 98% mass conservative for possible slope and roughness configurations, which may occur for hillslopes in a natural catchment. Fluctuations in the simulated discharge may occur in response to discontinuities in rainfall excess representation if Courant number Cu during the simulation exceeds a threshold of about 3. Catchment scale simulations with different temporal resolution show that the model response is independent of structural parameters (model consistency). Also, the overall accuracy is preserved for computationally inexpensive space-time discretizations (for which Cu > 3) because fluctuations that may occur at the local scale are dampened when propagating downstream. Comparison of model results with observed outlet hydrographs of the Rio Missiaga experimental catchment (Eastern Italian Alps) show this approach to be capable of describing both overland and channel phases of surface runoff in mountainous catchments
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