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Climate change of the Sardinian hydrology: the NAO impact on the precipitation and runoff regimes
No full textIn the last decades, climate change and human activities increased desertification process in Mediterranean regions, with dramatic consequences for agriculture and water availability. In Sardinia a dramatic reduction of water in dam reservoirs has been observed, due to the decrease of runoff.
The climate in Sardinia is typically Mediterranean maritime, characterized by a marked rainfall seasonality with wet winters and dry summers. The winter seasons play a key role for the dam water supply, and a systematic decrease of runoff during consequent years can dramatically impact on the management of the Sardinian water resources system. An analysis of the precipitation and runoff regimes for the whole Sardinia has been performed, highlighting the significant role of the coastal exposition and the orography.
We collect an innovative database of rainfall and runoff observations from 1922 to 2011, including data of more than 400 rain stations and 30 discharge stations. Hystorical trends are detected using the Mann Kendall, with a significance level of 5%, showing a decrease of the rainfall of the winter trimester (January-February-March) and, more marked, of the runoff, for the whole Sardinia generally. Interestingly, the decrease is more marked for the rain and discharge stations of the Sardinian west coast, which is exposed to the west european climate dynamics.
In this sense, several studies have shown a significant correlation between the main meteorological variables and indices related to fluctuations in global scale, for example NAO (North Atlantic Oscillation), which is a climatic phenomenon that represents the fluctuations in the difference of atmospheric pressure at sea level between the Icelandic low and the Azores high, and controls the direction and strength of westerly winds and storm tracks into Europe. A negative NAO brings to an increased storm activity and rainfall to southern Europe and North Africa. Finally, an analysis of hystorical storm tracks over the Mediterranean basin is also performed, for analysing the correlation between storm tracks and extreme events in Sardinia.
We found high negative correlations between NAO and precipitation and runoff during the winter season, and the correlations decrease with the increase of rain station longitudes. The results show a strong negative correlation at the stations and basins of the Sardinian west coast, which is due to the exposure to the mistral winds and the storm track dynamics. Instead a less negative correlation has been estimated for the east coast stations and basins due to the impact of the horography which attenuate the large scale atmospheric dynamics
Potential links between the North Atlantic Oscillation and decreasing precipitation and runoff on a Mediterranean area
No full textIn the Mediterranean region, the reduction in precipitation and warmer temperatures is generating a desertification process, with dramatic consequences for both agriculture and the sustainability of water resources. On the island of Sardinia (Italy), the decrease in runoff impacts the management of water resources, resulting in water supply restrictions even for domestic consumption. In the 10 Sardinian basins with a longer database (at least 40 complete years of data, including data from the past 10 years), runoff decreased drastically over the 1975â2010 period, with mean yearly runoff reduced by more than 40% compared to the previous 1922â1974 period. Trends in yearly runoff are negative, with Mann-Kendall Ï values ranging from â0.39 to â0.2. Decreasing winter precipitation over the 1975â2010 period everywhere on Sardinia island has led to these decreases in runoff, as most yearly runoff in the Sardinian basins (70% on average) is produced by winter precipitation due to the seasonality typical of the Mediterranean climate regime. The trend in winter precipitation is not homogenous; the negative trend is higher (around â0.25) on the west Sardinian coast, becoming lower across the island toward the east coast (around â0.14). Winter precipitation is highly correlated with the North Atlantic Oscillation (NAO), a weather phenomenon in the North Atlantic Ocean that controls the direction and strength of westerly winds and storm tracks into Europe. High negative correlations (up to â0.45) between winter NAO index and winter precipitation are estimated along the west coast. Meanwhile, these correlations decrease east across the island toward the high mountain in the center of Sardinia, reaching the lowest values along the east coast (about â0.25). The generally decreasing correlation between winter NAO index and winter precipitation in the longitudinal direction (from the North Atlantic dipole to the east) here accelerates due to local-scale orographic effects that overlap the large-scale NAO impact on the winter precipitation regime, thus softening the precipitation reduction due to the NAO. Such local topographic effects that may attenuate large-scale climate change effects must be considered in water resource planning and management alongside such climate change effects related to large-scale circulations, such as NAO
Trend storici e scenari di cambiamento climatico dei deflussi superficiali del bacino del Flumendosa tramite modellistica idrologica
No full textNegli ultimi decenni, i cambiamenti climatici e le attività dell’uomo, quali il cambiamento dell’uso del suolo e il sovra-sfruttamento delle risorse idriche stanno determinando un sempre più evidente processo di desertificazione che sta interessando le aree del Mediterraneo; con drammatiche conseguenze per l’agricoltura e la disponibilità idrica.
Nel sistema di invasi artificiali del Flumendosa, di fondamentale importanza per la gestione della risorsa idrica, in quanto soddisfa il fabbisogno idropotabile di Cagliari, si è evidenziato una riduzione del 35% dei deflussi annui. A fronte di una diminuzione dei deflussi è stato registrato un lieve decremento dei valori annui della precipitazione. Tali trend hanno messo in evidenza l’insostenibilità dell’ipotesi di stazionarietà del clima, e la necessità di tenere conto degli effetti di possibili scenari climatici futuri. Per la comprensione ed interpretazione dei processi idrologici in atto è necessario analizzare le dinamiche delle interazioni tra atmosfera, suolo e vegetazione, che influenzano il processo di trasformazione afflussi-deflussi.
In tal senso è stato sviluppato un nuovo modello ecoidrologico distribuito a scala di bacino, ottenuto dall’implementazione a scala di bacino del modello accoppiato di bilancio idrico (LSM) e di dinamica della vegetazione (VDM) di Montaldo et al. (2008). Infatti LSM stima per ciascuna cella di discretizzazione del bacino idrografico i flussi energetici e idrici, risolvendo le equazioni di bilancio idrico ed energetico, e l’infiltrazione tramite il metodo di Philips. Il VDM consente di simulare le complesse dinamiche della vegetazione andando ad effettuare il bilancio tra i termini che producono biomassa (fotosintesi) e i termini di consumo (respirazione, senescenza), e fornisce il valore del LAI utilizzato nel modello idrologico per la stima dell’evapotraspirazione e intercezione vegetale. L’implementazione a scala di bacino è stata realizzata includendo un modulo per il trattamento ed elaborazione del modello digitale delle quote e del reticolo idrografico ed uno per la propagazione del deflusso superficiale secondo il metodo Muskingum-Cunge a celerità variabile nella versione di Montaldo et al. (2007).
Il caso di studio considerato è il bacino del Flumendosa (bacino di circa 1200 kmq), bacino di primaria importanza per la gestione delle risorse idriche in Sardegna in quanto provvede al soddisfacimento del fabbisogno idrico di Cagliari. Infatti, la sezione di chiusura del bacino considerata è quella del medio Flumendosa in corrispondenza dei grandi bacini artificiali di Nuraghe Arrubiu (316 milioni m3) e Mulargia (347 milioni m3). Per tale bacino sono disponibili dal 2003 dati di portata di un idrometro a monte delle dighe a risoluzione oraria e i dati della stazione micrometeorologica sita nel Comune di Orroli, oltre ai dati delle 42 stazioni pluviografiche presenti nel bacino. Le informazioni sulla copertura vegetale sono state elaborate sulla base di una immagine satellitare Ikonos ad elevata risoluzione spaziale (2.8 m), mentre sono disponibili le mappe pedologiche e delle caratteristiche dei suoli. Per tale periodo è stata eseguita la calibrazione del modello, che è stato poi validato con i dati del periodo 1922-2002 per i quali sono disponibili
oltre i dati delle precipitazioni i dati giornalieri di portate in corrispondenza delle dighe. Dalla analisi del bilancio idrico del bacino si è dimostrato che il decremento del deflusso è dovuto al cambiamento del regime delle piogge con un decremento delle piogge durante i mesi invernali ed un incremento durante la primavera. L’incremento delle piogge durante la primavera ha generato esclusivamente un incremento del processo di traspirazione senza alcun impatto sul regime dei deflussi. Invece la riduzione delle piogge nel periodo invernale ha prodotto una notevole riduzione dei deflussi superficiali, determinando il conseguente decremento dei deflussi annui, fondamentali in quanto vengono accumulati nei principali invasi artificiali della Sardegna. Conseguentemente una riduzione dei deflussi va a generare una riduzione della risorsa disponibile per i centri abitati.
Si è poi eseguita la previsione dei deflussi superficiali futuri per gli anni 2081-2100 utilizzando scenari di cambiamento climatico. Infatti partendo dalle previsioni meteorologiche del IPCC (che prevedono decrementi della precipitazione fino al 20% nel trimestre Dicembre-Febbraio ed un aumento della temperatura fino a 3 °C) si è eseguita la previsione del bilancio idrico futuro del bacino idrografico, tenendo conto anche delle possibili modifiche della vegetazione, che ha conseguenze non solo sulla traspirazione ma anche sull’infiltrazione. Lo scenario climatico evidenzia una ulteriore riduzione dei deflussi superficiali futuri (di circa il 30%) e scenari di desertificazione con il decremento della percentuale di vegetazione sul bacino idrografico
A new eco-hydrological distributed model for the analysis of the climate change impact on water resources of Mediterranean ecosystems: the Flumendosa basin case study in Sardinia
No full textIn the last three decades, climate change and human activities increased desertification process in Mediterranean regions, with dramatic consequences for agriculture and water availability. For instance in the Flumendosa reservoir system in Sardinia the average annual runoff in the latter part of the 20th century was less than half the historic average rate, while the precipitation over the Flumendosa basin has decreased, but not at such a drastic rate as the discharge, suggesting a marked non-linear response of discharge to precipitation changes.
With the objective of analyzing and looking for the reasons of the historical runoff decrease a new ecohydrological model is developed and tested for the main basin of the Sardinia island, the Flumendosa basin.
The eco-hydrological model developed couples a distributed hydrological model and a vegetation dynamic model (VDM). The hydrological model estimates the soil water balance of each basin cell using the force-restore method and the Philips model for runoff estimate. Then it computes runoff propagation along the river network through a modified version of the Muskingum –Cunge method (Mancini et al., 2000; Montaldo et al., 2004). The VDM evaluates the changes in biomass over time from the difference between the rates of biomass production (photosynthesis) and loss (respiration and senescence), and provides LAI, which is then used by the hydrological model for evapotranspiration and rainfall interception estimates.
Case study is the Flumendosa basin (Sardinia, basin area of about 1700 km2), which is characterized by a reservoir system that supplies water to the main city of Sardinia, Cagliari. Data are from 42 rain stations (1922-2008 period) over the entire basin and data of runoff are available for the same period. The model has been successfully calibrated for the 1922 – 2008 period for which rain, meteorological data and discharge data are available.
We demonstrate that the hystorical strong decrease of runoff is due to a change of rainfall regime, with a decrease of rainfall during the winter months, and a little increase of rainfall during spring-summer months. Indeed, the higher Spring rainfall produced an increase of transpiration mainly, whithout any impact on runoff. Instead the decrease of rainfall in winter months produces a strong decrease of runoff. This trend impacts significantly on monthly runoff production, and, more important, on yearly runoff production, because most of the yearly runoff contribution comes from the winter months. Yearly runoff is more important in Sardinia water resources systems, because runoff is accumulated in dam reservoirs, and is the main water resources of the island. Hence, due to the change of rainfall regime in last decades we are observing a dramatic decrease of runoff, which is reaching to impact on the water availability of the Sardinian major city, Cagliari
A distributed hydrologic model for the estimation of the design flood and for the hydraulic safety of existing dams in Sardinia (Italy)
No full textThe recent floods observed in 2004 and 2008 in Sardinia (Italy) showed a significant flood increase in magnitude
and frequency and put the attention on the problem of the hydraulic safety of existing dams. Thirty dams built for
both electric production and water supply for irrigation and civil uses during the 1920-1960 are present in Sardinia
(Italy).
The distributed hydrologic model is an event model (FEST) which assesses runoff through a simplified approach
based on Soil Conservation Service equations and runoff propagation through the Muskingum-Cunge approach.
The FEST needs the calibration of different parameters: 1) the critical support area that defines the minimum
drainage area required to initiate a channel, 2) the ratio between cross-section width and flood flow depth for
hillslope, 3) the hillslope values of the Gaulckler-Stickler roughness coefficient, and 4) the Curve Number
obtained/extracted from the landscape characteristics of each basin.
The data collection of numerous historical flood observations of the Sardinian basins allow for the first time to
accurately calibrate an hydrologic model for the Sardinian basins, characterized by hortonian runoff mechanisms
typically, steep hillslopes and thin soils. Areas of the catchments are between 4 and 3147 Km2 and the basins are
characterized by an high variability of the landscape characteristics such as geology, topography, soil, land use
and vegetation.
Using the calibrated hydrologic model and the synthetic design hyetograph the design floods of the dams are
estimated. The comparison between peak flows of the generated design floods and those estimated using typical
regionalized statistical methods (based for Sardinian studies on two components extreme value (TCEV) and
logarithmic probabilistic distributions) shows contradiction and interesting results. For large scale basin (>
200 km2) the two contrasting methods agree, while for small basins the proposed method underestimates the
peak respect to the values estimated by the regionalized statistical methods, suggesting a strong revision of the
regionalized statistical methods now available over Sardinia
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Sulla variazione dei regimi idrologici nei bacini Sardi e l’impatto potenziale del futuro cambiamento climatico in un tipico bacino mediterraneo sardo
Full text linkIn the last three decades, climate change and human activities increased desertification process in Mediterranean regions, mainly due to a reduction in rainfall and even greater of runoff, with dramatic consequences for agriculture and water availability. Two key themes in the hydrological field, namely hydrological scale modeling and basin survey, and the study of climate and its impact on the territory have been studied. First, a historical analysis of weather data was carried out by verifying a negative trend of rains (with variability linked to wind and morphology) and even greater than the outflows (with reductions in the last thirty years between 35 and 60%) highlighting a marked rainfall-outflow non linearity, due both to flow-outflow processes and to variation in yearly rainfall seasonality; trying to understand the causes of such variations through the study of the main climatic patterns among which the North-Atlantic oscillation (NAO) and realizing an eco-hydrological model to understand the dynamics of the soil-vegetation-atmosphere system at local (cell and basin) scale. The aim of this thesis is to use a calibrated and validated eco-hydrological model (for the Mulargia basin) to have a tool for predicting future outflows through the generation of future climatic scenarios obtained through a Stochastic analysis of historical weather data, Based on the studies proposed by the IPCC, highlighting for Mulargia a reduction of future outflows, especially during the winter, with a reduction much more drastic then the reduction evaluate for the same basin considering the hystorical data. Learn how the system responds to climate change scenarios and how the outflows and water levels that are expected to have in the main invasions will vary and will provide a useful tool for the governance of the territory
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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