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Distributed hydrologic modeling of a sparsely monitored basin in Sardinia, Italy, through hydrometeorological downscaling
Full text linkThe water resources and hydrologic extremes in Mediterranean basins are heavily influenced by climate variability. Modeling these watersheds is difficult due to the complex nature of the hydrologic response as well as the sparseness of hydrometeorological observations. In this work, we
present a strategy to calibrate a distributed hydrologic model, known as TIN-based Real-time Integrated Basin Simulator
(tRIBS), in the Rio Mannu basin (RMB), a medium-sized
watershed (472.5 km2) located in an agricultural area in Sardinia, Italy. In the RMB, precipitation, streamflow and meteorological data were collected within different historical periods and at diverse temporal resolutions. We designed two statistical tools for downscaling precipitation and potential
evapotranspiration data to create the hourly, high-resolution forcing for the hydrologic model from daily records. Despite the presence of several sources of uncertainty in the observations and model parameterization, the use of the disaggregated forcing led to good calibration and validation performances for the tRIBS model, when daily discharge observations were available. The methodology proposed here can be also used to disaggregate outputs of climate models and conduct high-resolution hydrologic simulations with the goal of quantifying the impacts of climate change on water resources and the frequency of hydrologic extremes within medium-sized basins
Hydrological impact of climate change in a Mediterranean catchment with limited data availability
No full textThe Mediterranean basin is one of the areas of the world where climate changes due to global warming are expected
to be more significant. Future scenarios predicted by global and regional climate models (GCMs and RCMs)
indicate a decrease in water availability, which will lead to social and economic consequences, mainly affecting the agricultural sector. Reducing the uncertainty in the quantification of the climate changes impacts in Mediterranean watersheds is the main goal of the Climate Induced Changes on the Hydrology of Mediterranean Basins (CLIMB) project, funded by the 7th EU Framework Programme. One of the study sites of CLIMB is the Rio Mannu at Monastir
(473 km2) basin, located in an agricultural area of Southern Sardinia, Italy, that has experienced severe drought periods during the last 30 years with dramatic decreases of crop productivity. To quantify the climate change impacts,
outputs of four RCMs are used to force a distributed and physically-based hydrologic model, known as TIN-based Real time Integrated Basin Simulator (tRIBS). In this study, we first illustrate the tRIBS model calibration,
using the limited dataset available in the Rio Mannu basin, a common feature in most regions of the world.
In our study site, hydrometeorological data (streamflow, precipitation, temperature and meteorological variables)
are available at different resolution and during non-overlapping periods. To create the database at hourly resolution required for tRIBS application, we designed two downscaling strategies, aimed at (i) disaggregating precipitation from daily to hourly resolution using a multifractal model, and (ii) obtaining reference evapotranspiration at hourly
time scale from daily records of minimum and maximum temperature.We demonstrate how the downscaling tools
are able to generate a reliable database to calibrate the hydrologic model, and how they can be used to disaggregate
coarse outputs of climate models. In a second part of the study, the downscaled outputs of the RCMs are used
to force the calibrated hydrologic model for the future (from 2041 to 2070) and reference (from 1971 to 2000) periods. The impacts on water budget and availability in the Rio Mannu basin under the future climatic scenarios
is quantified through several techniques and metrics computed from the time series and the spatial maps of the
hydrologic variables simulated in the two periods
Impacts of climate change on precipitation and discharge extremes through the use of statistical downscaling approaches in a Mediterranean basin
No full textMediterranean region is characterized by high precipitation variability often enhanced by orography, with strong seasonality and large inter-annual fluctuations, and by high heterogeneity of terrain and land surface properties. As a consequence, catchments in this area are often prone to the occurrence of hydrometeorological extremes, including storms, floods and flash-floods. A number of climate studies focused in the Mediterranean region predict that extreme events will occur with higher intensity and frequency, thus requiring further analyses to assess their effect at the land surface, particularly in small- and medium-sized watersheds. In this study, climate and hydrologic simulations produced within the Climate Induced Changes on the Hydrology of Mediterranean Basins (CLIMB) EU FP7 research project were used to analyze how precipitation extremes propagate into discharge extremes in the Rio Mannu basin (472.5 km2), located in Sardinia, Italy. The basin hydrologic response to climate forcings in a reference (1971-2000) and a future (2041-2070) period was simulated through the combined use of a set of global and regional climate models, statistical downscaling techniques, and a process based distributed hydrologic model. We analyzed and compared the distribution of annual maxima extracted from hourly and daily precipitation and peak discharge time series, simulated by the hydrologic model under climate forcing. For this aim, yearly maxima were fit by the Generalized Extreme Value (GEV) distribution using a regional approach. Next, we discussed commonality and contrasting behaviors of precipitation and discharge maxima distributions to better understand how hydrological transformations impact propagation of extremes. Finally, we show how rainfall statistical downscaling algorithms produce more reliable forcings for hydrological models than coarse climate model outputs
Investigating parameter transferability across models and events for a Semiarid Mediterranean Catchment
Full text linkPhysically based distributed hydrologic models (DHMs) simulate watershed processes by applying physical equations with a variety of simplifying assumptions and discretization approaches. These equations depend on parameters that, in most cases, can be measured and, theoretically, transferred across different types of DHMs. The aim of this study is to test the potential of parameter transferability in a real catchment for two contrasting periods among three DHMs of varying complexity. The case study chosen is a small Mediterranean catchment where the TIN-based Real-time Integrated Basin Simulator (tRIBS) model was previously calibrated and tested. The same datasets and parameters are used here to apply two other DHMs-the TOPographic Kinematic Approximation and Integration model (TOPKAPI) and CATchment HYdrology (CATHY) models. Model performance was measured against observed discharge at the basin outlet for a one-year period (1930) corresponding to average wetness conditions for the region, and for a much drier two-year period (1931-1932). The three DHMs performed comparably for the 1930 period but showed more significant differences (the CATHY model in particular for the dry period. In order to improve the performance of CATHY for this latter period, an hypothesis of soil crusting was introduced, assigning a lower saturated hydraulic conductivity to the top soil layer. It is concluded that, while the physical basis for the three models allowed transfer of parameters in a broad sense, transferability can break down when simulation conditions are greatly altered
Sensitivity of the hydrological response in a Mediterranean catchment to different climate model forcing
No full textThe Climate Induced Changes on the Hydrology of Mediterranean Basins (CLIMB) Project is a multi-institutional research project, funded by 7th EU Framework Programme, which has the main goal of reducing uncertainties in the quantification of climate change impact in Mediterranean basins. Current studies including IPCC indicate, in fact, that the water budget in these areas will be very likely affected by climate change, with severe impacts on agricultural productivity and drinking water supply. One of the CLIMB study sites is the Rio Mannu at Monastir, a catchment located in an agricultural area in southern Sardinia (Italy) with gently rolling topography. In this study, we show preliminary results on the sensitivity of hydrological response in this basin under climatic changes conditions. For this aim, outputs of several climate models are used to force the TIN-based Real-time Integrated Basin Simulator (tRIBS), a fully distributed, physically based model able to continuously simulate hydrological processes occurring in a basin, by explicitly taking into account variability of meteorological forcing and basin properties. We first present results of the calibration effort, based on a relatively limited dataset consisting of: (i) hydrometeorological data available over 26 years in the period 1925-1964 and including daily rain gage observations, daily streamflow data at the outlet and temperature observations from four stations, (ii) a 10-m Digital Elevation Model, (iii) a digitized soil texture map, (iv) the CORINE land cover map. Once calibrated, we use tRIBS to simulate the hydrological response in the Rio Mannu basin, under a number of climate change scenarios, generated by several numerical climate models collected by the PRUDENCE project of the FP5, the ENSEMBLES project of the FP6 and the US project PCMDI/CMIP3. Finally, we present and discuss preliminary comparisons and analysis of the hydrological impacts of the different climatic scenarios
Evaluation of climate change effects on the hydrology of a medium-sized Mediterranean basin affected by data sparseness
No full textMany studies based on global and regional climate models agree on the prediction that the Mediterranean area will be most likely affected by climate changes with consequent reduced water availability and intensified hydrologic extremes. This study evaluates the effects of climate changes on the hydrologic response of a medium-sized Mediterranean basin through downscaling techniques and hydrologic simulations. The watershed is the Rio Mannu at Monastir basin (473 km2), located in an agricultural area of southern Sardinia, Italy, which has suffered drought
issues in the last decades. It is one of the seven study cases of a multidisciplinary European research project, CLIMB (Climate Induced changes on the Hydrology of Mediterranean Basins). In such basins, characterized by
strong climate variability and by a complex hydrologic response, process based distributed hydrologic models,
DHMs, combined with regional climate models, RCMs, and downscaling techniques can help in the evaluation of the local impacts of climate change on water resources decreasing the uncertainty. Since the Rio Mannu basin
is affected by data sparseness (meteorological and streamflow data are collected in non overlapping time periods and at diverse time resolutions), two statistical downscaling strategies for precipitation and potential evapotranspiration have been designed which allow to obtain the high-resolution input data required for the calibration of our hydrologic model, the TIN-based Real time Integrated Basin Simulator (tRIBS). We show how the DHM has been calibrated and validated with reasonable accuracy using the disaggregation tools. Next, the same downscaling algorithms have been used to fill the resolution discrepancy between RCMs and the hydrologic model. The
outputs of four RCMs, selected as the best performing and bias corrected within the CLIMB project, have been downscaled and used to force the tRIBS during a reference (1971-2000) and a future (2041-2070) period. Several hydro-climatic indicators have been computed based on the time series and spatial maps produced by the DHM to assess the variation in Rio Mannu water resources budget and hydrologic extremes in the future period as compared to the reference one. Our results confirms what is generally predicted for the Mediterranean area, showing a basin future condition of more water shortages due to both reduced precipitations and increased temperatures
Valutazione dell’impatto del cambiamento climatico sulla risposta idrologica di un bacino scarsamente monitorato della Sardegna attraverso simulazioni idrologiche distribuite e downscaling idrometeorologico
No full textIl Mediterraneo è considerato un’area a rischio a seguito dei cambiamenti climatici
specialmente per quanto riguarda la disponibilità di risorse idriche e l’intensificazione degli
eventi estremi, come dimostrato da numerosi studi basati su modelli climatici globali e regionali (Giorgi, 2006; IPCC, 2007 e Cudennec et al., 2007). In questo lavoro sono stati valutati gli effetti dei cambiamenti climatici sulla risposta idrologica di un bacino del
Mediterraneo di medie dimensioni, affetto da una scarsa disponibilità di dati, attraverso l’utilizzo di tecniche di downscaling e simulazioni idrologiche distribuite.
Il bacino considerato è il Rio Mannu di San Sperate, localizzato in un’area agricola della Sardegna meridionale. Esso rientra tra i sette casi di studio del progetto di ricerca FP7 CLIMB
(CLimate Induced Changes on the hydrology of Mediterranean Basins, Ludwig et al., 2010), all’interno del quale è stato sviluppato questo lavoro. Il Rio Mannu, che drena un’area di
472.5 km2, ha sofferto diversi periodi di siccità negli ultimi decenni del secolo scorso con rilevanti perdite nel settore agricolo e turistico e può, pertanto, essere ritenuto un interessante caso di studio nell'area mediterranea.
Per simulare la complessa risposta idrologica di questo bacino, è stato utilizzato il modello distribuito TIN-based Real-time Integrated Basin Simulator, tRIBS (Ivanov et al. 2004a, b). Come spesso accade anche in altri bacini, l'applicazione di modelli idrologici che simulano
fisicamente i processi, come il tRIBS, è però ostacolata dalla limitata disponibilità di dati idrometeorologici. Nella fattispecie, nel bacino in esame, i dati meteorologici e di deflusso
osservati sono stati raccolti in periodi di tempo non sempre coincidenti e con diverse risoluzioni temporali. Inoltre l’utilizzo degli output dei modelli climatici, tipicamente
disponibili con passo temporale giornaliero e risoluzioni spaziali ampie, come forzanti di modelli idrologici distribuiti, con passi di integrazione temporale e spaziale assai più piccoli, richiedono l’adozione di procedure di downscaling per trasferire correttamente le informazione tra le diverse scale. Per affrontare queste problematiche è stata proposta una nuova metodologia basata sull'applicazione di due strategie di downscaling che hanno permesso di creare le forzanti (precipitazione ed evapotraspirazione potenziale) ad elevata risoluzione temporale (1 ora) e spaziale (qualche chilometro), necessarie per le simulazioni idrologiche. Le procedure di downscaling proposte sono state utilizzate sia nella fase di calibrazione con i dati osservati giornalieri, che in quella di simulazione con le forzanti climatiche. Per le precipitazioni, è stata usata una tecnica di downscaling statistico basata sul modello multifrattale STRAIN (Deidda et al., 1999, Deidda, 2000, Badas et al., 2006). Partendo dall’informazione sulla precipitazione prevista o osservata a scale spazio-temporali estese il modello multifrattale genera in modo stocastico diverse realizzazioni ugualmente probabili dello stesso evento a piccola scala. In questo lavoro, la tecnica di downscaling è stata calibrata con dati di precipitazione ad elevata risoluzione temporale misurati da 208 pluviometri su un dominio di 104 km x 104 km, che contiene il bacino in studio. E’ stato pertanto possibile disaggregare la
pioggia misurata su un dominio di grande scala spaziotemporale di 104 km x 104 km x 24 h fino alla piccola scala di 13 km e 1 h. Per l’evapotraspirazione potenziale è stata invece
elaborata una procedura che consente di ottenere stime a scala oraria partendo dalle
temperature giornaliere minime e massime. A questo scopo, sono state usate le formule di Penman-Monteith (Allen et al., 1989, 2006) e di Hargreaves (Hargreaves, 1994; Hargreaves and Allen, 2003) per stimare funzioni adimensionali che rappresentano il ciclo diurno del processo mese per mese.
Usando i dati disaggregati come input, il modello idrologico è stato calibrato e validato in un periodo di tre anni, durante i quali i dati di deflusso disponibili presentavano le minori incertezze. La calibrazione è stata effettuata manualmente cambiando i parametri che più
condizionavano il modello tRIBS nel caso in studio. Le prestazioni sono state valutate in termini di coefficienti di Nash-Sutcliffe tra volumi di deflusso osservati e simulati. I risultati
ottenuti hanno permesso di affermare che, nonostante le diverse fonti di incertezza nelle osservazioni e nella parametrizzazione del modello, l’uso delle strategie di downscaling ha
consentito di ottenere delle buone prestazioni del modello nei periodi di calibrazione e
validazione (Mascaro et al., 2013).
Le strategie di downscaling sono state adottate anche per trasferire gli output di diversi modelli climatici regionali alla scala spazio-temporale richiesta dal modello tRIBS al fine di
valutare gli impatti dei cambiamenti climatici nel ciclo idrologico del bacino in esame. In particolare, sono stati dapprima selezionati i quattro migliori modelli climatici regionali per le aree di interesse, confrontando criticamente le prestazioni dei modelli del progetto
ENSEMBLES (Deidda et al., 2013). Gli output dei quattro modelli climatici selezionati sono stati quindi usati come forzanti di simulazioni idrologiche durante un periodo di riferimento
(1971-2000) e un periodo futuro (2041-2070). La risposta del bacino del Rio Mannu ai cambiamenti previsti dal set di modelli climatici è stata valutata elaborando le serie temporali e le mappe spaziali fornite dal modello tRIBS. Questo ha permesso di quantificare gli impatti
nella disponibilità di risorse idriche e negli eventi estremi che si potranno verificare nel
bacino nel periodo futuro. Le variazioni nelle forzanti climatiche (diminuzione delle
precipitazioni e aumento delle temperature) influenzano le diverse componenti della risposta
idrologica del bacino mostrando anche la sua non linearità. I risultati delle simulazioni
idrologiche, infatti, hanno indicato per il periodo futuro: (i) una diminuzione del deflusso medio annuo, con variazioni nel meccanismo di generazione, confermato da uno spostamento verso il basso delle curve di durata del deflusso in corrispondenza di tutte le probabilità di superamento; (ii) un abbassamento dell'evapotraspirazione reale, dovuta probabilmente alla ridotta umidità del suolo; (iii) una riduzione del livello medio della falda
acquifera. In breve, è previsto che il bacino subirà maggiori ristrettezze idriche nel futuro con suolo asciutto per periodi più lunghi pur avendo quasi le stesse perdite di evaporazione del
periodo di riferimento
Quantification of hydrologic impacts of climate change in a Mediterranean basin in Sardinia, Italy, through high-resolution simulations
Full text linkFuture climate projections robustly indicate that the Mediterranean region will experience a significant decrease of mean annual precipitation and an increase in temperature. These changes are expected to seriously affect the hydrologic regime, with a limitation of water availability and an intensification of hydrologic extremes, and to negatively impact local economies. In this study, we quantify the hydrologic impacts of climate change in the Rio Mannu basin (RMB), an agricultural watershed of 472.5 km2 in Sardinia, Italy. To simulate the wide range of runoff generation mechanisms typical of Mediterranean basins, we adopted a physically based, distributed hydrologic model. The high-resolution forcings in reference and future conditions (30-year records for each period) were provided by four combinations of global and regional climate models, bias-corrected and downscaled in space and time (from ~25 km, 24 h to 5 km, 1 h) through statistical tools. The analysis of the hydrologic model outputs indicates that the RMB is expected to be severely impacted by future climate change. The range of simulations consistently predict (i) a significant diminution of mean annual runoff at the basin outlet, mainly due to a decreasing contribution of the runoff generation mechanisms depending on water available in the soil; (ii) modest variations in mean annual runoff and intensification of mean annual discharge maxima in flatter sub-basins with clay and loamy soils, likely due to a higher occurrence of infiltration excess runoff; (iii) reduction of soil water content and actual evapotranspiration in most areas of the basin; and (iv) a drop in the groundwater table. Results of this study are useful to support the adoption of adaptive strategies for management and planning of agricultural activities and water resources in the region
Validation of hydrological models: conceptual basis, methodological approaches and a proposal for a code of practice
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