Comparing the Hydraulic Properties of Forested and Grassed Soils on an Experimental Hillslope in a Mediterranean Environment

AbstractThis experimental research compares the physical and hydraulic properties of two adjacent soils, one covered with a native forest of Mediterranean maquis, and the other with spontaneous grass. The latter replaced the previous natural forest. The aim is to quantify the significant differences in the soil properties caused by the removal of the natural vegetation. Although the soil texture was similar in the different land uses, the soil under the forest had a higher organic matter content, a lower apparent density and a higher water content at saturation than the grassed soil. The analysis of the water retention characteristics indicated that the retained water content of the forest soil exceeded that of the grassed soil in the range from saturation to -50cm of water tension. This suggests that changing the land use altered the soil pore structure within this range. The hydraulic conductivity of the forest soil exceeded that of the grassed soil at water tensions of -10, -5 and -3cm. Conversely the hydraulic conductivity of the grassed soil was similar to that of the forest soil at -1cm of water tension and at saturation. This result was probably due to the hydraulic activation of the desiccation cracks in the grassed soil. This increased the amount of infiltrated water in saturated and near-saturated soil conditions.This work shows that changes in land use have an unfavorable impact on the physical and hydraulic properties of the soil. Soil covered with grass is more vulnerability to water erosion than that under forest, and there is likely to be general worsening of flow regimes

Evaporation in a Mediterranean environment by energy budget and Penman methods, Lake Baratz, Sardinia, Italy

Abstract. In Mediterranean environments, evaporation is a key component of lake water budgets. This applies to Lake Baratz in Sardinia, Italy, a closed lake that almost dried up in 2008 after a succession of years with low seasonal rainfall. We used the energy budget method and Penman's equation to estimate evaporation over Lake Baratz. We measured, using a raft station, water temperature at the surface, at 1, 2, 4, 6 m depth and at the bottom of the lake, as well as air temperature, relative humidity, wind speed, and net radiation over a period of 3 years. We also compared Penman's equation and the energy budget method in two other climatic zones using published data. Our results indicate that mean yearly evaporation over Lake Baratz was 950 mm. On an annual scale, evaporation estimated by Penman's method omitting heat storage as is usually done was 18% higher than by the energy budget method that included heat storage, with monthly differences ranging between −38 and +60%. Including the heat storage term in Penman's equation changed the monthly values but did not change the yearly value significantly. Solar radiation and heat storage were found to be the most important energy fluxes to and from the lake and had the greatest effect on evaporation rates for the energy budget method. The bias between the two methods has a seasonal cycle due to the storage and release of energy from the lake. Energy advected to and from the lake by precipitation, surface water and ground water had minor effect on evaporation rates. Lake Baratz, like other lakes in a Mediterranean environment, is particularly sensitive to the summer hot and dry climate. In contrast, we found that rates of evaporation estimated from Penman and the energy budget methods over tropical African lakes were nearly constant over the entire year and the difference between the two methods smaller. Difference between the two methods for North American lakes is also smaller probably owing to the ice-cover season and to lower radiation and lower temperatures during summer

Osservazione e modellazione del deflusso sottosuperficiale laterale saturo in un ripido versante naturale

I versanti sono unità morfologiche fondamentali che governano la risposta idrologica dei bacini idrografici durante le precipitazioni più intense. Nonostante la loro riconosciuta importanza idrologica, è ancora necessario condurre approfonditi studi, sia a carattere sperimentale che teoretico, per meglio comprendere i meccanismi di generazione e trasporto del deflusso sottosuperficiale laterale saturo nei versanti. In questo contributo sono presentati i risultati di un monitoraggio di lungo periodo del flusso laterale di falda intercettato per mezzo di una trincea drenante installata su d’un ripido versante naturale nel bacino del Lago di Baratz, Sardegna. I dati raccolti vengono utilizzati per sviluppare una relazione di tipo esponenziale tra la conducibilità idraulica laterale satura (KS) e lo spessore della falda (T), valida alla scala di versante. I flussi osservati e i livelli di falda sono simulati con un modello numerico orizzontale 1D, basato sulla soluzione simultanea della legge del moto e dell’equazione di bilancio idrico, e implementato tramite uno schema risolutivo alle differenza finite. Le principali assunzioni del modello sono che nel suolo il flusso sottosuperficiale saturo sia regolato dalla legge di Darcy, e che si verifichi sempre l’istantaneo raggiungimento di un profilo di equilibrio idrostatico del carico di suzione nella zona vadosa. Il modello richiede la specifica della curva di ritenzione idrica del suolo e della relazione KS(T) ottenute sperimentalmente. I risultati della simulazione indicano che il modello è in grado di riprodurre in modo adeguato i dati osservati. Il flusso di falda è ben riprodotto, sia in termini di tempi di risposta che di portate drenate. Pertanto, il modello sviluppato può essere un utile strumento di previsione della risposta idrologica in versanti naturali a forte pendenza

SCHEMA SPERIMENTALE PER LA STIMA DELLA CONDUCIBILITÀ IDRAULICA LATERALE ALLA SCALA DI VERSANTE

La conducibilità idraulica alla saturazione, KS, è un parametro fondamentale per la simulazione del flusso idrico nel suolo. La memoria illustra un approccio sperimentale per la stima della KS laterale alla scala di versante. La metodologia proposta è stata applicata in due aree contigue ricoperte, rispettivamente, da macchia mediterranea e prato. Sono stati registrati i livelli di falda e i deflussi idrici sottosuperficiali, e queste misure sono state usate per il calcolo della KS del suolo tramite l’equazione di Darcy. Il monitoraggio è stato effettuato durante il periodo piovoso compreso tra gennaio e giugno 2014. In aprile, inoltre, sono state eseguite prove di pioggia artificiale con intensità di 30 e 70 mm h-1. Durante le precipitazioni naturali, il valore massimo di KS stimato nel prato è stato di 2870 mm h-1, mentre è stato pari a 2400 mm h-1 nel corso delle prove di saturazione artificiale. Il valore massimo di KS pari 4000 mm h-1 è stato ottenuto, nella macchia, durante le prove di pioggia artificiale. Per contro, valori minori di KS sono stati ottenuti durante le piogge naturali a causa delle basse portate sottosuperficiali registrate. La metodologia proposta è risultata idonea per la stima di valori della conducibilità idraulica rappresentativi per le aree di interesse. Tali informazioni potranno garantire una maggiore attendibilità della modellizzazione dei processi idrologici a scala di versante e di bacino

Short-term impact of Coppice management on soil in a Quercus ilex l. stand of Sardinia

The short-term impact of coppice-with-standards management on soil in a Mediterranean holm oak forest was assessed to contribute to address appropriate recommendations to minimize possible negative effects of the silvicultural practices. For this purpose, soil surface features and topsoil properties were investigated in two representative areas located in a public forest in southwest Sardinia (Italy) and coppiced in the periods November 2012-March 2013 and November 2011-March 2012, respectively. Regardless of differences in soils and slope gradient, the same management, in terms of final density of trees standing after the clear-cut and accumulation of brushwood in strips along the maximum slope gradient, was applied in both areas. Field observations and laboratory data highlighted the disturbances caused to the soil by the silvicultural practices in the stands when compared with the undisturbed stands. These disturbances involved the almost complete removal of organic horizons, with consequent negative impact on organic carbon content, and the activation of erosion processes, mostly related to rainsplash erosion. Although soil mobilization locally largely exceeded the tolerable erosion rates, no extreme rainfall events occurred after the coppicing to produce critical situations at the catchment level. The adjustment of the final density of trees standing after the clear-cut in relation to soil properties, slope gradient and the possibility of extreme rainfall events, a different brushwood management and the restriction to the passage of wild animals would have strongly reduced the negative impacts on soils

Flash Flood simulation and valve behavior of Mytilus galloprovincialis measured with Hall sensors

Mussels close their shell as a protective strategy and the quantification of this behavioral marker may represent an alarm signal when they are exposed to environmental stressors. In the present study, we investigated the ability of the Mediterranean mussel Mytilus galloprovincialis to recover and then the resilience or inertia of valve activity after a pulsing exposition to diverse levels of salinity (5, 10, 20 and 35 PSU as reference value). The trial simulated an event of drastic and sudden reduction of seawater salinity thus mimicking an event of Flash Flood from intense rain. Valve gaping and movements were measured in continuous cycle for ten days using a customized magneto-electric device which uses Hall sensors. Results showed that under normal conditions of salinity (35 PSU) the general pattern of valve movements was a continuously open state with sporadic spikes indicating a closing motion. At salinity of 5 PSU mussels reacted by closing their valves, leading to a 77% mortality on the fourth day. At salinity of 10 PSU animals were observed with closed valves for the entire duration of the exposure and no mortality occurred, they showed a significant reduction in the valve activity once the reference value of salinity was re-established. In contrast, salinity of 20 PSU did not trigger a significant behavioral response. Interestingly, there no define rhythms of valve movements were recorded during salinity challenges

Tidal, seiche and Wind Dynamics in a Small Lagoon in the Mediterranean Sea

Environmental monitoring and hydrodynamic studies were carried out to understand the forcing mechanisms that are responsible for the circulation patterns of the several small coastal lagoons of Sardinia, in the center of the Mediterranean sea, Italy. The water level observed in the Calich lagoon showed a free-surface oscillation with a semidiurnal period (tide) and a second order oscillation with higher frequency and lower amplitude (seiche). The wind has a local effect on these oscillations and generates a secondary circulation. Numerical analysis confirmed that the dynamic of the lagoon is mainly controlled by a balance between accelerations, barotropic pressure gradients, and bed and wind stresses, and highlighted that the surface seiche is one of the mechanisms responsible for driving residual currents. The employed 2D-horizontal model of lagoon hydrodynamics, forced with tides only at the basin boundary, reproduced the natural mode oscillation period of 0.5 h of the basin. Besides, stronger seiches were simulated by strength and variability in the wind field, in close agreement with the measured strong seiche and wind data. Our study evaluated the accuracy of the description of the seiche and wind effects in the formulation of a plane 2D model. The 5 min time step of our monitoring was, however, too large to quantify accurately the relationship between residual seiche-induced circulation and wind stress. Simulation results indicate that the wind variations at the time scale of minutes could affect the strength of the oscillation. Nevertheless, this variability had insignificant effect on the frequency, suggesting that the oscillatory response is predominantly linear. This analysis shows that tidal, atmospheric and seiche forcing mechanisms are responsible for the circulation patterns of these small Mediterranean wetland areas, and generate water level oscillations with specific frequencies but with amplitudes of the same order

STUDIO DEL FLUSSO INSATURO NELLE CALCARENITI: CARATTERIZZAZIONE IDRAULICA, MONITORAGGIO IN CAMPO E SIMULAZIONI NUMERICHE

Un sito per lo studio sperimentale delle interazioni tra il substrato roccioso calcareo e i processi idrologici superficiali e sottosuperficiali \ue8 stato strumentato nella Sardegna nord-occidentale, in un ambiente tipicamente mediterraneo. Lo schema sperimentale consiste nel monitoraggio integrato dell'umidit\ue0 del suolo e della tensione idrica nel suolo e nel substrato calcareo insaturo per il primo metro di profondit\ue0. La caratterizzazione idraulica del calcare \ue8 stata eseguita in laboratorio tramite una metodologia ampiamente utilizzata nell'idrologia del suolo. Il monitoraggio in campo ha messo in evidenza che il flusso idrico nella roccia insatura pu\uf2 verificarsi attraverso la matrice calcarea e attraverso le fratture, nella forma di flusso preferenziale. L'applicazione del modello di simulazione a permeabilit\ue0 duale MACRO 5.0 suggerisce che il contributo della matrice porosa della roccia calcarea ai processi di ricarica della falda \ue8 quantitativamente paragonabile al contributo delle fratture

APPLICAZIONE DI UN MODELLO 2D PER ACQUE POCO PROFONDE PER LA SIMULAZIONE DI INONDAZIONI FLUVIALI

Uno schema risolutivo ADI-lagrangiano delle equazioni per acque poco profonde in forma completa, insieme con un algoritmo di "wetting-drying" e con un supporto topografico basato sul DEM, \ue8 stato applicato per simulare l'inondazione fluviale di una piana alluvionale. La simulazione \ue8 stata condotta su un'area di circa 15 km2, suddivisa con celle di lato pari a 5 m, con un passo temporale di 1 s e con una portata di piena di 1500 m3/s. L'esondazione nell'ampia area golenale in corrispondenza di una morfologia complessa, situazione che rende l'approssimazione 1D fortemente carente, \ue8 stata efficacemente rappresentata dal modello di simulazione 2D. La forte pendenza del profilo in alcune sezioni ha evidenziato l'influenza dei termini inerziali delle equazioni per acque poco profonde nella propagazione del fronte dell'onda