Energy balance and evaporation loss of an irrigation reservoir equipped with a suspended cover in a semiarid climate (south‐eastern Spain)

The main objective of this study was to assess the impact of a suspended cover on the evaporation loss of an agricultural water reservoir (AWR). To this aim, a detailed data collection was carried out in a typical AWR located in south‐eastern Spain during 2 consecutive years. During the first year, the reservoir remained uncovered, while during the second year it was covered with a double black polyethylene (PE) shade cloth. On an annual scale, it was observed that the cover can provide a reduction of evaporation loss of 85%. Two approaches, energy balance and mass transfer, were used to analyse the effect of the cover on the evaporation process. Important modifications were observed on the magnitude, sign, annual trend and relative weight of the components of the energy balance. The changes were ascribed to the strong reduction of net radiation and to the substantial weight of the heat storage and sensible heat flux in the energy balance. A relevant finding was the contrast between the patterns of the annual evaporation curve for open‐water and covered conditions. The mass transfer approach allowed discriminating between the wind‐ and radiation‐shelter effects on the evaporation term. The reduction in water‐to‐air vapour deficit was the main factor explaining the high efficiency of the cover, whereas the reduction of the mass transfer coefficient was a modulating factor that accounted for the wind‐shelter effect. Overall, both approaches provided a sound basis to describe and explain the physical mechanisms underlying the high performance of the tested cover. Copyright © 2010 John Wiley & Sons, Ltd.     

Contrasting palaeosol development in two different tectonic settings: the Upper Buntsandstein of the Western Iberian Ranges, Central Spain

Palaeosols may offer excellent evidence for the development of sedimentary basins but few studies have used diagenetically altered material: here we show that material of this sort can also reflect the sedimentary environment. The Lower to Middle Triassic red beds of the western Iberian Ranges were deposited in a tectonically active half-graben in which subsidence rates varied along the basin as a response to differential fault movements. During this period the basin was filled by a set of fluvial units with interbedded palaeosols. The palaeosols show typical pedogenic calcrete profiles, although extensive dolomitization has deleted part of their microstructure; however, macrostructure and morphology are preserved. Differences in the maturity stages of the palaeosols are related to the changes in subsidence and sedimentation rates along the basin. Thus, two different scenarios are recognized at: (i) the hanging wall, Riba de Santiuste area, where palaeosols reach stage III as the episodic tilting of the floodplain inhibited the development of more mature soils and (ii) the footwall, Cercadillo area, where palaeosols attain stage V maturity, favoured by prolonged periods of tectonic stability resulting in lower sedimentation rates over the floodplain areas.     

Floodplain hydrology of the Mekong Delta,Vietnam

The Mekong Delta is one of the largest and most intensively used estuaries in the world. Each year it witnesses widespread flooding which is both the basis of the livelihood for more than 17 million people but also the major hazard. Therefore, a thorough understanding of the hydrologic and hydraulic features is urgently required for various planning purposes. While the general causes and characteristics of the annual floods are understood, the inundation dynamics in the floodplains in Vietnam which are highly controlled by dikes and other control structures have not been investigated in depth. Especially, quantitative analyses are lacking, mainly due to scarce data about the inundation processes in the floodplains. Therefore, a comprehensive monitoring scheme for channel and floodplain inundation was established in a study area in the Plain of Reeds in the northeastern part of the Vietnamese Delta. This in situ data collection was complemented by a series of high‐resolution inundation maps derived from the TerraSAR‐X satellite for the flood seasons 2008 and 2009. Hence, the inundation dynamics in the channels and floodplains, and the interaction between channels and floodplains, could be quantified for the first time. The study identifies the strong human interference which is governed by flood protection levels, cropping patterns and communal water management. In addition, we examine the tidal influence on the inundation in various parts of the Delta, since it is expected that climate change‐induced sea level rise will increase the tidal contribution to floodplain inundation. Copyright © 2011 John Wiley & Sons, Ltd.     

Tephra stratigraphy on the North Icelandic shelf: extending tephrochronology into marine sediments off North Iceland

Tephra stratigraphical and tephrochronological studies of marine core MD99‐2275 on the North Icelandic shelf have revealed 58 new tephra horizons within the last 7050 cal. a BP, bringing the total number of identified tephra layers to 76. So far, over 100 tephra layers have been identified in the entire core spanning the last 15 000 years. The majority of the newly identified tephra layers are basaltic in composition and originate from the most active volcanic systems in Iceland, namely Grímsvötn, Veidivötn‐Bárdarbunga and Katla. A total of 40 tephra layer land–sea correlations have been made within this time period, of which 16 represent absolutely dated tephra markers. In addition, two tephra marker series are revealed in the marine sediments and in the terrestrial tephra stratigraphy, located between c. 2300–2600 and between 5700–5900 years. For the last 15 000 years, 21 tephra markers have been recognized. The marine tephra layer frequency (TLF) reveals two peaks, within the last 2000 years, and between 5000 and 7000 years ago. It shows the same general characteristics as the terrestrial TLF curve in Iceland, which indicates that marine sediments can yield important information about volcanism in Iceland. This is useful in time segments in which terrestrial records are poor or non‐existent. The study contributes to a high‐resolution tephrochronological framework on the North Icelandic shelf, with core MD99‐2275 representing a potential stratotype section in the area, and for the northern North Atlantic–Nordic Seas region, as well as being an important contribution to the Lateglacial–early Holocene volcanic history of Iceland.     

Cohesive‐frictional model for bond and splitting action of prestressing wire

This paper presents a numerical procedure for bond between indented wires and concrete, and the coupled splitting process of the surrounding concrete. The bond model is an interface, non‐associative, plasticity model. It is coupled with a cohesive fracture model for concrete to take into account the splitting of such concrete. Bond between steel and concrete is fundamental for the transmission of stresses between both materials in precast prestressed concrete. Indented wires are used to improve the bond in these structural elements. The radial component of the prestressing force, increased by Poisson's effect, may split the surrounding concrete, decreasing the wire confinement and diminishing the bonding. The combined action of the bond and the splitting is studied with the proposed model. The results of the numerical model are compared with the results of a series of tests, such as those which showed splitting induced by the bond between wire and concrete. Tests with different steel indentation depths were performed. The numerical procedure accurately reproduces the experimental records and improves knowledge of this complex process. Copyright © 2010 John Wiley & Sons, Ltd.     

Vegetation in western Central Mexico during the last 50 000 years: modern analogs and climate in the Zacapu Basin

The late glacial and the transition towards the Holocene marked a period of significant environmental change at a global scale. In western Central Mexico, few records span beyond the Holocene, and little is known about Pleistocene climatic and environmental variability. Here we report on the pollen record of a composite sequence made of three cores that cover the last 50 ka (thousands of calibrated radiocarbon years before present) in the Zacapu Basin, western Central Mexico (～1970 m asl). The conjunction of modern pollen rain from the area with the composite fossil pollen sequence from cores Cantabria, Cantabria 1 and CEMCA Point‐1 provided a detailed history of the vegetation of the area. Recognition of modern environmental patterns through individual modern pollen taxa proved difficult, but multivariate analysis separated different vegetation types. No‐modern‐analog communities relative to the modern vegetation cover of the area were evident, especially during periods of drought. Apparently, summer precipitation was significant before 40 ka and after the deglaciation. However, this moisture source declined from ～40 to 10 ka because tradewinds from the Gulf of Mexico were blocked by the eastern mountains of Central Mexico as a result of lower temperatures. By contrast, westerly and northerly winds were enhanced by maximum ice cover from ～19 to 17 ka, providing abundant winter precipitation and therefore preventing extreme drought in the area. Copyright © 2012 John Wiley & Sons, Ltd.     

Sediment bioavailable organic matter,deposition rates and mixing intensity in the Setúbal–Lisbon canyon and adjacent slope (Western Iberian Margin)

The role of the Setúbal–Lisbon canyon in accumulation and transport of labile organic matter from the coastal sea and ocean surface water towards the deep sea was assessed by investigating the distribution of organic matter of different quality in sedimentary aggregates and surface sediments of the canyon and adjacent slopes. Total hydrolysable amino acids (THAA) and organic carbon (Corg) were measured from aggregates, and contents of Corg, chlorophyll a (chl a), phaeopigments (phaeo), chloroplastic pigment equivalents (CPE) from sediments. As indices of organic matter (OM) quality THAA:Corg, degradation index (DI), chl a:phaeo, chl a:Corg and C:N ratio were determined. Sediment profiles of chl a and the isotope 210 of lead (²¹⁰Pb) were used as tracers in a transport model to estimate deposition rates and background levels of the tracers, and sediment mixing rates (Db). Whereas bulk Corg contents of canyon and slope sediments were practically similar at all depths, higher contents of THAA, chl a and CPE, as well as higher THAA:Corg, DI and chl a:Corg, in aggregates and sediments from the upper reaches of the canyon indicate that labile organic matter accumulates in the upper canyon. This is confirmed by higher chl a and ²¹⁰Pb deposition and Db calculated from the model. Hence, the Setúbal–Lisbon canyon, specially the upper region, acts as a natural trap of organic matter that is transported to the region via lateral transport and vertical settling from primary productivity. Organic matter might be further transported in downward canyon direction via rebound processes. The chl a and ²¹⁰Pb profiles reveal active sediment mixing by physical processes and/or animal reworking.     

Coastal vulnerability in barrier islands: The high risk areas of the Ria Formosa (Portugal) system

Coastal vulnerability in barrier island systems is extremely high. The barrier island shoreline is exposed to many threats, such as storm erosion, reductions in sediment longshore drift and sea level rise. Many of these threats to coastal areas, such as Ria Formosa, are likely to increase in the near future. The main objectives of the present study are to identify the areas in the system that are at most risk by assessing the evolution of the Ria Formosa barrier island system since the 1940s, and determine which interventions would be necessary to protect these areas.The coastal vulnerability of the system was assessed based on current literature, data available from official institutions, and aerial and terrestrial in situ examination. The results obtained for the evolution of the Ria Formosa coastal system reveal that there are several vulnerable areas that have very dynamic processes, such as extremely high evolution rates of islands and inlets. Human actions that affect the whole system, namely the construction of many physical structures, are the main cause of the instability. Moreover, as several stakeholders are involved, efficient management of the coastal system is imperative. Extensive and careful interventions are urgently needed to avoid irreversible negative impacts on the barrier island system. Three types of techniques could be adopted to deal with the threats to barrier islands: hard stabilization, soft techniques, and non-structural alternatives. However, using hard techniques has many negative effects compared to soft techniques, and as a result it is necessary to remove existing structures from the areas at most risk.     

A mass‐balance approach to estimate in‐stream processes in a large river

A mass‐balance approach was used to estimate in‐stream processes related to inorganic nitrogen species (NH₄⁺, NO₂^? and NO₃^?) in a large river characterized by highly variable hydrological conditions, the Garonne River (south‐west France). Studies were conducted in two consecutive reaches of 30 km located downstream of the Toulouse agglomeration (population 760 000, seventh order), impacted by modification of discharge regime and high nitrogen concentrations. The mass‐balance was calculated by two methods: the first is based on a variable residence time (VRT) simulated by a one‐dimensional (1‐D) hydraulic model; the second is a based on a calculation using constant residence time (CRT) evaluated according to hydrographic peaks. In the context of the study, removal of dissolved inorganic nitrogen (DIN) for a reach of 30 km is underestimated by 11% with the CRT method. In sub‐reaches, the discrepancy between the two methods led to a 50% overestimation of DIN removal in the upper reach (13 km) and a 43% underestimation in the lower reach (17 km) using the CRT method. The study highlights the importance of residence time determination when using modelling approaches in the assessment of whole stream processes in short‐duration mass‐balance for a large river under variable hydrological conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

Jurassic‐to‐present thermal history of the central High Atlas (Morocco) assessed by low‐temperature thermochronology

Apatite fission track (AFT) and (U–Th)/He data from the High Atlas have been obtained for the first time to constrain the tectono‐thermal evolution of the central part of the chain. Results from Palaeozoic basement massifs indicate long residence at low temperatures, consistently with their original location out of the deepest Mesozoic rift troughs and indicating minor exhumation. The best rocks for extracting the Alpine history of the Atlas Mountains are Jurassic intrusives, which yield AFT ages centred on c. 80 Ma; thermal models based on AFT data and constrained by (U–Th)/He suggest that these ages are included in a slow cooling trend from intrusion age to c. 50 Ma ago that we attribute to post‐rift thermal relaxation. This is followed by a stability period of c. 30 Ma and then by a final exhumational cooling until present exposure. Eocene intrusives yield AFT ages similar to those of Rb–Sr and K–Ar suggesting rapid emplacement in the uppermost crust.