Propagation transhorizon au-dessus de la Méditerrannee Sud-Orientale (Mer de Libye)

Sommaire Une étude radiométéorologique en mer close, a été initiée par le Groupe Radiométéorologique de l'OTAN. Pour completer les resultats des mesures au dessus du Canal d'Otranto (Martina Franca-Corfou) publiées en collaboration avec le Groupe Italien du Service de l'Aeronautique Militaire, des mesures au dessus de la mer de Libye sur la partie SE de la Mediterranée, d'une liaison transhorizon de 725 km entre le Caire et la ville d'Ierapetra, ont été enterprises.Pour expliquer le mecanisme de cette propagation transhorizon, on a appliqué une méthode d'analyse, basée sur les courbes de distribution du champs reçue en fonction du pourcentage du temps, indiquée par l'un des auteurs (Anastassiadis). Les courbes de distribution Rayleigh ou SuperRayleigh, obtenues pendant la période chaude, indiquent un mécanisme de reflection diffusée, tandis que les courbes avec des pentes superieures d'une Rayleigh pure, pendant la periode froide indiquent un mécanisme de guidage. L'hypothèse ci-dessus est supportée par le fait que les variations en altitudes des surfaces isobares suivant les periodes chaudes ou froides, presentent un fort coefficient de correlation avec les intensités du champ reçu.

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Summary A radiometeorological study of VHF propagation along transhorizon paths over several parts of the Mediterranean sea, was initiated by the Radiometeorological Panel of NATO's Science Committee. After a study over the Otranto channel (Martina Franca-Corfu) performed in collaboration with the Italian group, in the present paper is investigated the 725 km, mainly over the Lybean sea transhorizon path Cairo City to lerapetra, located in the South cost of Creta island. Three years fieldstrengths measurements were analysed in Ierapetra center. Following the observed results, during the warm period of the year it is a good reception, while during the cold period the reception was very poor or inexistant.In order to explain the mechanism of this transhorizon propagation, a method of analysis based on the distribution curves, as indicated by one of the authors (Anastassiadis) was used. Rayleigh or super-Rayleigh distribution curves of the received signalstrength versus time percentage observed during the warm period indicate a diffused reflection mechanism while Sub-Rayleigh distribution observed during the cold period indicate a ducting mechanism. These results are supported by the close correlation observed between the variation in altitudes of isobaric surfaces and the location of volume following the warm or cold period of the year in which the diffused reflection mechanism is produced.

     

An empirical model for the polarization of pulsar radio emission

We present an empirical model for single pulses of radio emission from pulsars based on Gaussian probability distributions for relevant variables. The radiation at a specific pulse phase is represented as the superposition of radiation in two (approximately) orthogonally polarized modes (OPMs) from one or more subsources in the emission region of the pulsar. For each subsource, the polarization states are drawn randomly from statistical distributions, with the mean and the variance on the Poincaré sphere as free parameters. The intensity of one OPM is chosen from a lognormal distribution, and the intensity of the other OPM is assumed to be partially correlated, with the degree of correlation also chosen from a Gaussian distribution. The model is used to construct simulated data described in the same format as real data: distributions of the polarization of pulses on the Poincaré sphere and histograms of the intensity and other parameters. We concentrate on the interpretation of data for specific phases of PSR B0329+54 for which the OPMs are not orthogonal, with one well defined and the other spread out around an annulus on the Poincaré sphere at some phases. The results support the assumption that the radiation emerges in two OPMs with closely correlated intensities, and that in a statistical fraction of pulses one OPM is invisible.     

Inferring lake depth using diatom assemblages in the shallow,seasonally variable lakes of the Nebraska Sand Hills (USA): calibration,validation, and application of a 69-lake training set

The Nebraska Sand Hills are a distinctive eco-region in the semi-arid Great Plains of the western United States. The water table underlying the Sand Hills is part of the High Plains/Ogallala aquifer, an important water resource for the central Great Plains. Lake levels are affected directly by fluctuations in the water table, which is recharged primarily by local precipitation and responds quickly to climatically induced changes in regional water balance. Instrumental records are available for only 50–100 years, and paleolimnological data provide important insights into the extremes and variability in moisture balance over longer time scales. A set of 69 lakes from across Nebraska was used to establish a statistical relationship between diatom community composition and water depth. This relationship was then used to develop a diatom-based inference model for water depth using weighted averaging regression and calibration techniques. Development of the inference model was complicated by strong intra-seasonal variability in water depth and the linkages between depth and other limnologic characteristics, including alkalinity, water clarity and nutrient concentrations. Analysis of historical diatom communities from eight lakes allowed for the reconstruction of lake-level fluctuations over the past several thousand years. Comparisons of the more recent portion of these reconstructions with the instrumental Palmer Drought Severity Index (PDSI) showed that sediment records may not faithfully reflect short-term fluctuations in water level, except where sedimentation rates are very high. However, large and persistent changes in moisture availability were discernible even in longer, low-resolution records. Thus, diatoms are a useful addition to the tools available for understanding past drought in the central Great Plains, especially when trajectories of change are constrained by data from multiple sites or other proxies.     

Surface water quality contamination source apportionment and physicochemical characterization at the upper section of the Jakara Basin, Nigeria

The present study investigates the surface water quality of three important tributaries of Jakara Basin, northwestern Nigeria to provide an overview of the relationship and sources of physicochemical and biological parameters. A total of 405 water samples were collected from 27 sampling points and analyzed for 13 parameters: dissolved oxygen (DO), 5-day biochemical oxygen demand (BOD₅), chemical oxygen demand (COD), suspended solids (SS), pH, ammonia-nitrogen (NH₃NL), dissolved solids (DS), total solids (TS), nitrates (NO₃), chloride (Cl), phosphates (PO₄), Escherichia coli (E. coli) and fecal coliform bacteria (FCB). Pearson’s product–moment correlation matrix and principal component analysis (PCA) were used to distinguish the main pollution sources in the basin. Four varimax components were extracted from PCA, which explained 84.86, 83.60, and 78.69 % of the variation in the surface water quality for Jakara, Tsakama, and Gama-Kwari Rivers, respectively. Strong positive loading included BOD₅, COD, NH₃NL, E. coli, and FCB with negative loading on DO attribute to a domestic waste water pollution source. One-way ANOVA revealed that there was no significant difference in the mean of the three water bodies (p?>?0.05). It is therefore recommended that the government should be more effective in controlling the point source of pollution in the area.     

Groundwater resources assessment using numerical model: A case study in low-lying coastal area

The impacts of climate change and human pressure in groundwater have been greatest threats facing small islands. This paper represents a case study of groundwater responses towards the climate change and human pressures in Manukan Island Malaysia. SEAWAT-2000 was used for the simulations of groundwater response in study area. Simulations of six scenarios representing climate change and human pressures showed changes in hydraulic heads and chloride concentrations. Reduction in pumping rate and an increase in recharge rate can alter the bad effects of overdrafts in Manukan Island. In general, reduction in pumping rate and an increase in recharge rate are capable to restore and protect the groundwater resources in Manukan Island. Thus, for groundwater management options in Manukan Island, scenario 2 is capable to lessen the seawater intrusion into the aquifer and sustain water resources on a long-term basis. The selection of scenario 6 is the preeminent option during wet season. The output of this study provides a foundation which can be used in other small islands of similar hydrogeological condition for the purpose of groundwater resources protection.     

Coastal morphodynamic analysis in Buleleng Regency,Bali—Indonesia

Natural Hazards - Sediment as erosion product can affect shorelines, making sediment transport a key process to consider in coastal and shoreline management. Field surveys and secondary data can...     

Monitoring land subsidence in Semarang,Indonesia

Semarang is one of the biggest cities in Indonesia and nowadays suffering from extended land subsidence, which is due to groundwater withdrawal, to natural consolidation of alluvium soil and to the load of constructions. Land subsidence causes damages to infrastructure, buildings, and results in tides moving into low-lying areas. Up to the present, there has been no comprehensive information about the land subsidence and its monitoring in Semarang. This paper examines digital elevation model (DEM) and benchmark data in Geographic Information System (GIS) raster operation for the monitoring of the land subsidence in Semarang. This method will predict and quantify the extent of subsidence in future years. The future land subsidence prediction is generated from the expected future DEM in GIS environment using ILWIS package. The procedure is useful especially in areas with scarce data. The resulting maps designate the area of land subsidence that increases rapidly and it is predicted that in 2020, an area of 27.5 ha will be situated 1.5–2.0 m below sea level. This calculation is based on the assumption that the rate of land subsidence is linear and no action is taken to protect the area from subsidence.