Romanian Academy

About: Romanian Academy is a archive organization based out in Bucharest, Romania. It is known for research contribution in the topics: Population & Nonlinear system. The organization has 3662 authors who have published 10491 publications receiving 146447 citations. The organization is also known as: Academia Română & Societatea Literară Română.

Hydrogen bond network topology in liquid water and methanol: a graph theory approach

Abstract: Networks are increasingly recognized as important building blocks of various systems in nature and society. Water is known to possess an extended hydrogen bond network, in which the individual bonds are broken in the sub-picosecond range and still the network structure remains intact. We investigated and compared the topological properties of liquid water and methanol at various temperatures using concepts derived within the framework of graph and network theory (neighbour number and cycle size distribution, the distribution of local cyclic and local bonding coefficients, Laplacian spectra of the network, inverse participation ratio distribution of the eigenvalues and average localization distribution of a node) and compared them to small world and Erdős–Renyi random networks. Various characteristic properties (e.g. the local cyclic and bonding coefficients) of the network in liquid water could be reproduced by small world and/or Erdős–Renyi networks, but the ring size distribution of water is unique and none of the studied graph models could describe it. Using the inverse participation ratio of the Laplacian eigenvectors we characterized the network inhomogeneities found in water and showed that similar phenomena can be observed in Erdős–Renyi and small world graphs. We demonstrated that the topological properties of the hydrogen bond network found in liquid water systematically change with the temperature and that increasing temperature leads to a broader ring size distribution. We applied the studied topological indices to the network of water molecules with four hydrogen bonds, and showed that at low temperature (250 K) these molecules form a percolated or nearly-percolated network, while at ambient or high temperatures only small clusters of four-hydrogen bonded water molecules exist.

Spatio-temporal changes of the climatic water balance in Romania as a response to precipitation and reference evapotranspiration trends during 1961–2013

Abstract: Changes in precipitation (P), evapotranspiration (ET) and, implicitly, in the climatic water balance (CWB), are imminent effects of climate warming. However, changes in the CWB, as a response to changes in P and ET, have not yet been analysed thoroughly enough in many parts of the world, including Romania. The present study explores the spatio-temporal changes of the CWB (difference between P and reference evapotranspiration, ETo) in Romania, based on a wide range of climatic data (P, as well as temperature, relative air humidity, sunshine duration and wind speed, necessary for computing ETo with the FAO-56 Penman-Monteith method) recorded at 70 weather stations across the country in the 1961–2013 period. As a secondary objective, the study attempts to identify the possible connections between the index's trends and large-scale atmospheric circulation, assessed based on the dynamics of certain European-scale relevant teleconnection indices. Thus, the Mann–Kendall test and Sen's slope methods were used to analyse CWB trends (but also P and ETo trends, in order to explain CWB pattern changes) annually, seasonally and in the maize and wheat growing seasons. Also, the Spearman correlation procedure and a composite analysis between interannual series of teleconnection indices and CWB were used to assess the influence of atmospheric circulation on the index's variability for all analysed time scales. The results generally showed CWB decreases (for the most part of up to −2 mm/yr, yet with a relatively low statistical significance) and highlighted an overall amplification of drier conditions on all time scales, except for autumn (CWB increases, generally of up to 1 mm/yr, but with low statistical significance). Moreover, net changes of even under −200 mm/53 yrs annually and −175 mm/53 yrs in summer and for the maize and wheat growing seasons were found in the CWB. Spatially, it was found that the country's southwestern and southeastern regions are the main epicentres of drier trends, while the northwest appears to have become wetter. Overall, the negative CWB trends are due to partial P decreases (statistically insignificant) and general ETo increases (highly statistically significant, even 100% in summer). It seems that the amplification of the climatic water deficit across the country is especially linked to the positive phases of the Arctic Oscillation and North Atlantic Oscillation, but also, in part, to those of several other teleconnection indices that affect Europe. Our results signal the necessity to adapt anthropic and ecological systems to future dryness trends countrywide, which will most likely intensify against the background of climate change expected to occur by the end of the century.