We report on a large-scale characterization of river discharges by employing the network framework of the horizontal visibility graph. By mapping daily time series from 141 different stations of 53 Brazilian rivers into complex networks, we present an useful approach for investigating the dynamics of river flows. We verified that the degree distributions of these networks were well described by exponential functions, where the characteristic exponents are almost always larger than the value obtained for random time series. The faster-than-random decay of the degree distributions is an another evidence that the fluctuation dynamics underlying the river discharges has a longrange correlated nature. We further investigated the evolution of the river discharges by tracking the values of the characteristic exponents (of the degree distribution) and the global clustering coefficients of the networks over the years. We show that the river discharges in several stations have evolved to become more or less correlated (and displaying more or less complex internal network structures) over the years, a behavior that could be related to changes in the climate system and other man-made phenomena.

ABSTRACT. This study analyzed the external factors that influence the yield obtained from photovoltaic modules (Solarex ® -MSX 56), as solar irradiance, temperature, placement angle and dust deposition on the photovoltaic modules installed at the facilities of the Medianeira campus of the UTFPR, working under real conditions. To obtain the data it was used a datalogger from Campbell Scientific, Inc, model CR23X. It was observed that under solar irradiance below 550 W m -2 the panel did not convert maximum power, and above this value the panel reached saturation levels. Temperature increase led to reduced voltage, and consequently lower module output power, decreasing the efficiency value by nearly 6% at temperatures 15 o C above the Standard Test Conditions (STC) temperature. These panels are usually placed at different angles according to local latitude, remaining fixed in that position. In comparison with a horizontally-placed panel, it was obtained a 4-hour increase in yield when the panel reached saturation value. Dust levels reduced electricity production levels by approximately 16%. These factors must be taken into account for placement and maintenance of photovoltaic systems, so they can function efficiently.Keywords: solar irradiance, temperature, deposited dust, positioning.Rendimento de módulos fotovoltáicos em situação real de trabalho no oeste do Paraná -Brasil RESUMO. Neste trabalho foram analisados os fatores externos que influenciaram no rendimento de painéis fotovoltáicos (Solarex ® -MSX 56) como irradiância solar, temperatura, ângulo de instalação e sujeira acumulada sobre os painéis fotovoltáicos, instalados nas dependências da UTFPR, Câmpus Medianeira, funcionando em condição real. Para obter os dados foi utilizado um datalogger da Campbell Scientific-Inc, modelo CR23X. Verificou-se que na irradiância abaixo de 550 W m -2 o painel não converteu sua potência máxima e, acima de 550 W m -2 , o painel atingiu o valor de saturação. O aumento da temperatura provocou redução na tensão e, portanto, na potência de saída do painel, diminuindo a eficiência cerca de 6% em temperaturas com 15 o C acima da temperatura Standard Test Conditions (STC). É usual instalar o painel inclinado conforme a latitude do local e mantê-lo fixo nesta posição. Comparando a um painel instalado na posição horizontal, obteve-se acréscimo de 4h no rendimento em ocasiões em que o painel atingiu o valor de saturação. A geração de energia elétrica reduziu cerca de 16% o nível de sujeira. Estes fatores devem ser considerados na instalação e manutenção de sistemas fotovoltáicos para que funcionem com eficiência.Palavras-chave: irradiância solar, temperatura, sujeira acumulada, posicionamento.

A nematic cell subjected to a large electric field undergoes a molecular reorientation that affects the electrical current flowing through it. To analytically establish the dependence of the current on the applied voltage, the cell is considered as parallel of resistance, R(t) , and capacitance, C(t) , that are connected with the nematic director profile. This profile is determined in the quasistatic regime in which the nematic orientation follows the time variation in the external field normal to the cell plates without delay. The analysis performed for a weakly anchored cell shows that the current presents a peak when the applied voltage overcomes the threshold voltage for the transition of Fréedericksz at a critical time t* as in the case of strong anchoring. For large voltages, R(t)-->R|| and C(t)-->C||, where parallel refers to the nematic director. We show that, for large enough time t>>t*, it is possible to connect the measured current with the extrapolation length characterizing the sample by means of simple analytical expressions. This connection can be used to experimentally estimate the anchoring energy by means of current measurements.

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