Yazar "Kahya, E." seçeneğine göre listele

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    Detection of Water Quality Trends in the Rivers of the Susurluk Basin
    (1998) Kalaycı, S.; Kahya, E.
    In recent years, some non-parametric tests to determine trends of surface water quality have been proposed due to the fact that assumptions in the classical parametric tests (i.e., normality, linearity, independence) are usually not met by a typical surface water quality data. At the same time, statistical tests for trends of water quality are commonly confounded by several of the following problems: missing values, censored data (i.e., values reported as less than a specified quantity), flow relatedness and seasonality. For these reasons, several non-parametric tests (Sen's T, Spearman's Rho, Mann-Kendall, Seasonal Kendall, Mann-Whitney and Kruskall-Wallis'H Tests) have been used or developed in the last two decades. In this study, the surface water quality data of rivers in the Susurluk Basin were used. Four different non-parametric trend tests were selected to detect linear trends of surface water quality. These tests are the Sen's T test, the Spearman's Rho test, the Mann-Kendall test and the Seasonal Kendall test. The linear slopes (change per unit time) of trends are calculated by using a non-parametric estimator. In addition, the homogeneity in monthly trends are tested by using a method developed by Van Belle and Hughes. The results of non-parametric tests indicate that discharge and concentration of sediment have downward trends while temperature and concentrations of specific conductance, sodium, potassium, calcium+magnesium, bicarbonate and chloride generally have upward trends. However concentrations of carbonate, pH, sulfate, organic matter, and boron show no trends.
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    Multivariate Stochastic Modeling of Monthly Streamflow of Rivers in the Sakarya Basin [Sakarya Havzasindaki Aylik Akimlarin Cok Degiskenli Stokastik Modellemesi]
    (1999) Karabörk, M. C.; Kahya, E.
    Mathematical expressions of multivariate periodic autoregressive (PAR) and periodic autoregressive-moving average (PARMA) models were obtained for monthly streamflow observations of 12 stations located in Sakarya Basin, Turkey. The methodology of both models was given in detail at five phases (preliminary analysis, estimation parameters, goodness of fit test, optional tests, and reliability of estimated parameters). For the purpose of methodological to be easily understood, the methodological procedures of annual multivariate AR and ARMA models were firstly presented. Because of being more practical PAR(1) model was initially considered in the analyses, but it was found that this model did not preverse the cross-correlation structure of historical series. Since a long-term dependency structure was observed in the correlograms of historical series in the phase of preliminary analysis, the modeling procedures were continued with the multivariate ARMA(1,1) model. This model was proven to be suitable for Sakarya Basin due to the fact that statistical moments of individual stations as well as cross correlation structures of all stations were satisfactorily preserved in the generated synthetic series.