جستجوی مقالات مرتبط با کلیدواژه « Copula functions » در نشریات گروه « اکولوژی »

In the recent years, the dependence structure among hydrological variables is taken into consideration and it led to employ the multivariate analysis as a suitable alternative for univariate methods. In this study, the copula functions were employed for multivariate frequency analysis of low flows of Dez River basin at Tange Panj-Bakhtiari (TPB) and Tange Panj-Sezar (TPS) hydrometric stations. At the first, at first 7-d series of low flow was extracted from measured daily flows at the studied stations in the period of 1956-2012. In the next stage, 11 different distribution functions were fitted onto the low flow data which logistic distribution had the best fit on the TPB station and the GEV distribution had the best fit on the low flow data of TPS station. After specifying the best fitted marginal distributions, the copula parameter should be estimated. In this study, two methods of inference function for margins (IFM) and particle swarm optimization (PSO) were used to estimate copula parameter. The results showed that the PSO method had outperform than IFM in estimating the copula parameter. Among the Ali - Mikhail – Haq, Clayton, Frank, Galambos and Gumbel-Hougaard copulas, the Frank copula function had lowest error and highest accuracy in constructing the joint distribution of paired 7-d low flows data and was used for calculating the joint return periods in two states of “OR” and “AND”.

Monitoring and prediction of droughts, especially accurate determination of its emersion time and duration, is very important in water resources management and planning drought mitigation strategies. In this study drought conditions in Northeast of Iran was evaluated by means of Joint Deficit Index (JDI). Monthly precipitation data from 6 synoptic stations including Torbat Heydariyeh, Sabzevar, Semnan, Shahroud, Gorgan and Mashhad during the period of 1971 to 2011 were used for calculating the JDI index. Results showed that in recent years the number of dry months across study area (especially at wet regions) increased, significantly. As in all of considered stations (except Semnan station) the percentage of dry months increased to over (more than) 50% during recent 10 years (2002-2011). Results showed that the JDI provides a comprehensive assessment of droughts and that it is capable of reflecting both emerging and prolonged droughts in an accurate manner and allows for a month-by-month drought assessment.