Estimation of Non-Stationary Behavior in Annual and Seasonal Surface Freshwater Volume Discharged into the Gorgan Bay, Iran

The surface freshwater volume discharged into the Gorgan Bay is a significant part of its ecological processes. The Ghareh-Sou River is the most crucial source of surface freshwater in the Gorgan Bay watershed. It is necessary to investigate the temporal changes of surface freshwater volume series at this river's outlet to the bay. In this study, changes in surface runoff volume discharged by the Ghareh-Sou River into the Gorgan Bay (seasonal and annual time series in 1971–2018) were studied using generalized additive models for location, scale and shape (GAMLSS). Probability distribution functions, including normal, gamma, Gumbel, Weibull, and log-normal as well as six different models were applied to estimate variance and mean changes. This study indicated that the dominant probability distribution function of the studied series was gamma. All annual and seasonal series showed significant non-stationary behavior. The magnitudes of the upper quantiles' changes were more powerful than the lower ones, indicating the importance of investigating the different quantiles' changes. The differences between different quantiles of annual, spring, autumn, and winter series of surface freshwater volume series were increasing. Moreover, the GAMLSS showed the suitable ability to estimate the variation over time, contributing to a deeper understanding of changes in surface freshwater volume time series discharged into the Gorgan Bay. Moreover, changes in climatic variables and human activities were driving factors of the revealed trends in freshwater volume series.