Changeability evaluation of hydro-climate variables in Western Caspian Sea region, Iran

Trends of the three hydro-meteorological variables, including precipitation, temperature, and river discharge have been studied to detect the impacts of climate change on water resources in western Caspian Sea region, Iran. We used Kendall's tau, Spearman's rho, Sen's slope estimator, and Mann-Kendall approaches to detecting the annual and seasonal trends over the period 1981-2010. The nonparametric tests detected statistically significant decreasing trends in all of the hydrometric stations. In general, magnitude of an annual downward trend in the hydrometric stations was 2.17 m(3)/s per decade at the 5% significance level. As the results of the Pettitt test for detecting abrupt changes (jumps), the change points (or trends started) in river flow series were found in all hydrometric stations in the years of 1994-1999 at the significance level of 0.05%. Meanwhile, the outputs exhibited that majority of the annual and seasonally precipitation trends were increasing in contrast to the river discharge trends. Furthermore, on average, magnitude of annual temperature increasing trend was found (+) 0.43 degrees C per decade at the 0.05 significant level. Abrupt changes in the annual temperature as same as river flow series were at the 1990s over the period 1981-2010. The Pearson correlation was used to explore the relationships between river flow, temperature, and precipitation time series. Finally, the high correlation was obtained from the area-averaged temperature and discharge series at the autumn and annual time series with the value of the (-) 59 and (-) 50%. It is concluded that decreasing the river discharge might be related to increasing temperature and decreasing precipitation and anthropogenic changes in the study area.