Analysis of hydroclimatic trends in the Atrak River basin, North Khorasan, Iran (1975 – 2008)

Hydrologic regime is highly dependent on climate change, as hydrologic cycle components are directly influenced by climatic conditions. This paper analyzes the impacts of climate change on the hydrologic regime of the Atrak River basin in the northeast of Iran. It drains to the Caspian Sea. The data collected at 11 hydrometeorological stations were examined to detect trends in hydroclimatological variables. Ten variables including annual mean discharge, annual maximum discharge, annual precipitation, maximum daily precipitation, annual minimum, mean and maximum temperatures, diurnal temperature range, annual pan evaporation and annual potential evapotranspiration during 1975 – 2008 were analyzed using the non-parametric test of Mann-Kendall. First, the long term observations of annual mean discharge at 8 hydrometric stations have been analyzed to find out any indication of increasing or decreasing trend. Second, the long term records of precipitation, temperature, pan evaporation and potential evapotranspiration were examined to detect signs of climate change. Finally, the relationship between trends in streamflow and meteorological variables were discussed. The results indicate that although the annual mean discharge at all stations across the Atrak River basin is decreasing, but it is statistically significant at 6 stations (75 percent of stations) . However, there is no evidence of significant increasing or decreasing trends in the precipitation variables. Analyses on temperature and evaporation variables show increasing trends in most of the stations. Therefore, it is concluded that decreasing trends of annual mean discharge is most likely related to an increase in the evaporative power of atmosphere.