It is important to assess possible temporal trends in streamflow discharge and the factors influencing it for optimum decisions in regional water resources planning. This study aims to investigate the possible trends in river discharge, land-use changes, and climate factors and their possible linkages. Therefore, streamflow records from eight sites of the Dez River Basin, Iran, were analyzed by a modified Mann-Kendall test to examine possible trends in annual and monthly time series of streamflow. The results showed significant negative trends in the streamflow time series of the warm months (April, May, June, July, August, and September) over 34 year-long records. Streamflow reductions in sub-basins that were unaffected by irrigation indicated that land-use change cannot cause the observed trend. Although no significant trends were observed statistically for precipitation data, streamflow reductions possibly caused by overall reductions in precipitation. Besides that, there were significant increasing trends in temperature and evaporation, especially in summer season. The Pearson correlation analysis was applied to explore the relationships between evaporation and streamflow time series during the warm months (May, June, July, August, and September). The results indicated strong correlations between river discharge and evaporation at the monthly scale for six of eight sites. Besides the powerful effects of temperature on water losses through evaporation, changes in temperature also can cause some changes in precipitation forms and patterns as well as basin responses. This highlighted the role of changes in temperature that intensifies the effect of overall decreasing changes in precipitation on the streamflow trends.
