Temperature and precipitation are among important atmospheric parameters for watershed planning. Assessment of temperature and precipitation trends is very important for future watershed planning. In this paper, trends of atmospheric parameters such as seasonal and annual temperature and precipitation were examined for the synoptic stations of Bandar Anzali, Rasht, Ramsar, Babolsar and Gorgan. In order to detect temperature and precipitation trends, homogeneous time series are needed. Expert judgment, metadata and standard normal homogeneity test (SNHT) were used to assess homogeneity of seasonal and annual time series. Some seasonal and annual time series were heterogeneous which were adjusted to homogeneous time series. The results show positive trends of annual and seasonal maximum and minimum temperature, and negative trends of annual and seasonal maximum and minimum precipitation. Also the trend of minimum temperature is higher than the trend of maximum temperature. Mean trends of annual minimum and maximum temperature and annual precipitation are 0.39 ◦c/decade, 0.05◦c/decade and -31/8mm/decade, respectively. The highest average trend of seasonal maximum and minimum temperature is related to the summer season, whereas the highest of average trend of seasonal precipitation is related to the winter season.
The lowest of average seasonal trend of minimum and maximum temperatures are related to the winter and spring seasons, respectively. Mean of seasonal precipitation trends of spring, summer and autumn are almost similar each other.