Precipitation can be considered as one of the non-thermodynamic and complicated weather elements that has significant temporal and spatial variations. Temporal variation is more prominent in arid and semiarid parts of the world. As most of area of the country is arid and semiarid, any changes in precipitation patterns like its type, amount, and dates are of crucial importance. Any significant changes in amount of precipitation and occurrence of its extreme values have remarkable influences on main development programs of the country.
Main reasons for inhomogeneity of precipitation of time series in reliable synoptic stations are trends and fluctuations rather than jumps (reverse is true for temperature). In spite of general opinion, we found both negative and positive trends for synoptic stations’ precipitation time series. For instance, Mashhad showed positive trend and Bandar Anzali showed negative trends for their annual precipitation time series. In addition, we did not find parallel trend lines for seasonal precipitations. Qualitative and quantitative changes of trends and ratio of trend to mean are shown and discussed.
)