Analysis of Temporal Distribution of Rainfall in Iran Over the Past four Decades

Precipitation is one of the most changeable meteorological and hydrological phenomena. Estimating the average annual number and the manner of rain fall time changes for management and planning changes of water resources in different regions is a necessary subject, So that the maximum or minimum exponential number with different objectives in water resources management has an important cultural, social and political consequences in a country. In many resources, always numerical value of 240 to 250 mm of rainfall in the past few decades has been introduced. So on this basis to explore the accuracy of the above number; the aim of this study was to calculate the mean and variance of rainfall during the final decades in Iran. In this study, changes in precipitation over the last four decades in 33 synoptic stations with proper spatial distribution were studied. For temporal analysis of precipitation data , firstly their preliminary nature from the view of the uniformity, adequacy and stochastic were statistically analyzed And the average precipitation was calculated by Thiessen polygons and Isohyetal in different ways of Kriging interpolation, inverse distance and natural neighborhood, and using Leuven statistics, the heterogeneity of variances were compared in different decades . Based on the results obtained, the average rainfall in Iran over the past four decades was estimated 421 mm in Thiessen polygons method, 232 mm in Kriging method, IDW 715 mm and Natural Neighbor Interpolation 715.5 mm .The results of the study of heterogeneity of variance in four decades showed that the variances are homogeneous and does not show a significant difference.Accordingly, it is proposed that the nominated method for estimating precipitation on the surface to be selected based on the considered objective and accuracy.