Geographical factors affecting variability of precipitation regime in Iran

This study compares the precipitation regimes by using harmonic analysis during the last four decades (1965-2004). We used the measured precipitation data from 428 rain-gauge sites and weather stations distributed across Iran by applying 15 x 15 km spatial grids to generate the interpolated data. Data validations were carried out by statistical tests. In this study, first three harmonics of precipitation variances were evaluated. Variability of precipitation regime was explored by using three harmonic analysis methods. In addition, the effect of geographical factors (GF) (site elevation, latitude, and longitude) affecting the precipitation regime (P) was verified by multivariate regression method. The resulted regression equation between P and GF for spring showed the highest correlation coefficient (r = 0.79). For other seasons, r was lower than for spring and varied between 0.26 (summer) to 0.58 (autumn). Analysis of the first harmonic proved that the main precipitation regime in Iran tends to concentrate in one specific season (winter) as a result of large-scale Mediterranean systems passing over the country. In other words, the first harmonic is able to explain most of the precipitation variations which are caused by large-scale atmospheric circulation. For all the three harmonics, variances of precipitation were mainly a function of the geographical factors. This effect was more evident in the third harmonic; in such a way that increasing the latitudes caused higher precipitation variance. This means that the precipitation regime in northern sites is more sensitive to the local factors than those of southern sites. The results of this research can be used for reliable estimation of precipitation in ungauged sites.