University of Tehran Press Desert 2008-0875 19 1 2014 01 01 Temporal-Spatial Variability of the Severest Dry Spells in the North-West of Iran 17 25 51050 10.22059/jdesert.2014.51050 EN Yadollah Yousefi Department of Geography and Urban Planning , Faculty of Humanities and Social Sciences, University of Mazandaran, Babolsar, Iran Jafar Masoompour Samakosh Department of Geography, Faculty of Humanities, Razi University, Kermanshah, Iran Journal Article 2011 09 18 The variability of temperature and precipitation is regarded as one of the main characteristics of the climate.<br />Precipitation and its results, especially results such as droughts, vary on different temporal and spatial scales. The<br />purpose of this paper is to determine the frequency of the inter-annual variability of the driest month in north-west<br />Iran. In order to obtain the best results, we used 11 synoptic weather stations and included 30 years of continuous<br />data records (1977-2006). Initially, the driest months were determined using the Standardized Precipitation Index<br />(SPI); thereafter, the harmonic method was applied for closer examination and demonstration of the frequency of<br />precipitation. The results derived from the harmonic method showed that the highest percentage of variance<br />distribution (approximately 80%) is found in the central sections of the region according to the first harmonic of the<br />Zanjan station. In these sections, the inter-annual changes of in the driest months typically fit a semi-annual pattern<br />during the examined period. In the northern parts, higher harmonics are required to explain variance, which implies a<br />higher variability and a lack of consistency in the occurrence of the driest month in these regions. The lowest PVR1<br />value (nearly 28%) was registered by the Ardebil station. Therefore, in Zanjan station, the inter-annual changes of the<br />examined component have more homogeneity owing to the high PVR value of the first harmonic, while the level of<br />variability in the driest months was higher in stations like Ardebil. Higher harmonics are required to explain this<br />variance.<br /><br /> https://jdesert.ut.ac.ir/article_51050_d169feac3aa8ac97498191c242aa34f6.pdf