Temporal-Spatial Variability of the Severest Dry Spells in the North-West of Iran

Abstract

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 />