Change of the Precipitation Regime in Northwest of Iran

IntroductionPrecipitation regime refers to the distribution of precipitation over time and depicts the number of cycles per year. Most of the economic activities (e.g. agriculture) in every given region are adapted to the precipitation regime. The change in precipitation regime can affect these activities as it affects water resources. Accordingly, it is important to analyze these changes in every geographical region.A change in the precipitation regime as a change in the temporal pattern of precipitation is one of the effects and results of climate change. This phenomenon is has received much attention from climatology experts. They have studied this phenomenon using many techniques. One of the most important features in analyzing the change of precipitation regime is to study the periodicity of different harmonics. The harmonic number shows the number of cycles during the year. For example first harmonic shows one cycle per year while the second harmonic shows two cycles and … in one year.Materials and methods In this research, 7.2 percent of lands in the northwest have been studied. This area is about 118670 square kilometers. This area contains two kinds of topographies: rough mountainous area and the flat areas among it. In order to accomplish this research, time series data of 260 well-scattered synoptic, climatology and rain gauge stations are analyzed. Monthly precipitation maps have been created to obtain an acceptable cover even for lake of data stations. The research has been done on the basis of harmonic analyses. As it has been mentioned before, harmonic analyses show the numbers of cycles during the year.For a given time series Xt, can be written as a combination of different sine signals as follows: Where is the arithmetic average of the original data series, while the rest of the terms represent the first, second. .. n/2th harmonic analysis of the period T. The amplitude can be written as The percentage of the total month to month variations explained by a given harmonic can be determined by forming a ratio of the square of the amplitude of that specific harmonic to the sum of all harmonics; as follow:In other words, the importance of the total month to month variation explained by a given harmonic can be determined by forming a ratio of that harmonic variation to sum of all harmonics. The main objective of this study is to investigate and analyze the first three harmonics of four successive decades (1966-2005) in northwest of Iran (West-Azarbayjan, East-Azerbaijan, Ardabil, and Zanjan). All calculating operations have been done on map cells in order to achieve the harmonic characteristics.Results and discussion Annual precipitation of northwest of Iran increased from northeast to southwest. Tremendous spatial change is dominant on precipitation. Unexpectedly, the maximum precipitation does not occur on the summits of the mountains. Based on the methods used in this paper, it is revealed that both systems, synoptic and local, explain more than 90% of precipitation variations.The first harmonic explains the maximum precipitation variation in southwest from which to northeast the ratio of precipitation is decrease in region that the ratios of synoptic systems are minimized. Therefore, local systems (convective, orographic and local frontally and so on) have greater roles here. However, as annual precipitation increased, the synoptic share in precipitation increased too. So, the southwest experienced more precipitation. This is true for all four decades by oscillate behaviors. Changing the first harmonic ratio, the others have changed too. All changes lead to an increase in annual precipitation concentration and a decrease in its distribution over time.ConclusionAccording to this paper, precipitation regime in northwest of Iran has experienced a big change especially during the last decade. This can be seen in the ratio of every cycle among the data. This change happened in the area with concentrated and non-concentrated precipitation. Accordingly, the areas changed from non-concentrate to concentrated regime and vice versa. So, it seems necessary to recognize these changes for water resources management and planning.