A Discrimination of Roles of Internal and External Factors on the Decadal Variation of Annual Precipitation in Iran over Recent Four Decades (1975-2016)

Document Type : Full length article


1 Professor in Climatology , Department of Geography, University of Zanjan

2 Department of Geography, Geography Department, University of Isfahan, Isfahan, Iran

3 Geography Department, University of Zanjan, Zanjan, Iran


A Discrimination of Roles of Internal and External Factors on the Decadal Variation of Annual Precipitation in Iran over Recent Four Decades (1975-2016)
According to Intergovernmental Panel on Climate Change (IPCC) the Earth's climate has been changed during recent century (IPCC, 2007). These changes may continues for the next century. The changes have happened in two ways, log- term change (trend) and variation (in form of Oscillations, phases, shifts, and persistence) (Asakereh,2017). Moreover, some of these changes are due to internal factors of a region, whilst some of them result from external culprits of a given region. The distinguish between these two groups of factors is an important scientific effort to understand the changes mechanisms governing them.
The emerge of climate changes and climate variation can be traced by investigation of some sensitive climate elements. One of those chaotic elements is precipitation which experiences changes in different tempo-spatial scales (Goudi, 1994). Most of these changes, specially the trends, are studied in global (Todorov, 1985; Vining and Griffiths, 1985; Diaz et al. 1989) and national (Askari and Rahimzadeh, 2003; Asakereh,2004; Zahedi et al., 2007; Katiraie , 2007; Mohamadi, 2012; Ekhtesasi et al. 2015; Nazeri Tahrudi, 2016) scals.
In the climatology literature (Singh et al. 1995; Ghayur and Masoodian, 1996; Glazirin, 1997; Mojarrad and Moradifar, 2003; Asakereh, 2004; Raziei and Azizi, 2009; Asakereh and Seifipour, 2013) spatial changes of precipitation was attributed to spatial coordination (longitude and latitude) and topographic features (elevation, slope magnitude and aspect). The temporal changes in precipitation in association with these topo-spatial factors has not been considered in details and in proper ways in climate researches.
In current study we put the spotlight on the decadal variation in relation with the topo-spatial features as a representative of climate change and as a vital context of research. Accordingly, a regression model is adopted so as quantify the effects of topo - spatial factors effecting variability of precipitation over recent four decades.
Materials and methods
In order to achieve the aim of current study, two dataset were adopted; a national precipitation dataset, Esfezari, and Digital Elevation Model (DEM):
The third version of Esfezari dataset is result from Kriging interpolation of daily record of 2188 synoptic, climatologic, and rain gauge stations for 46 years (1970-2016) with 10 km resolution. The 16801 daily maps with 167×205 pixels were created accordingly. Consequently, the dataset include dimensions of 167×205×16801.
The DEM data with 10 meters spatial resolution was adopted from The U.S. Geological Survey which was provided by "Astet" satellite images. This dataset was applied to extract the topographic features (elevation, slope and its aspect) for Iran.
To start with, the annual precipitation for the entire under investigation period (1970-2016) was analyzed according to abovementioned data. In the second place, the topo-spatial distribution of precipitation for four decades ended up to 2007-2016 was compared in an analyzing effort by using linear and non-linear correlation. In this stage, according to Law of Parsimony, it is revealed that linear correlation illustrate the relation between precipitation and topo - spatial factors in proper way. Moreover, A multivariate linear regression fitted on the five topo-spatial elements for every under study decade so as to detect precipitation accordingly. The regression model can be expressed as follow:

where refers to annual precipitation in th pixel which is detected by the topo - spatial factors and considered the internal- cause precipitation. and are latitude and longitude, respectively. In the above regression model, , and are elevation, slop, and aspect of slop, respectively. Consequently, the external - cause precipitation is the model residual. Finally, the descriptive characteristics of precipitation maps of internal-cause and external - cause were analyzed.
Results and discussion
General Features of Iran Precipitation
The general feature of precipitation over Iran shows a decreasing trends from west to east and from north to south. The coast of Caspian Sea and the summits of Zagros Mountain chain receive the highest values of precipitation. The spatial average of annual precipitation is about 250.5 mm. The strongest relationship between precipitation and topo-spatial factors is related to longitude in negative way. This feature is reflected in eastward decreasing of precipitation. The determine coefficients for latitude and elevation are 13% and 4.5%, respectively.
The variation of effects of internal and external factors
In spite of the stability of the determine coefficients of all regression models for all decades, some of topo - spatial factors have noticeable variation. This feature refers to the fact that increasing in the effect of one factor may decrease the effect of other one(s). The models, however, illustrate a stable feature for precipitation over all four under study area during four decades. Consequently, the external factors are the main culprits in decadal changes of annual precipitation. Our finding showed that in the first decade the ratio of factors was 52.38% which increased gradually to 54.08% , 58.44%, and 59.72% among of four under the rest decades (Table 1). This refer to the fact that the changes in variation of precipitation is in association of global changes.
Table 1: The areas under the effects of variation due to internal and external factors (%)
Priod The percent of the country due to ---effects
internal external
First decade 47.62 52.38
Second decade 45.92 54.08
Third decade 41.56 58.44
Forth decade 40.28 59.72
Entire period 44.38 55.62

The climate and precipitation climatology of Iran is effected by internal and external factors, due to geographic features of the country. The internal factors include spatial (latitude and longitude), and topographic (elevation, slop and its aspect) features. In current study, in order to discriminate internal and external characteristics which effects precipitation variation we fitted a multivariate linear regression on internal factors to separate them out and distinct external factors. Accordingly, our finding revealed that the ratio of external factors in variability of precipitation increased from the first decade (52.38%) to the last decade (59.72%). This result is in line with previous studies (Khodadi et al. 2013; Farajzadeh and Ahmadian, 2014; Darand, 2015; Karimi et al., 2018). The most influenced area from the variations of external factors are internal parts rather than marginal parts of the country.
Keywords: Iran, Precipitation, Climate Change, Decadal Variation, Precipitation Variability


Main Subjects

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