Land degradation process that affects the arid, semi-arid and sub-humid zones of the globe has been interpreted as desertification that great many debates have grown up around the concept. A fundamental debate has been whether desertification actually exists? If so, how it might be defined, measured and assessed (Herrmann and Hutchinson, 2005). In fact, the term "desertification" was used by Aubreville (1949) to describe the change of productive land into desert, which was the result of human activities in the tropical forest zone of Africa (Tavousi, 2010).However, the United Nations Conference on Desertification (UNCOD), held in Nairobi in 1977, launched the desertification issue into the global arena (Herrmann and Hutchinson, 2005). Desertification as defined in the United Nations Conference on Environment and Development (UNCED) and also in the United Nations Convection to Combat Desertification (UNCCD) is land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors, including climatic variations and human activities (Cardy, 1993). Also, on the basis of this Convention, arid, semi-arid and sub-humid arid regions are regions in which the ratio of precipitation to potential evapotranspiration is in the range of 0.05 to 0.65 (Tavousi et al, 2010).
Determining the contribution of climatic variability to desertification is very complicated, and it is virtually impossible to separate the impacts of drought and desertification, because these processes often work together (Nicholson et al., 1998). Although now a more understanding of climatic variability has emerged, the understanding of the causes of this variability is still unfolding.
Two prevalent paradigms are expressed for climatic variability: One Internal feedback mechanisms such as Biophysical feedback mechanisms between land surface and precipitation due to modification of land cover characteristics in dry land regions and the other are External forcings, such as influence of the El-Nino Southern-Oscillation phenomenon and other major driving forces that promote changes in atmospheric circulations. Most probably, nor of these two prevalent paradigms (internal and external forcings) are mutually exclusive. Relative contributions of climate variability and human agency to desertification will likely depend on specific regional contexts (Herrmann and Hutchinson, 2005).
On the basis of UNEP index we observed that most areas of Iran have arid and semi-arid climates. With respect to the desertification intensity class, these two kinds of climates have classes of severe and very severe conditions. After those two kinds of climates, ultra arid, dry sub-humid, very humid and sub-humid climates cover most areas in Iran respectively (Alijani et al, 2015).
The purpose of this study was to investigate the trend of fluctuations in annual precipitation and the trend of UNEP aridity index of diverse climatic zones in the west and northwest of Iran.
Materials & Methods
In order to study the increase of aridity index in diverse climatic zones of the west and northwest of Iran, in the first step, the area was isolated by cutting 32 N latitude and 50 E longitude. Then, annual temperature average and total annual precipitation data was provided from 43 meteorological stations in the study area during the period of (1981-2010).
This period was divided into three decades: 1981-1990, 1991-2000 and 2001-2010. Then, for each decade, a zoning map was drawn.
In order to classify the climate, evaluate the Aridity Climatic Index and displacement of climatic zones in the northwest of Iran, the aridity index of UNEP (United Nation Environment Program) was used. Also, Kendall's nonparametric test was used to determine the significance of changes in annual precipitation.
Since the air temperature determines the potential evapotranspiration, the UNEP relationship is expressed based on the average total of annual precipitation relative to the average total of annual evapotranspiration.
Discussion and Results
In order to analyze the change in the Aridity Coefficient for each year, the UNEP index was calculated for 43 weather stations in the west and northwest of Iran. Based on the average UNEP index in each decade, the zoning map of the Aridity Index was drawn for three consecutive decades. Then, the UNEP Aridity index was subtracted in successive decades and the change occurred in the studied area was investigated. The spatial displacement of climatic zones over these three decades, represents the increase in the aridity coefficient and expansion of the territory of arid and semiarid climate in the area.
The results clearly indicate climate change from humid climate to semi-humid arid climate and semi-humid arid climate to arid climate. Based on Aridity Index of UNEP, in most parts of the northwest of Iran investigated in this study, the coefficient of Aridity has increased from the moderate risk class to severe and very severe Aridity. Although the results of Mann-Kendall test showed that 32 stations have a negative trend, this trend is significant for the 6 stations of Urmia, Tabriz, Khoy, Miandoab, Piranshahr and Sanandaj at = 0.05 .