The incidence of drought phenomenon is one of the realities of the Urmia Lake Drainage Basin and can be stemmed up from periodical climatic fluctuations and lack of humid and rainy Mediterranean air masses. In this research the data relating to the annual precipitation have been studied for Urmia Lake Drainage Basin in a time period of 43 years (1960-2002) and with the purpose of analysis and calculation of dry and wet years. The main methodology being used in this research is the Dependable Rainfall (DR) criterion. The results of this study show the occurrence of drought phenomenon in all stations generally showing a degree of weakness or effectiveness. The Dependable Rainfall (DR) Index is recognized as a better and more accurate model compared to the other models applied in this research, i.e., Z Score Index of Annual Precipitation (ZSIAP), and Percent of Normal Precipitation Index (PNPI).
Keywords: Annual precipitation, Drought threshold, Dependable Rainfall Index (DRI), Drought, Wetness, Urmia lake drainage basin.
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