ABSTRACT
Climate changes are one of the most important environmental challenges at present. Our understanding of the human impacts on the environment, particularly those associated with the warming due to the increasing greenhouse gases, shows that several parameters are most likely changing. According to scientific reports, the average surface temperature of Earth has increased about 6°C during the 20th century. It is expected that the amount of evaporation will have an ascending trend. Therefore the atmosphere will be able to move greater amounts of water vapor and hence the amount of precipitation will be affected (Tabari 2011). Insufficient precipitation and its severe fluctuations on daily, seasonal, and annual scale are amongst the inherent climatic characteristics of Iran. The precipitation patterns have changed under the influence of global warming and led to the occurrence of extreme weather events such as floods, drought, rain, storm, etc. For instance a significant reduction in the number of rainy days has been confirmed in many parts of the world, including China (Zhang et al. 2008, Ren et al. 2000, Gong and Ho 2002, Zhai et al. 2005). One of the most important aspects of climate change that requires closer examination is to review the temporal distribution of precipitation and its historical changes. With regard to the effect of precipitation on water resources like groundwater, surface water, and snow reserves, it seems necessary to use indices for the expression of changes. Some of these indicators include Standardized Precipitation Index (SPI), Precipitation Concentration Index (PCI), and Density Index (DI).
