عنوان مقاله [English]
Taking into consideration that Iran is the world's desert belt and has a semi-arid and dry climate, the most important climatic issues focus on lack of rainfall, shortage of water resources and atmospheric elements of climatic instability. Because rainfall is the most variable climatic element in Iran and supplies water resources, natural ecosystems and the environment, the change in the characteristics of precipitation due to unstable sources is dependent on it. Since in Ilam province, the economy is based on agriculture, change in the amount and time of rainfall affects the economy and society. For awareness of this important subject some studies cited explore the spatial variations of one-day rainfall in providing rainy days and amount of precipitation in Iran which showed that in a quarter of the whole of Iran had a decreased share in providing rainy days and only 3 percent of Iran increased their share (Nazarpour et al.,2012).The variance in the retention of precipitation in Kurdistan Province indicate that the share of short-term survival of rainy days and providing the amount of precipitation in the eastern half of the province is declining (Darand, 2014).The results of studies carried out in the context of changes in seasonal and annual rainfall in Iran have indicated rainy season precipitation decrease and increase of seasons with low precipitation (Asakereh and Razmi, 2012, Mofidi,et al., 2012, Negaresh et al., 2012, Mohammadi., 2013, Movahdi et al., 2013, Masoodian and Darand, 2013).The findings of the study of precipitation and drought in southern Italy showed that annual rainfall has decreased and the process of decreasing rainfall in winter is greater than other seasons (Marco and Federico, 2004). Studies examining rainfall trends in the Czech Republic and Slovakia has demonstrated an overall increase in rainfall variability at all scales (Bodri et al., 2005). Evaluation of heavy rains in China has revealed an increase in average annual rainfall in the south west, North West and east of China and a decreasing trend of rainfall in central and north eastern parts (Gong et al., 2004). Evaluation of daily precipitation in the Yangtze River basin illustrated increased summer rainfall, particularly in June and July (Jing Tong et al., 2007). Examination of the two scales of monthly and seasonal rainfall in Turkey showed more stations to record negative winter precipitation trends while higher precipitation rates was observed in other seasons (Kahya and Partal, 2007).
Data and Methodology
In this study, daily precipitation data of Ilam synoptic station in the period 22/05/1986 to 21/09/2012 was used. Homogeneous and heterogeneous precipitation data was evaluated using two tests: the cumulative deviation test and Worsley's Maximum Test. Significant data trends were calculated using the nonparametric Mann- Kendall test.A slope estimator was used to estimate the slope. Leap year in the time series of monthly precipitation amounts was identified with the help of the homogeneity tests mentioned and significant differences in mean time series before and after leap year was evaluated using the Mann Whitney test.
Resultes and Discusión
In this study, variance estimation of the amount and time of precipitation at Ilam synoptic station was performed. The results showed that the amount and time of rainfall during cold and rainy seasons in Ilam station decreased. Thus, precipitation rates of months with higher precipitation also decreased. By comparing the findings of previous research conducted, the results of this study can be confirmed. Exploring the spatial variations of one-day rainfall in providing rainy days and amount of precipitation in Iran showed That in a quarter of the extent of Iran their share in providing rainy days decreased and only 3 percent of Iran on their share has increased(Nazarpour et al.,2012). The variance in the retention of precipitation in Kurdistan province indicate that the share of short-term survival rainy days and providing the amount of precipitation eastern half of the province is declining (Darand., 2014). The results of studies carried out in the context of changes in seasonal and annual rainfall in Iran indicated procedure rainy season precipitation decrease and increased their season with low precipitation(Kaviani and Asakereh, 2005, Mofidi, et al., 2012,Negaresh et al., 2012, Mohammadi., 2013, Movahdi et al., 2013, Masoodian and Darand, 2013). The findings of the study of precipitation and drought in southern Italy using data annually, monthly, daily, showed that annual rainfall has decreased and process of decreasing rainfall in the winter is more than other seasons (Marco and Federico, 2004). Evaluation of daily precipitation in the Yangtze River basin by Mann-Kendall and regression Increased summer rainfall, especially in June and July revealed (Tong Jing et al., 2007). Examination of the two scale monthly and seasonal rainfall in Turkey, more Stations in winter precipitation trends were negative. However, increasing precipitation was obtained in other seasons (Kahya and Partal, 2007).
The findings of this study showed that the amount of precipitation received during the study period in months of high precipitation at Ilam station has fallen while the amount of precipitation received in months of low rainfall and dry months, showed an increasing trend. The maximum reduced rate of precipitation received was in March. In this month, the average rate of rainfall decreased by 3/61 mm for each year. In addition, the reduction in rainfall in November was significant and on average 2/43 mm of annual rainfall amount was reduced in this month. In contrast, in September, the amount of precipitation at Ilam station rose. The time series of annual total precipitation received at Ilam station showed that on average 12/57 mm of its rainfall decreased each year. The findings indicated a positive and increasing trend of rainfall in the warm and dry period of the year. While showing a declining trend for most of the other months of the year. Evaluation time index of rainfall showed a greater reduction of rainfall rates in May compared to other months of the year, and on average 2.12 percent of time of rainfall received in this month was reduced every 10 years. However, rainfall times increased by 0.2 and 0.28 percent per decade in September and October, respectively