عنوان مقاله [English]
Background and objectives: Concentration Index (CI) is actually an index to examine the statistical characteristics of daily precipitation. This concept could be also used in irrigation planning and designing new systems. Unbalanced distribution of precipitation could reduce the agricultural crops yield through reduction of moisture in soil and increment of irrigation periods. Besides, unbalanced distribution of precipitation means drought. The precipitation less than regional average may cause dangerous floods since soil moisture will reduce and vegetation will be destroyed. The aim goal of this study is evaluation the distribution and concentration of daily rainfall in the basin of Lake Urmia.
Materials and Methods: In this study used the 42 rain gaging station in Urmia Lake basin to evaluation the daily precipitation concentration. Concentration Index (CI) and related concentration curve are in fact used to define inequality quantity of a parameter from a specified variable. For example, it could be used in statistics to determine improved level of health subside for poor people in different countries as well as precipitation dispersion in an area or country. Concentration Index is defined by a concentration curve (Lorenz Curve). The Lorenz curve is actually a concept used in economic theories to examine the temporal-spatial changes in time series of daily precipitation and concentration
Results: The results of CI index in Lake Urmia basin during the statistical period in spring showed that CI>0.6 is not observed in none of the areas under study. Majority of Lake Urmia areas in spring had the CI values between 0.4 and 0.5. In this season (spring), aggressive and terrifying precipitations were observed in none of the basin areas. As spring in the study under area in autumn, CI> 0.6 was not observed in any of the stations studied. In winter as well as spring and autumn, a part of the South and the Northeast of Lake Urmia and also an area of the Northeast of basin had average CI between 0.5 and 0.6 in statistical period and show the high concentration of daily precipitation and irregularity in distribution of daily precipitation in these areas. It can be concluded that the amounts of daily rainfall in listed areas were not divide into rainfall days. The CI in most of the areas under study were between 0.4 and 0.5. The southern areas of Lake Urmia basin had severe irregular precipitation in summer and it could be concluded that 70% of rainfalls in this season occurred only in 25% of rainfall days. In this season the Northeast and South regions of Lake Urmia, an area in Northeast of Lake as well as a station in the Southwest of lake had irregular distribution of precipitation in summer. In annual scale, most of the Lake areas had average concentration. In annual scale, similar to seasonal scale, Northeast and South of lake had more irregular concentration compared to other areas.
Conclusion: In this study to evaluation the concentration and pattern of daily precipitation in Urmia Lake basin in period of 1984-2013 used the concentration index (CI). The results of concentration index in basin area showed that daily precipitation in Lake Urmia basin was not in regular and severe irregular situation in none of the stations under study. The entire stations surveyed were in relatively regular, average concentration and relatively irregular situation in terms of daily precipitation distribution. Most of the studied stations were in average concentration category in terms of daily precipitation concentration. In autumn, winter, spring and summer in annual scale, almost 74, 90, 81, 74 and 84% of stream-gaging stations showed average precipitation concentration.