Investigating the trend and time of precipitation and river flow rate changes in Lake Urmia basin, Iran

Abstract

In this study, the trend of precipitation and river flow rate changes in Lake Urmia basin (LUB) were investigated at two monthly and annual scales using the modified Mann-Kendall test (MMK) with complete removal of autocorrelation process. To this purpose, 42 rain gauge and 25 hydrometric stations were selected in Lake Urmia basin during the statistical period of 1984–2013. The results of studying the precipitation changes in the LUB indicated that this basin experienced a combination of decreasing and increasing trend during the studied statistical period. The decreasing precipitation trend was often observed at west and southwest stations of the basin. The increasing trend was observed in the southern and northeastern regions. The results of studying the changes in river flow rate of the studied stations in the LUB indicated that the trend of river flow rate is decreasing in most of the months. At an annual scale, the decreasing trend of river flow rate was also clearly observed across the basin at all stations. In addition, the decreasing trend of river flow rate is more severe in autumn and winter. The results of studying the time of changing point in the precipitation series showed that the time of changing point occurred between 1992 and 1998 at most of the stations that experienced a decreasing precipitation trend. The time of changing point of river flow rate in LUB occurred between 1994 and 1998. Two years after the reductions in precipitation, the decreasing trend of river flow rate occurred in the rivers.