عنوان مقاله [English]
The proper management and utilization of rivers requires the correct estimation of river discharge. Due to the importance of Dez River in the study area, the trend analysis of streamflow changing of North watershed of Dez River in three times span namely monthly, seasonally and annually were investigated by trend-free pre-whitening- Mann Kendall (MK-TFPW) approach. The data of 14 hydrometric stations were selected in 40-years period (1969-2009). The slope of the linear trend of data estimated using the Theil–Sen approach (TSA) and then using trend-free pre-whitening (TFPW) approach the effect of coefficient of self-correlation of data was eliminated. The trend of streamflow changing were analyzed with MK test. The results show that the decreasing trend rates of more than 70 percent of the stations on annual basis are significant at 5% level. On seasonal basis the maximum and minimum negative changes related to spring and winter seasons, and the negative trend discharge of 10 stations were significant at the level of 10%. On monthly basis the most negative trend related to April month, as in this season the negative trends of 8 stations were significant at 10% level. The lowest negative changes are related to the October and February, as in this month, the negative trends of only 3 stations were significant at the 10% level.