Trend of runoff changes in the watershed of Kordan River

In the present paper, trends of rainfall, runoff and temperature in the Kordan sub-basin which is one of the sub-basins of Qazvin basin were investigated for period of 1966-95. This sub-basin with 360 km2 area is located in the north eastern part of Qazvin basin with a different range of elevation from 1410m to 4108m. Results show although there is not any change in the trend of annual rainfall, but a significant increase and decrease in total amount of precipitation in the second half of year (cold months) and in the first half of year (warm months) and increase in the monthly temperature in most of months were observed. Also trend of moving average of ratio runoff/rainfall (coefficient of runoff) shows significantly a sharp increase annually and also for seasons of spring and autumn. A comparison for coefficient of runoff between first and third decade of period shows different amounts of increase annually and also for seasons of spring and autumn as 58.9%, 12.9% and 5.1% respectively. Increasing of coefficient of runoff in spring is related to general increase of temperature and also precipitation increase in winter season. Increasing of coefficient of runoff in autumn is related to precipitation increase in this season and also occurrence of mismanagement in plant, soil and water resources. Therefore the volume of runoff in this sub-basin which is evacuated and wasted more than before, reaches to 9.4 million m3 per year.