Keywords: 
• Hydrometeorological variables /
• Mann-Kendall trend test /
• Multiple temporal scales /
• The Upper Reach of the Heihe Watershed

Abstract: This paper analyzed the long-term trend of the hydrometeorological variables including air temperature, precipitation (for the period of 1959-2009)and discharges (for the period of 1978-2007) at the monthly, seasonal and annual scales in the Upper Reach of the Heihe River Watershed, Northwest China, using the nonparametric Mann-Kendall trend test. The effects of air temperature and precipitation on the discharges were also examined in the study watershed. The results indicate that the hydrometeorological variables had all increased at the annual scale during the study period, and annual mean temperatures in the Sunan and Tuole Weather Stations significantly increased. The seasonal mean temperatures, precipitation and discharges had also shown an increased trend, with a larger rise of the mean temperatures in autumn and winter than those in spring and summer. The increase in seasonal mean precipitation mainly occurred in summer. At the monthly scale, the monthly mean temperatures had increased at all four weather stations except at the Sunan Weather Station in October. The monthly mean precipitation displayed a mixed pattern during the study period, showing an increasing trend during the months of January, May through October, and December, and a decreasing trend during the other months. The monthly mean discharges showed an increasing pattern throughout the year at the Yingluoxia Gauge Station (located at the outlet of the Upper Reach of the Heihe Watershed) while the monthly discharges at the Zamashike Gauge Station (located in the upstream of the Upper Reach) exhibited increasing trend during the months of April through November. The increased mean temperatures in autumn and winter and the rising mean precipitation in summer contributed to the increasing mean discharges significantly.