Abstract: Evapotranspiration (ET) is an important component of water cycle, but its measurement in high altitude mountainous region is quite difficult, resulting in the poor understanding of the temporal and spatial variations of actual ET in high altitude mountainous region. In this paper, a weighing lysimeter was used to measure the hourly ET in a grassland in the Pailugou basin in the upper reach of the Heihe River, Northwest China. Based on the measured data, diurnal variations of grassland ET over different periods were analyzed. Results indicated that snow and ice sublimation appeared during the freezing period, with a very different diurnal variation pattern compared with other three periods. During the period without sunshine, the amount of snow and ice sublimation was nearly constant. When the highest global radiation and lowest relative humidity appeared in the same period, the amount of snow and ice sublimation increased a little. The early growth period was a period when snow and ice started to melt, during which snowmelt evaporation and soil evaporation occurred at the same time. The growth period had the highest ET rate. Due to continuous rainfall events, maximum and minimum ET values appeared at the same hour. ET in the late growth period mainly came from soil evaporation, producing 3 peaks in diurnal variation, which was different from only one peak in both the early growth period and the growth period.

Key words: grassland evapotranspiration, upper reach of the Heihe River, weighting lysimeter, diurnal variation