The Study Of Diurnal Variations Of Grassland Evapotranspiration In The Pailugou Watershed In The Upper Reach Of The Heihe River Basin

The Heihe River basin,the second largest inland river basin in China,is facing a severe problem of water scarcity,which makes the Heihe River basin an extremely hot study area.The streamflow of the Heihe River basin mainly originates from the Qilian Mountain,the upper reach of the Heihe River basin,providing water for the domestic water supply,economic development,and ecosystem services in the middle and lower reaches of the Heihe River basin.Hence,studying the hydrological processes in the upper reach of the Heihe River basin is of prime importance for understanding the hydrological cycle of the Heihe River basin,and supporting effective management of the water resources.Evapotranspiration(ET)is an important component of hydrological cycle,but its measurement in the upper reach of the Heihe River basin 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,an automatic weighing lysimeter was used to measure the hourly ET in a grassland field in the Pailugou Watershed in the upper reach of the Heihe River basin.Other observation equipment included soil moisture probes(ECH2O)and a weather station.Based on the measured data,diurnal variations of grassland ET over different periods(freezing period,early growth period,growth period,and late growth period)and its relationship to the environmental factors were analyzed by statistical methods,which included box plots,correlation coefficients and path coefficients.The main findings from the research are as follows:Firstly,snow and ice sublimation appeared during the freezing period,with a very different diurnal variation pattern compared with other three periods’.During nighttime,the amount of snow and ice sublimation was nearly constant;when the highest solar radiation and lowest relative humidity appeared in the same period,the amount of snow and ice sublimation increased a little.Energy factors,which included solar radiation and soil temperature,had the strongest positive effect on the ET rate.At the same time,soil temperature had the highest direct path coefficient to the ET rate.Secondly,the early growth period was a period when snow and ice started to melt,at which snowmelt evaporation and soil evaporation occurred at the same time with one peak in diurnal variation.Solar radiation was the most important environmental factor during the early growth period.However,unlike the freezing period,correlation coefficient between ET and soil temperature was insignificant.Besides,the direct path coefficient between soil temperature and ET was-0.058.And compared to the freezing period,the influence of aerodynamic component(wind speed and relative humidity)on ET increased a lot.Thirdly,the growth period had the highest ET rate,which was 0.61 mm·h-1 occurred at 14:00.Due to consecutive rainfall events,maximum and minimum ET values appeared at the same hour in the box plots.This phenomenon revealed that rainfall events would exert obvious impact on the diurnal variation of ET through affecting soil water content.The correlation coefficient between solar radiation and ET was 0.850.This highest correlation coefficient was mainly contributed by grass transpiration.Furthermore,the indirect path coefficients of solar radiation by other factors to ET were very high.Fourthly,the late growth period mainly produced on soil evaporation,producing three peaks in diurnal variation,which was different from only one peak in both the early growth period and the growth period.The occurrence of this phenomenon was due to the constantly upward heat transfer in the soil during nighttime and the existence of the zero-flux plane.Compared to the early growth period and the growth period,correlation coefficient between ET and soil temperature was significant again.Soil temperature also had the highest direct path coefficient to the ET rate.And the indirect path coefficients of soil temperature by other factors(air temperature,relative humidity,and soil moisture)to ET were very high.