The Evapotranspiration Of Alpine Meadow And Its Complementary Relationship At Point Scale In Permafrost Region Of Qinghai-Tibet Plateau

Evapotranspiration (ET) is linked with the water-heat exchange process between biosphere and atmosphere.It not only directly influences the precipitation,surface runoff,and soil moisture content,but also plays a key role in changes of the water resource and ecosystem environment.The alpine meadow is one of the important ecosystems on the Qinghai-Tibetan Plateau,and researching ET of alpine meadows is beneficial for understanding how alpine ecosystems respond to climate change.In this paper,we selected alpine meadow where in Fenghuoshan region on the three river sources as a study area,and analyzing the actual evapotranspiration(ETa)and potential evapotranspiration(ETp)of alpine meadow and its influence factors based on the data by eddy covariance and meteorological station;and using the CR principle to simulate the ET process.The results showed that:(1)on the daily/monthly scale,the changes of ETa and ETp were both fluctuated,and the maximum value was reached in July,3.34 mm/d and 4.38 mm/d respectively.In the precipitation period,the ETa is slowly and persistently response to precipitation,but ETp is severly and transiently response to precipitation.(2)The ETa and ETp's influence factors were variation with the change of seasonal.The feature of surface soil freezing in soil completely frozen period and not completely frozen/thawed period,leads to no significant relationship between ETa and meteorological factors,including relative humidity and wind speed,these were different with ETp.In the soil completely thawed period,ETa and ETp were both have a negative relationship with wind speed.The feature which with the increasing of wind speed,the precipitation and solar radiation decreasing,was the main reason.(3)The results of simulating the alpine meadow based on tradition CR principle as follow: there was no existing complementary relationship of ET in the soil completely frozen period,but there was an asymmetric complementary relationship when HI(where HI is equal to ETa/ETp)> 0.2,and ? = 0.58(where ? is a coefficient that express the proportion of the sensible heat that increases ETp).There was an asymmetric complementary relationship in the soil completely thawed period and the soil not-completely frozen/thaw period,?=0.84 and 0.63,respectively.These results show that the complementary relationship of ET on the Qinghai Tibetan Plateau is affected by season,and it is necessary to consider the influence of seasons when using the CR principle to estimate ETa.(4)The results of simulating the alpine meadow based on the latest CR principle which consider physical boundary as follow: due to the special alpine environment,the alpine meadow evapotranspiration process under original P-T formula condition was not only deviation from the latest CR principle,but also can't be simulated by tradition CR principle.However,the evapotranspiration process is close to the CR principle's results after revised P-T formula and has a significant asymmetric complementary relationship(where ? < 1)in the soil completely thawed period.Besides atmospheric condition,vegetation,soil and etc.,the permafrost maybe was a not ignorable influence factor for the CR of alpine meadow.