Environmental Response And Partitioning Of Evapotranspiration Of A Populus Euphratica Forest In Extremely Arid Areas

Water resource is the main limiting factor for vegetation growth in extremely arid areas in northwest China.The study of water consumption of vegetation is of great significance to improve the efficiency of water resource utilization and alleviate the tension of water resource.Evapotranspiration(ET),including evaporation(E)and transpiration(T),is an important part of the terrestrial water cycle.As one of the most important ecosystems in arid areas,desert riparian forests play an important role in maintaining ecosystem balance and can promote regional sustainable development.As the representative vegetation of desert riparian forest in the extremely arid areas of the lower reaches of the Heihe River,Populus euphratica has the important value for soil and water conservation,wind prevention and sand fixation and ecological environment improvement.During the main growing seasons of P.euphratica in 2018 and 2019,we measured ET of P.euphratica based on eddy covariance system,and T by scaling cup from sap flow of individual trees,and then we can partition ET.The changes of ET,T and the ratio of T to ET(T/ET)in different time scales(diurnal and phenological periods)were analyzed.Combined with environmental factors,including net radiation(R_n),air temperature(T_a),vapor pressure deficit(VPD),relative humidity(RH),wind speed(u)measured by micrometeorological observation system,and soil heat flux(G)by soil heat flux plates,we conducted correlation and path analyses to analyze the influence of each environmental factor on ET.Based on the data of groundwater table observed continuously,the influence of surface water discharge on T/ET of P.euphratica in the lower reaches of the Heihe River was explored.Main results are as follows:(1)Diurnal variations of ET and T of P.euphratica forest showed a"parabolic"trend on the whole-increasing firstly,and then decreased,and the peak values occurred around14:00.During the main growing seasons of P.euphratica,ET rised with fluctuations during leaf expansion and fruit periods,while decreased at seed dispersal and leaf discoloration periods.ET in fruit and seed dispersal periods were the main parts of the whole growing season of P.euphratica,total ET were 285.88 mm and 293.03 mm,accounting for 52.1%and 52.2%of the total amounts in 2018 and 2019,respectively.Total ET during seed dispersal periods were 223.24 mm and 227.59 mm,accounting for40.7%and 40.6%of the total amounts in both years.T increased at leaf expansion and fruit periods,reached peak values at the beginning of seed dispersal period,and decreased at the middle and late stages of seed expansion period and leaf discoloration period.T/ET increased slowly at first,and then decreased gradually,while during the whole growing season of P.euphratica,T/ET increased gradually at the beginning,and peaked at the end of fruit and early seed dispersal periods.The daily mean values of T/ET were 0.42 and0.45 in the main growing seasons of P.euphratica in 2018 and 2019,respectively.(2)Correlation analysis showed that ET had the highest correlation with R_n(correlation coefficients were both 0.97 in 2018 and 2019),followed by T_a(0.58 in 2018and 0.46 in 2019),and ET had extremely significant positive correlations with R_n,T_aand G(P<0.01),and negatively correlated with RH and u(P<0.01).In different phenological periods,the correlation coefficients between R_nand ET(0.983 in 2018 and 0.987 in 2019)were higher than those in other periods.Path analysis showed that R_nhad a direct effect on ET during the main growing seasons of P.euphratica,T_a,VPD,RH and G mainly had indirect effects on ET through R_n,and u had a direct effect on ET.According to the determination coefficients,R_nis always the main determinant of ET(0.934 in 2018 and0.931 in 2019),and RH and G are the limiting factors.(3)T/ET of P.euphratica increased firstly,and then decreased during the whole growing season of P.euphratica.T/ET increased continuously from May to early August,and then decreased before flood irrigation.After September,P.euphratica entered the late growing season.Although the groundwater level in the test site increased as a result of surface water discharge of the Heihe River from September 13,T/ET kept a downward trend,indicating that T would not increase continuously with the further increase of water resources under the condition that the growth of P.euphratica did not face water shortage,and the influence of surface water discharge on T/ET was small.