Dynamics And Environmental Control Of Nocturnal Vapor Exchange In A Desert Shrub Ecosystem In Mu Us Desert

Nocturnal vapor exchange is very important in ecosystem hydrological cycle,and it has great influence on ecosystem water balance and physiological development of plants.However,the understandings of its dynamics and mechanism in desert ecosystems were still weak.This thesis measured water and energy fluxes of a desert shrub ecosystem in Mu Us desert using eddy covariance method and corresponding environmental variables from 2012 to 2014 to examine the dynamics and abiotic controlling mechanism of nocturnal vapor exchange.The main results are as follows:(1)The average annual air temperature(Ta),relative humidity of air(RH),vapor pressure deficit(VPD),volumetric water content at 10 cm and 30 cm below the ground(VWC10 and VWC30)were 9.5?,48.44%,0.73 kPa,0.069 m3 m-3 and 0.082 m3 m-3,respectively.RH/was higher during the mid-and late-growing season.VPD was higher in spring and early summer from April to June.The year 2013 was dry with VWC30<0.08 m3 m-3.The year 2014 had spring drought.Wind speed(Ws)ranged 1.0?3.0 m s-1,being lower during the peak-growing season.(2)Nocturnal Rn,G and H were almost negative.Positive values of nocturnal LE represent nocturnal evapotranspiration(ETN)and negative values represent nocturnal condensation(CN).The minimum of monthly means of H/Rn and bowen ratio(HILE,P),and the maximum of monthly means of evaporative fraction(LE/Rn,EF)and Priestley-Taylor coefficient(?)were in July,indicating the strong evapotranspiration(ET).Annual EBR from 2012 to 2014 were 0.79,0.79 and 0.72,respectively,and the respective annual nighttime EBR were 0.48,0.56 and 0.54,thus nighttime LE obtained by EC well representing the dynamics of nocturnal vapor exchange.(3)Daily mean of CN was 0.15±0.08 mm.Number of days with CN accounted for 78%of the whole year.CN was more on summer and autumn than on spring and winter.CN showed positive correlation with Ta,RH,Ws and rainfall,and negative correlation with G,VWC10 and VPD,being dominantly controlled by RH.CN increased slowly at RH<75%,while it increased significantly when RH>75%.Mean annual CN amount over three years was 12.5%of rainfall.The water input met the ET demand in this ecosystem.CN can relief the water loss caused by ET during the non-rainfall period.Penman-Monteith equation estimates the CN well.Method combing leaf wetness sensor and Penman-Monteith equation would underestimate CN compared with EC method.(4)ETN followed the pattern of first rising then falling in a year,and it peaked during the summer month(June to August).ETN showed positive correlation with Ta,VPD and Ws,and negative correlation withVWC10 and VWC30.Growing-season ETN was mainly controlled by regardless of years,with its interannual variation being controlled mainly by soil water.The responses of ETNto environmental factors varied with phenophases.Ta,VPD,VWC10 VWC30 and Wswere the main environmental factors affecting ETNin the leaf expending stage,while Ta,VPD and VWC30 in the leaf expended stage,and Ta,VPD,VWC10 in the leaf coloring stage.Overall,Ta dominant in different phenophases and growing season in different years.Mean annual total ETN over three years was 10.0 mm,accounting for 3.7%of ET.(5)The net nocturnal vapor exchange(NNVE)showed as CN,and was higher during the mid-and late-growing season.The annual amounts of net nocturnal vapor exchange(NNVE)from 2012 to 2014 were 42.9 mm,28.0 mm and 19.4 mm.Mean growing-season NNVE over three years accounted for 66.2%of the mean annual total.The results above reveal the dynamics and mechanism of nocturnal vapor exchange(including CN and ETN)in a desert shrub ecosystem in Mu Us desert,and the influence of CN and ETN on the water balance,and this will provide supplement for the research of water balance and support for the decision making of water management and recovery of vegetation in arid area.