Temporal Variation Of Canopy Conductance And Its Effect On Evapotranspiration For Aremisia Ordosica Community In Mu Us Desert

Canopy conductance?gc?is the most important variable in the exchange of carbon and water.Evapotranspiration and environmental factors of a shrub ecosystem,which was dominated by Artemisia ordosica in northwest China,were continuously measured using eddy covariance technique in growing season?April-October?from 2013-2015.Meteorological variables including air temperature?Ta?,relative humidity?RH?,photosynthetic active radiation?PAR?,soil volumetric water content?VWC?,and precipitation?PP?were also measured.Canopy conductance was calculated using the inverted Penman-Monteith equation.The results suggest that:?1?Canopy conductance of Artemisia ordosica community showed obvious diurnal and seasonal dynamics.Diurnal dynamics of canopy conductance showed a single peak curve,with similar diurnal pattern to PAR and vapor pressure deficit?VPD?.Diurnal pattern in gc peaked around 12:00,1-2 h and 3-4 h being ahead of PAR and VPD.The maximum value of gc generally appeared in July,August and September.The gc showed interannual variability,with annual mean being highest in 2013,and lowest in 2015.?2?PAR and VPD had significant influences on canopy conductance.Canopy conductance increased with inceasing PAR,then decreasing when PAR was greater than this threshold.The response of canopy conductance to VPD was different among three years.In 2013 and 2014,canopy conductance decreased with increasing VPD,while it increased with increasing VPD in 2015,saturating at 1.5 kPa,then decreasing when VPD was greater this threshold.?3?VWC is the key limited factor at our site.The gc values had positive relationship with VWC under high VWC?VWC₁0>0.16 m3·m-3,VWC₃0>0.18 m3·m-3?.Under low VWC,the gc values remained smaller.The gc was more sensitive to V WC₃0 than VWC₁0.The gc was more responsive to environmental factors?PAR,Ta and VPD?when VWC was high.?4?Diurnal evapotranspiration?ET?showed a single peak curve,increasing from 06:00 in the morning,peaking at 12:00 and then decreasing to 0 at 20:00.ET was lowest in April over three years,highest in July in 2013 and 2014,and September in 2015.ET increased with increasing canopy conductance,saturating at 2.5 mm s-1,and no correlation when gc was greater than this threshold.?5?The sensitivity of ET to gc varied in different time scales.The sensitivities of ET to gc were in the order of daily scale,hourly scale.Our results provided more understanding of gc dynamics and its controls on ET in semi-arid land.Moreover,our results could potentially provide information towards hydrological modellingof arid and semi-arid ecosystems.