Studies On Stomatal Conductance And Evapotranspiration Of Maize Field In Semi-arid Area

Evapotranspiration,which is comprised of evaporation from soil surface and transpiration from vegetation,plays an important role in maintaining global energy balance and regulating climate.Using eddy covariance measurements?small automatic weather station and portable photosynthesis instrument in maize fields for the growing season at the Experiment Station of Agro-ecosystem in Semiarid Area(ESASA)of Lanzhou University,we constructed stomatal conductance model with Jarvis model and Ball-Berry model.We also ran the revised Shuttleworth-Wallace model(S-W model),partitioned evapotranspiration in maize fields under plastic film mulch conditions into evaporation and transpiration,validated the performance of the model for different time scales and under different weather conditions with measured eddy covariance values,analyzed the driving factors of ET and its components.Results were as follows:(1)The diurnal variation of stomatal conductance in sunny days is double-peak curve,and multi-peak curve in cloudy days.Main meteorological elements that affect stomatal conductance are photosynthetically active radiation,atmospheric CO2 concentration,temperature and saturated vapor pressure difference.Jarvis model performs well than Ball-Berry model,and the Jarvis model constructed with photosynthetically active radiation and atmospheric CO2 concentration performs the best.(2)In the prevailing wind direction,the upwind footprint tail of the growing season is longer than that of non-growing season.However,the reverse is the case for the vertical upwind direction.The comparison between non-growing season 1 and non-growing season 2 show a stable area of flux footprint for the non-growing season.In the prevailing wind direction,flux footprint decreases from seedling stage to tasseling stage while it increases from tasseling stage to maturity stage.Under stable stratification,source areas are larger than those under unstable conditions.The energy closure are 0.671 and 0.869 at half-hour scale and daytime scale,respectively.It can be considered that eddy covariance data of the experiment station is reliable.(3)The simulated ET in the study area is in good agreement with the measurements in both sunny and cloudy days,but the model performed poorly in rainy days.In diurnal timescale,the modified model performs well when ET is larger than 2 mm·d-1 in both sunny and mostly cloudy days.But the model slightly underestimates ET in rainy days.Diurnal variation in E/ET followed a single-peak curve,the low point is observed at night.At seasonal timescale,E/ET decreases from 18% to 8% at jointing stage,and keeps at 8% level at tasseling stage and filling stage.The E/ET is controlled by canopy stomatal conductance at diurnal timescale,while at seasonal timescale,it is mainly controlled by leaf area index and soil moisture content which regulates transpired water from leaf stomata and evaporated water from bare soils.