Study On The Variation Characteristics Of Water Use Efficiency On The Typical Tobacco Field Ecosystem In Pan-xi

Water use efficiency(WUE)reflects the relationship between crop material production and water consumption,and is an important indicator of the deep understanding of the coupling relationship between water and carbon cycle in cropland ecosystem.Pan-xi is one of the main producing areas of Clear Flavor Flue-cured Tobacco in China.It plays a very important role in tobacco production in China.It is also a typical climate sensitive and fragile area.This study is based on the eddy covariance technology and MODIS products,to simulate,verificate,resolute and evaluate the variation characteristics and main controlling factors of water-carbon flux and WUE.Meainwhile,evaluating the applicability of MODIS products in the simulate of WUE,and analyzing the spatial and temporal distribution and change trend of WUE under the background of climate change in Pan-xi tobacco planting area.Thus,it provides a scientific basis for improving the ecological system WUE and alleviating the ecological problems caused by the complex climate in the Pan-xi tobacco area.The main results are as follows:1.The WUE in tobacco ecosystem showed obvious diurnal and seasonal variations derived from dynamics of water-carbon flux.In daytime,WUE increased rapidly to maximum at 8:00~9:00,then decreased to minimum before evening.For the seasonal variation,the highest WUE was found in the stage of long term and mature harvest period.Statistical analysis indicated that there were quadratic relationships between WUE and air temperature as well as between WUE and net radiation,but there was an exponential decline relationship between WUE and saturated vapor pressure(VPD).The driving effects of air temperature,VPD and net radiation on the increase of WUE showed temporal differences.During growing period,VPD was the main driving factor on increasing WUE at the beginning,followed by air temperature and net radiation.The temporal differences were mainly related to different sensitivity of gross primary production(GPP)and evapotranspiration(ET)to air temperature,VPD and net radiation.2.Except for seedling and maturity ? stages,the variation of both CO2 flux and ET presented a bimodal curve during a day at the other tobacco growing stages,which became slightly decline or slowdown from 11:00 to 13:00.At the same net radiation intensity,the values of CO2 flux and ET in the afternoon were significantly higher than those in the morning,which was contrary to canopy conductance.In the morning,lower temperatures and VPD inhibited the positive regulation effects of canopy conductance and net radiation on water and carbon exchange.However,in the afternoon,higher temperature and VPD compensated the effects of decrease of canopy conductance and net radiation on water and carbon exchange.3.The ET values calculated by MODIS-ET model with defaulted parameters were about 12.8% lower than the observation values.The efficiency coefficient was 0.767,and the average error in the early growth period was the highest.After parameterization on the basis of the field observation data,the dynamic curve of ET was consistent with the measured dynamic curve,and the model has efficient applicability after parameterization in the study area.4.The GPP simulation value with default parameters is lower than the observed value;the 1:1 straight slope is 0.769;the determination coefficient is 0.776;the efficiency coefficient is 0.253;the root mean square error is 0.268 g·m-2·8d-1.Based on the flux observation data,the GPP simulation effect is obviously improved after the local parameterization of the MODIS productivity model with the default parameters.5.From 2001 to 2016,the annual mean WUE in Pan-xi tobacco planting area was 1.77 gC·kg-1H2 O,and showed an increasing trend with a rate of 0.0537 g C·kg-1H2O· year-1.especially in the last few years,the rate was relatively high.The annual mean WUE and its slope from 2001~2016 showed a single peak pattern with elevation,and the maximum WUE and the minimum slope appeared near the elevation of1500-2000 m ASL.Temperature was the dominant factor affected the WUE in the study area,and the relationship between WUE and temperature was positive.While the correlation between WUE and precipitation were different in different elevations.In the elevation where the WUE were relatively high and its slopes were relatively low,the WUE had strong correlation and sensitivity to temperature,especially the dominant factors.However,the responses of the WUE to temperature was reduced in the lower and higher elevations which away from the center of high WUE distribution.