Characteristics And Uncertainty Analysis Of Different Estimation Methods Of Chinese Fir Plantation Evapotranspiration In Huitong

Chinese fir(Cunninghamia lanceolata)plantation is the subtropical timber plantation with the largest artificial planting area and high economic benefits in China.Therefore,accurate observation of evapotranspiration from Chinese fir plantation plays a key role in the scientific utilization and planning of its water resources and plantation land.However,there is still a lack of accurate estimation methods for how to evaluate the evapotranspiration water consumption of Chinese fir plantation.Based on the observation data of water vapor flux and meteorological gradient from the National Field Scientific Observation and Research Station of Chinese fir plantation ecosystem in Huitong,Hunan Province from 2016 to 2019,this study uses the eddy covariance method(EC)to study the temporal variation characteristics of Chinese fir plantation evapotranspiration(ET)in this area,and its response to meteorological factors,and compared the ET obtained with Penman Monteith Method(PM),Water Balance Method(WBM),and Bowen Ratio Energy Balance Method(BREB)on different time scales,discussed the effect of different estimation methods in ET research on Chinese fir plantation,and identified the best estimation method,the main results are as follows:(1)The ET of Chinese fir plantation showed single-wave average diurnal variation characteristics of high in the afternoon and low in the evening in four seasons,the maximum values all appeared at 13:00,and the values were 0.259,0.396,0.250,and 0.128 mm·h-1,respectively;the minimum values appeared at night,the time is uncertain,and the values were close to 0.The ET showed single-wave annual variation characteristics with high summer and low winter,the maximum values concentrated in July and August,and the values were 105.11,103.54,107.02,113.86 mm·mon-1,respectively;the minimum values were concentrated in winter,and the values were 30.00,30.62,31.41,29.06 mm·mon-1.(2)The main meteorological factors affecting ET of Chinese fir plantation are net radiation(Rn),air temperature(Ta)and vapor saturation difference(VPD).On the hourly,daily and monthly scales,the relationships between ET and meteorological factors were basically the same.There is a very significant linear function relationship between ET and Rn(P<0.001),a very significant exponential function relationship with Ta(P<0.001),a very significant quadratic polynomial function relationship with VPD(P<0.001),and a very significant linear function relationship with soil water content(SWC)(P<0.001).(3)On the hourly,daily and monthly scales,the ET obtained by PM and EC had good consistency in spring and autumn,followed by summer and worst in winter.The diurnal variation trends of ET obtained by PM and EC were relatively consistent in spring,summer and autumn,but there is obvious phase drift in winter,and the difference is obvious;the annual trend of ET obtained by PM and EC was basically the same,with obvious overestimation in summer and obvious underestimation in winter,so that the annual total difference is very small;the difference of ET obtained by PM and EC showed a very significant linear positive correlation with Rn,Ta and VPD(P<0.001),and a very significant linear negative correlation with SWC(P<0.001).(4)On the monthly scale,the consistency of ET obtained by WBM and EC in four seasons was not good,especially in spring,the monthly ET obtained by WBM was significantly higher than that obtained by EC.The annual trend of ET obtained by WBM and EC was similar,but there were significant differences in spring,early summer and annual total;the difference of ET obtained by WBM and EC had no significant correlation with Rn,Ta and VPD(P>0.05),but had a very significant linear positive correlation with SWC(P<0.01).(5)On the hourly,daily and monthly scales,the consistency of the ET obtained by BREB and EC in the four seasons was poor,showing that the ET obtained by BREB was significantly higher than that obtained by EC.Although the daily trends of ET obtained by BREB and EC were similar in four seasons,the difference is significant in most of the time;the annual trend of ET obtained by BREB and EC was basically the same,but the ET obtained by BREB in each month was larger than that of EC,which makes the annual total vary greatly;the difference of ET obtained by BREB and EC showed a very significant linear positive correlation with Rn,Ta and VPD(P<0.001),and a very significant linear negative correlation with SWC(P<0.001).In summary,the ET of Chinese fir plantation showed a single-wave diurnal variation with high in the afternoon and low at night and a single-wave annual variation with a high in summer and low in winter;the main meteorological factors affecting ET were Rn,Ta and VPD;the ET obtained by EC and PM had good applicability in Chinese fir ET research,but the applicability with WBM and BREB was poor,and the ET obtained by different estimation methods was higher than that obtained by EC.