Simulation On Evapotranspiration And Analyse Of The Plant Water Stress Index Over A Mixed Plantation In The Southern Foot Of The Taihang Mountain

In this paper,the Penman-Monteith(P-M)model and the Shuttleworth-Wallace(S-W)model are used to simulate ET over a mixed plantation in the southern foot of the Taihang Mountain.The mixed plantation is mainly composed of Quercus variabilis,Platycladus orientalis and Robinia pseudoacacia.In order to evaluate the model applicability,ET simulated by the S-W model and the P-M model was compared with the measured one obtained by the eddy covariance technique.In addition,the sensitivities of P-M and S-W models to their resistance parameters were analyzed and the diurnal and seasonal variations in ET were explored.This study also calculate the plant water stress index(PWSI)from evapotranspiration ratio method to analyze the relationship between PWSI with soil water content(SWC)and evapotranspiration(ET)and net Ecosystem productivity(NEP)and water use efficiency(WUE)in sunny or cloudy conditions for mixed plantation.ET simulated by the S-W model and the P-M model was about 6%and 21%lower than the measured ET,respectively.Compared with the S-W model,ET simulated by the P-M model was less than the measured one.Compared with the P-M model,the correlation coefficient,mean absolute error between measured ET and simulated ET by the S-W model were lower,agreement index were higher.The fitted curve of the relation between ET simulated by the S-W model and measured ET was closer to the 1:1 line than that of P-M model.Accordingly,ET simulated by the S-W model was higher than that of the P-M model.It is suggested that the S-W model was more suitable for simulating ET in this region,and ET simulated by the P-M model was low.ET simulated by the P-M model and the S-W model was most sensitive to the change of canopy resistance.The second sensitive parameter was the aerodynamic resistance from canopy to reference height for the S-W model,and it was insensitive to the aerodynamic resistance from the soil to the canopy.Transpiration and evaporation were most sensitive to the canopy resistance and soil surface resistance,respectively.PWSI decreased with the increase of SWC,whatever regardless of weather conditions or distinguish between sunny and overcast weather.The downward tendency that PWSI decreased with the increase of SWC was more obvious in sunny day.Both ET and NEP showed a significant decline trend with the increase of PWSI.ET did not decrease with the increase of PWSI When PWSI increased to a certain value.Stomatal conductance stoped decrease as PWSI increased to a certain value.In sunny sky conditions,ET and NEP and WUE were higher than the ones in cloudy sky conditions.ET remained stable with the increase of PWSI.ET quickly decline with the increase of PWSI but NEP reduced with the increase of PWSI when PWSI was more than 0.6...Compared with the cloudy sky conditions,the decrease tendency of ET and NEP in sunny sky conditions was more evident.There was a significant increasing linear relationship between WUE and PWSI in two weather conditions.In sunny sky conditions,WUE was no significant change when PWSI was less than 0.4,but significant increasing linear relationships between WUE and PWSI were found when PWSI was greater than 0.4.Meanwhile,WUE increased slowly with the increased of PWSI.