Estimation Of Cropland Daily Evapotranspiration

Water vapor and heat flux density in winter wheat and mung bean field were measured using eddy covariance system in Luoyang. Energy balance closure were evaluated by ststatistical regression of turbulent energy fluxes (sensible and latent heat (LE+H)) against available energy (Rn-G-Ss) and by solving for the energy balance ratio, the ratio of turbulent energy fluxes to available energy. The characteristic of energy exchange and transfer in winer wheat and mung bean field ecosyctem were discussed. Empirical model to estimate daily evapotranspiration of winter wheat and mung bean field were established using the transient observation data of net radiation and the difference of canopye and air temperature.Main research results are as follows:1,Energy balance closure were evaluated. The methods used to evaluate energy balance closure indicate a general lack of closure in winter wheat–mung bean field in Luo yang. The the mean EBR was 0.78. The imbalance was greater during the early stage of winter wheat and nocturnal periods. The OLS slope were 0.4773 and 0.6443, EBR were 0.72 and 0.84 before and after canopy closed, respectively. The OLS slope and EBR increased with friction velocity, LAI and vegetation height. EBR increased with air temperature and VPD and decreased with RH. There was no obvious effect of bowen ratio on EBR.2,The characteristic of energy exchange and transfer in winer wheat and mung bean field ecosyctem were discussed. The differences of diurnal and nocturnal bowen ratio was obvious. Most anomalous valuses of bowen ratio were found at night. The diurnal bowen ratio in different growing stage were different, which was higer in the early growing stage of crops and lower in the peak season. The change of EF was more stable then the bowen ratio. The characteristics of the diurnal cycle of EF in different growing stage of winterwheat and mung bean were same. There was no obvious difference of nocturnal EF in different growing stage of winter wheat and mung bean. The diurnal EF in peak season of winter wheat and mung bean were greater.3,The semi-emperical model of daily evapotranspiration were established. The daily net radiation and daily maximum canopy and air temperature were used in the semi-emperical model. Observation data of were used for verifying the proposed model. Verification results showed the semi-emperical model simulated daily evapotranspiration in winter wheat and mung bean field successfully.4,The emperical model of daily evapotranspiration of winter wheat-mung bean field in Luo yang based on transient net radiation, transient canopy and air temperature were established. And the emperical model of daily evapotranspiration in sunny and cloudy days during different growing stage were discussed. Results show that the simulation of emperical model in suuny days after canopy closed were satisfied and better then the simulation results of the emperical model in cloudy days before canopy closed.5,The main creative point : (1) The transient net radiation, transient canopy and air temperature were used to establishe the emperical model of transient evapotranspiration in winter wheat-mung bean field,and Observation data of transient net radiation, transient canopy and air temperature were used to for verifying the proposed model. Verification results showed the simulation of the emperical model were satisfied. (2) The emperical model to estimate daily evapotranspiration based on transient evapotranspiration and sine relation were established. The transient net radiation, transient canopy and air temperature were used to calculate the daily evapotranspiration. The emperical model to estimate daily evapotranspiration using transient net radiation, transient canopy and air temperature provided an easy way to estimate field daily evapotranspiration over large areas using remote-sensing information.Our study on corners needs to be improved. The observed altitude of eddy covariance system depends on fetch and the vegetation height. The observed altitude of eddy covariance system should be adjusted to accommodate the vegetation height in cropland. The feasible observed altitude of eddy covariance system in crop field were supposed to be 0.5m-1.0m above the canopye。But in our study the observed altitude of eddy covariance system were same during different growing stage of crops, which may be the main reason for the imbalance.