Abstract: By using eddy covariance and remote sensing techniques, the relationships between winter wheat soil moisture content and farmland evapotranspiration or canopy temperature were analyzed at field scale under various environmental conditions in the North China Plain. The results showed that when the soil moisture content was below 65% of field capacity, the evaporative fraction under full canopy was low and stable during the middle part of clear days. Under clear sky condition, there was a good non-linear correlation between latent heat flux and crop canopy temperature with diurnal and seasonal patterns. The temperature difference between crop canopy and air as well as the relative evapotranspiration had a close link to the relative moisture content of 0～100cm soil layer. Based on the in situ measurements of daily evapotranspiration amount (ET_d), daily net radiation flux(Rn_d, mm)，average canopy temperature (T_c, ℃) from 13:30 to 14:00, and daily maximum air temperature (T_{a max},℃) during the field experiment, the parameters of simplified estimation model for daily evapotranspiration were established.

Key words: Constant charge soil colloid, Adsorption, Desorption, Lead(Ⅱ), Low-molecular-weight organic acids