【Objective】 Evapotranspiration (ET) is a major part of water balance and energy balance, and is the main pathway of water consumption in farmland ecosystems. In Xianyang District of Shaanxi, the annual evapotranspiration accounts for about 90% of the annual precipitation. There are quite a few methods for evaluating or measuring evapotranspiration, such as eddy covariance, Bowen ratio and microclimate. Currently the eddy covariance method is more widely used than the other two because it can be used to measure evapotranspiration continuously on an ecosystem scale, and to measure simultaneously various meteorological factors and various components of energy, as well, in most of the eddy-covariance-related systems, which is conducive to analysis of relationships of various impact factors with components of energy balance. The present study is oriented to explore basic laws of water movement in soils different in land use, but under a similar plant growth environment and also to find a scientific method for estimating water consumption in soil. 【Method】In order to explore laws of diurnal dynamics of evapotranspiration and its influencing factors in farmlands on loess tableland, with the help of some soil water and conventional micrometeorological observatory systems, the eddy covariance method was used to analyze characteristics of the evapotranspiration of the crops (winter wheat and spring corn) in the farmland studied during the growing season (from April to October) of 2013 and their influencing factors. 【Result】Closure analysis of energy balance in the studied area using the eddy covariance method shows that the regression line was 0.54 in gradient and the determination coefficient R² 0.80, both being in the ranges reported in most papers in the literature. Soil moisture is an important factor affecting evapotranspiration, and precipitation is another. After a rainfall event, daily evapotranspiration would increase to a certain extent. Soil water content in the 0~100 cm soil layer varied drastically with CV being quite high, while crop roots were distributed mainly in the 0 ~ 80 cm soil layer, so the soil water in the 0~100 cm soil layer was closely involved in evaporation process. Cumulative evapotranspiration was higher in a sunny day than in a cloudy day, while evapotranspiration started later in a cloudy day than in a sunny day. The daily evapotranspiration was 4.5 mm d^-1 on average in the sunny day and 3.8 mm d^-1 in a cloudy day, with a difference being 0.7 mm d^-1. Moreover, evapotranspiration in a cloudy day was more susceptible to the disturbance of meteorological factors. Net radiation was the main factor affecting evapotranspiration rate, regardless of weather conditions, and the two varied in a similar trend, but the latter lagged somewhat behind the former temporally in variation because it takes some time for energy to flow. The regression equation for analysis of relationships between evapotranspiration rate and its influencing factors in a sunny day does like: ET = 0.833R_n + 0.513T_a -0.333v-0.486; and that in a cloudy day does like: ET = 0.858R_n + 0.219VPD-0.001. Evapotranspiration also varied drastically in process and intensity as affected by soil water regime. 【Conclusion】The influence of weather on evapotranspiration is also obvious and varies sharply with weather conditions. Cumulative evapotranspiration is higher in a sunny day than in a cloudy day, while evapotranspiration starts later in a cloudy day than in a sunny day. And the evapotranspiration under a cloudy weather was more susceptible to the disturbance of meteorological factors. Net radiation is the main factor influencing evapotranspiration rate under all weather conditions. In fields under water stress, the daily evapotranspiration is relatively low because the crops try to maintain their physiological and biochemical processes, thus forming an extensive “evapotranspiration plateau”, while in fields sufficient in soil water, the daily evapotranspiration is high, and the “evapotranspiration plateau” lasts short in duration, and evapotranspiration rate stays high for a long time.