Abstract[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 coefficientR2 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 = 0.833Rn + 0.513Ta-0.333v-0.486;and that in a cloudy day does like:ET = 0.858Rn + 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.%蒸散是水量平衡和能量平衡的重要组成部分，也是农田生态系统水分消耗的主要途径。为探究黄土塬区农田蒸散的日动态变化规律，运用涡度相关法、土壤水分及常规微气象观测系统等，于2013年作物生长季（4—10月）对试验区农田作物（冬小麦、春玉米）蒸散特征及影响因素进行分析。结果表明，降水对蒸散的影响较为显著，降水过后的日蒸散量较降水前会有所增加；农田0～100 cm土壤含水量变异系数较大，土壤水分变化剧烈，作物根系的集中分布范围在0～80 cm之间，因此0～100 cm土壤水分主要参与蒸散过程；晴天蒸散的累积量大于阴天，晴天和阴天的日均蒸散量分别为4.5、3.8 mm d-1，相差0.7 mm d-1。阴天蒸散开始的时间较晴天晚，阴天条件下的蒸散更易受到气象因子的扰动；不同天气条件下净辐射均为蒸散的主要影响因子，蒸散速率与净辐射变化趋势一致，但在时间上滞后于净辐射；在不同的土壤水分环境条件下，蒸散的过程和强度差异较大，水分胁迫条件下，全天蒸散量水平较低，“蒸散高地”的持续时间较长；而水分相对充足时，全天蒸散水平较高，“蒸散高地”持续时间较短，维持较高的蒸散速率的时间较长。
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