明渠水流机械能损失的确定是水力学乃至工程流体力学中的重要内容。从明渠均质不可压缩液体恒定总流出发，以流体力学中黏性流体运动数学模型为基础，本质上揭示了机械能损失的构成和相互转化关系。成果表明矩形明渠均匀流紊流的机械能损失由时均流速梯度引起的损失与紊动能耗散引起的损失组成，当雷诺数较小时，时均流速梯度引起的损失是机械能损失的主要来源，而雷诺数较大时，紊动能耗散引起的损失是机械能损失的主要来源。在宽浅明渠中，雷诺数约为9.5×10³时，均流速梯度引起的损失和紊动能耗散引起的损失相等。

The calculation of mechanical energy loss of flow in open channel is an important part both in hydraulic and fluid mechanics. In this paper the mechanical energy loss is obtained based on the viscous fluid model and the equation of total flow of constant homogeneous incompressible fluid in open channel. The constitutions of mechanical energy loss and their mutual transformation are revealed. Result shows that the mechanical energy loss are resulted from time-averaged velocity gradient and dissipation of turbulent kinetic energy. When the Reynolds number is small, the time-averaged velocity gradient is the main cause of mechanical energy loss; whereas when the Reynolds number is large, the dissipation of turbulent kinetic energy is a major contributor. In conclusion, in shallow and wide open shallow with a Reynolds number of 9.5×10³, the mechanical loss caused by time-averaged velocity gradient is equal to that caused by kinetic energy dissipation.