Hi, The outlet of my simulated lake was a weir. When the water level was bigger than 130m, it wiil has outflow. I only define this weir in fvc file as follow: structure == Nodestring, 4 !Weir Flux function == weir !Weir flag Properties == 130,1.0 !Weir elevation, weir coeffecient end structure But when I run the model, only very small inflow can make the water level of lake increase sharply. So I suspect a problem with the weir. I need your help to define a outflow boundary as a weir. Thanks very much. Best wishes. Lei