Thank you so much for your answer. Effectively, this kind of model is pretty difficult to work with completely dry conditions near the boundary with the stage discharge condition. I added a volume the water and it starts to work. But now I think I have another problem. The water intake seems like the discharge curve is adding volume to the domain, but my intention is that the boundary extracts volume of the domain following the stage discharge curve.
Please is just matter of change the sign of the flow rate in the curve, because now is with positive and as I have mentioned the flow rate is been added to the domain.
Best regards
Ricardo