Ok.
If We select a surface, in the riboon we can select "AddSurfaceSlopeLabel" to add labels in the surface.
However, I don´t like the behaviour of this label, because the slope value will always be according to the direction from one picked point to second picked point.
In my case, I always show in these labels the negative slope (direction of the water), and the arrow in that direction.
Now, the direction of the arrow is according to the direction from one point to second point too.
I can´t change this behaviour in the label style.
Perhaps, it would be interesting a new label type, such as AddSurfaceSlopeNegativeLabel... independent of picking the first point and second point.