1. The problem statement, all variables and given/known data It's stated in the notes that line fg is the flow line . Why is it so ? 2. Relevant equations 3. The attempt at a solution I think it's wrong . The flow line is defined as a line along which the water paticle travel from upstream to downstream in the permeable medium . However , in my textbook , it's stated that the water can move only thru the flow net , not in one direction only . So , how could the line fg be the flow line ? How could water flow thru line fg ( horizontally in one direction only) ?