Here, the water 'leaks' out of the sidehills near the top of the hill.  Very important to tile to the top of the hill to make sure it is dry all the way at the top.  Water that runs off the top of the hill is going to run all the way to the bottom.  If you can stop it by having that area dried out and allow the water to infiltrate, you decrease erosion by a lot.  Used to be, they would stop when they could see over the hill.  Bad mistake, you can see where the tile stops and we are now going back in and starting to fill these areas in.