I don't worry about erosion... I put tarps on my mile of trails if we're going to get more than a drizzle. Actually, when I grade my trails using the box blade, I pitch them so water runs off. Most of my trails traverse the hillside so when I pitch it to the downhill side, it runs off pretty quickly. The trails are about 20 years old and I'd guess I've lost about 2-3" average due to erosion.