We put the tarp down then we stuck Ice and Water sheild onto the tarp. It makes it very water tight. We used chicken wire to hold the straw mat in place. The straw mat does not last all that long so now each year we cover it like we would cover a duck blind. Grasses, tumble weed, and pin oaks.
The UV rays will just trash the tarp if you leave it out. I have also used Perma Barrier to cover the tarp. It is a 18" wide roll of rubber that is have one sticky side.