I experimented with a tarp to save weight in my backpacking days. In one outing to the mountains of New Hampshire, it began to rain just as I was setting up the tarp and then rained steadily most of the night. As the camp site was on soft, absorbent forest duff, I was completely dry in a down bag. In another outing in a tarp, I was plagued all night by hoards of hungry mosquitoes. Bring a good supply of bug repellent.