Backwoods Savage said:
Why on earth would you "drop" the tarps fully? At least is sounds like you will be encasing the entire wood pile. That certainly is not necessary and if you do it that can trap moisture for sure. Better to just top cover. Rain or snow hitting the sides of the wood pile does absolutely no harm.
Wood is like a bundle made up of hundreds of thousands of ultra-fine drinking straws running the length of the piece. Water is drawn through these "straw"s through capillary action, and can soak quite deeply into the end grain of a piece of lumber or a split. In fact, the drier the wood, the faster this will occur. Last fall when we had 9" of rain in a single day, my basement flooded unexpectedly, leaving a lot of my good hardwood planks standing on end in the water. After only a few hours with the ends submerged, this kiln-dried hardwood was up over 40% MC (maxed out my meter) 2" up the planks (where I had cut them off to prevent further swelling and damage). On a 16" split, 2" from each end would soak 1/4 of the split's volume to over 40% MC. Long periods of heavy rain might cause even further soaking.
Once the wood is fully seasoned, the ends are the only areas you really need to worry about. Rain will soak the ends of your wood about 10-15 times as fast as it will from the top. That's why it makes sense to drop the tarp down to the ground. Before it is fully seasoned, however, I agree that all you will do is to trap the moisture inside, so top covering only is best at that point.