Heat/cooling load calcs for finished semi-conditioned basement

Here's the deal: Dad's got camping/fishing stuff. A LOT of stuff. And my parents are adding on to my house, and my dad needs a place for his stuff that is reasonable climate controlled. So we're looking at a basement.



In this area, we should do fine with a dehumidifier and no A/C in the basement storage, but I need to find out what kind of heat load I should be calculating for. Obviously, the ground temp will be considerably higher than the air temp in the winter, but I don't know how to account for that with heat load calcs.



Help, please? How can I find out what ground temperature to use?