If you are already seeing bats on your land then I wouldn't worry too much about distance from water. I would guess that a properly constructed and placed bat house would be used by the bats on your land.
If water is scarce in your area, it would benefit the bats to have a close water source. Do you have a stock tank on your land? Some of us here have purchased or built small ponds for use by bats, you might think about that. Anything the size of a bathtub or larger will do. Just keep it filled with water and keep the mosquito larvae down by circulating the water or using those mosquito eating fish. You will find that a pond will attract many other creatures as well, e.g. birds are attracted to the sound of running water. I have just a small pump on my pond which circulates the water through a biological filter (i.e. the roots of water plants). This keeps the mosquitoes and algae at a minimal level.
If you do have or provide a water source, make sure that there is a way for bats and other creatures to crawl out if they happen to fall in. See the "Water for Wildlife" booklet from BCI:
http://www.batcon.org/news2/pdf/bciwaterforwildlife.pdfBefore purchasing or building a pond for bats, let me know so I can give you my "lessons learned".
Dave