I'm sorry to see that no one has responded to your question earlier. Different experts come online at various times; I just came online and saw your question.
It's wise of you to research this before starting. Ponds are an excellent habitat for turtles, but there are some considerations. In the north, where it gets and stays cold, turtles hibernate for the winter. In the south, where temperatures fluctuate, things are harder on them. They wake up, go back to hibernation, spend a few days being sluggish, and repeat this over and over. Hibernation is a dangerous time, even for wild turtles who have adapted to outdoors and can find the optimal places for hibernation. For domestic turtles in man-made ponds, the risks are even greater. Turtles can suffocate, starve if they don't have enough fat built up, develop fatal bacterial infections if they eat too close to hibernation time, and be killed by toxins that build up in the water. There is no way to absolutely ensure that your turtles will survive the winter. There are ways to increase the chances that they will. There must be at least one foot of unfrozen water at the bottom of the pond. There must be something to dig in on the bottom, so an earth-filled box would work. The water must have adequate oxygen in it - turtles absorb it through their skin when hibernating. It's a good idea to have an electric de-icer on the pond's surface to allow oxygen in and the detrimental gases out. The following site has more details on turtle ponds:
Turtles also must be properly conditioned for hibernation, and should be examined by a reptile vet in the fall to be sure they're fit to hibernate. There's more about that here:http://happyturtle.ms11.net/hibernation.html
The time leading up to hibernation is also risky. If you ahve an extended period of damp weather with temperatures in the 40's to 60's, the turtle will become less active, but it's not cold enough to trigger hibernation. Skin and shell infections and respiratory infections are common in those circumstances.
Besides hibernation, there are a couple of other problems you may encounter with outdoor housing. One is that your turtle is likely to simply leave. They go off seeking mates the first chance they get. A small fence a few feet out from the pond will be needed. Many people make a rock or concrete block wall just tall enough to keep in a turtle. Predators often injure or kill turtles that live outdoors. Raccoons are the most common offenders, and they will chew off feet, and destroy shells. Dogs will grab a basking turtle and carry it off, thinking it's a toy. The site I gave you above on pond design has some tips to minimize these problems.
With hot summers, the pond needs to be deep enough to provide cooler water, and it would be good if one part of it is in the shade. Three feet of water int he deepest part should be adequate.
If you have more questions, just let me know by clicking on REPLY. I'm not trying to discourage you, but just want you to be aware of potential problems so you can circumvent them.
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