Ok, I hadn't thought about that. It's summer right now, so quite warm. It seemed like it was digging more when I took it out of the water in the evening, so I was thinking "ok, it prefers to lay its eggs in the evening." But, maybe I misunderstood and it is not trying to nest after all - perhaps it is colder in the evening, so it is trying to dig a hole to keep warm? Thanks again. I will identify the gender and probably stop taking it out as much in the evening, in case it is getting cold.

As for the digging, it seemed to be trying to dig with its hind legs into the lawn, trying to make a hole. It is possible it was trying to make a hole to hide in, as you suggest. I will look into how to identify the gender and whether or not it is carrying eggs.

Thanks for your answer, this is really helpful. In terms of water, right now we are keeping it in a plastic storage container that we have part filled with water. I am letting it out a couple of times a day to walk around the garden and get some exercise. I am feeding it some mulberries from a tree we have, which it seems to be eating. I think we will have to make a decision soon about whether we will keep it. We don't have a pond in our garden and I don't know if we have space for a tank that large in the house. There is a large pond close to our house, which is probably where it came from.