Heat the butter in a large Dutch oven or pot over medium heat. Add the onion, celery, and flour and cook for 1 minute. Add in the potatoes, sage, and pepper. Pour in 2 cups water, chicken broth, and the half-and-half. Cover and bring to a boil. Reduce the heat to medium and simmer, partially covered, for 10 minutes. Add the carrots and simmer until the vegetables are tender, about 6 minutes. Add the shredded turkey and cook until warmed through, about 1 minute.