It is from the book called "Your bodies many cries for water". Basically you drink half you body weight in ounces. If you weigh 100lbs you drink 50 oz in water a day. He also suggests that you add 1/4 tsp of salt per quart of water or make sure to salt your food. Only use real, unrefined (grey) salt. table salt is not good.
This made a huge difference in me, and I think is one reason my cleanse was so successful.