When I worked at a water treatment plant, we produced about 160 million gallons/day of water in the summer time, and only about 80 million gallons/day in the winter time. Now ask yourself what water-consuming activities happen in summer that don't happen in winter.

Primarily, lawns. It's lawns. Most of the international difference in water consumption I would chalk up to lawns, given that the US has much larger average lot sizes and a much larger proportion of detached single-family houses (i.e., houses sitting in the middle of a lawn) than European countries have.