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.
I have a modest front and back yard in the "hot" part of the Bay Area. Summer months I'm pouring 5000 gallons a week to keep it moderately brown. It's insane.
Not really just lawns, but irrigation specifically. I know a lot of people in my country that have detached houses with lawns. I know of no one that owns a sprinkler system. Dry years the lawn just goes a bit brown and no-one cares.
Everyone has a hose, they wash their car and water their flowers by hand.
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.