Extreme events of water such as floods and droughts form another important characteristic of water. Most of the annual water flow may come as floods following snowmelt or heavy rains, and unless captured by reservoirs, it flows to the seas, sometimes causing seasonal flooding. Such areas may suffer droughts later in the year. These characteristics show the first particularity of water: its quantitative limitation. Water can be misused, polluted or its flow pattern and chemical properties can easily be changed by human activities and other factors. Hence water vulnerability is another key characteristic of the world’s freshwater resource.