Water moves in an endless cycle from the sea and land to the atmosphere and back again. The sun’s energy converts water from lakes, oceans, rivers and the surface layers of soil into water vapour. This process is called evaporation. Plants also add water vapour through transpiration. The water vapour rises into the atmosphere and condenses into clouds and it then precipitates and falls back to the land or sea as rain, sleet, hail or snow.

Most of the rain that falls on land either soaks into the soil in a process called infiltration or runs off the land surface into rivers, lakes and eventually out to sea. A dam can catch the intermittent flood flows that run off the catchment and store it for use during dry periods.