Drivers of Water Movement
Water is in constant motion throughout the hydrosphere, the collective term for all water found on, beneath, and above Earth's surface. Two forces keep it moving: solar radiation and gravity.
Solar energy drives water upward. When the Sun heats a water surface, molecules gain enough energy to escape into the atmosphere as vapour, a process called evaporation. Plants add to this through transpiration, releasing water vapour from leaf stomata as part of their metabolism. Together, these are referred to as evapotranspiration, and they account for the vast majority of water entering the atmosphere at any given moment.
The transformation of liquid water into water vapour due to solar heating.
The release of water vapour from plant leaves through stomata into the atmosphere.
Gravity then returns water to Earth's surface. Once water vapour condenses and falls as precipitation, gravity pulls it downslope, through the soil, and eventually back toward the oceans. Water at altitude holds gravitational potential energy that converts to kinetic energy as it flows downhill through rivers, channels, and groundwater systems.
Heat is absorbed during evaporation and released during condensation. This energy exchange is not incidental: it drives atmospheric circulation and shapes regional climate patterns.
These two forces are complementary. Solar radiation lifts water; gravity returns it. Neither operates alone, and the cycle they sustain is continuous.
The Hydrological Cycle as a System
The global hydrological cycle is a system of stores and flows. Water moves continuously between stores, changing state and location, but the total volume of water on Earth stays constant over geological time. In that sense, the cycle is a closed system with respect to matter. It remains open with respect to energy, drawing on solar radiation and gravity to keep flows moving.
A place where water accumulates or is held for a period of time, such as an ocean, glacier, lake, or aquifer.
The movement of water between stores, either as a transfer (movement without a change of state) or a transformation (movement involving a change of state).
Flows fall into two categories:
- Transfers move water from one store to another without a change of state. Examples include precipitation, surface runoff, and groundwater flow.
- Transformations involve a change of state. Examples include evaporation (liquid to vapour), condensation (vapour to liquid), freezing (liquid to solid), and melting (solid to liquid).
The cycle links four major Earth systems: the atmosphere, biosphere, hydrosphere, and lithosphere. When water evaporates from the ocean, condenses into cloud, precipitates over land, infiltrates the soil, and drains back to the sea, it has passed through all four.
Application of skills: Creating a systems diagram
A systems diagram of the hydrological cycle follows a specific convention:
- Draw each store (ocean, atmosphere, glaciers, groundwater, surface water, organisms) as a labelled box.
- Draw each flow as an arrow connecting two boxes, with the flow process labelled on or beside the arrow.
- Where a transformation occurs (state change), indicate the process name on the arrow, for example "evaporation" between the ocean box and the atmosphere box.
- Check that every arrow has a clear origin store and a clear destination store.
- Review the diagram: any flow listed in 4.1.4 that is missing represents a gap.