Hydrological Cycle as a Closed System
- The global hydrological cycle is a system.
- There are open and closed systems:
- Open systems have external inputs and outputs of energy and matter exchange at its boundaries
- Closed systems only have energy as its input and output, matter is contained within the system boundary
- Energy is from the sun which is irradiated back from Earth to space
- Within global systems there are usually numbers of smaller subsystems e.g. drainage basin of a river
- The global hydrological system is a closed system meaning there are no external inputs or outputs, water is not lost or gained from space
- It is defined as :
'the continuous movement of water on, above and below the Earth's surface'
- The cycle is a series of processes in which water is constantly recycled through the system
- Evaporation - the sun evaporates surface water into vapour
- Condensation - water vapour condenses and precipitates
- Flows - water runs off the surface into streams and reservoirs or beneath the surface as ground flow
- The hydrological cycle is powered by the sun.
- The sun heats water in the oceans or on land and evaporation occurs
- Water vapour then rises into the atmosphere and condenses to form clouds and this drives the atmospheric circulation
- Cloud droplets then fall back to earth as precipitation
- The hydrological cycle involves energy exchange, leading to local temperature fluctuations
- As water evaporates, it uses energy from its surrounding to perform this process
- This effectively cools the environment
- The reverse happens when water condenses (heat is released)
- This heat exchange influences the local climate
- When a water droplet falls on a mountain or slope, high above sea level, it has a high amount of gravitational potential energy (GPE)
Gravitational potential energy of a raindrop
- As the water droplet is in motion, the GPE turns to kinetic energy
Kinetic energy of a raindrop on a hill slope
- This allows the water to move over the surface
Exam Tip
You could be asked to explain what a closed system is or how it operates. Try to give a definition of a closed system and then go on to describe how the flows and stores of water change despite the amount of water in the system staying the same. Then explain the reasons for those changes.