The Hydrological Cycle

The Global Hydrological Cycle is a closed system - there are no inputs or outputs. There are however, local hydrological cycles e.g. drainage basin hydrological cycles.

Important Key Definition:

A river's drainage basin is the area surrounding the river where the rain falling on the land flows into that river (also known as the river's catchment). 

The boundary of the drainage basin is the watershed - any precipitation falling beyond the watershed enters a different drainage basin. 

Drainage basins are open systems with inputs and outputs

Water comes into the system as precipitation and leaves via evaporation, transpiration and river discharge.

Inputs (water coming into the system)

Precipitation: all the ways moisture comes out from the atmosphere e.g. rain, snow, hail, dew, frost.

Storage (water stored in the system)

Interception: when precipitation lands on vegetation or other structures before it reaches the soil. It creates a significant store of water in wooded areas and is only temporary because the collected water evaporates quickly.

Vegetation storage: water taken up by plants - all the water contained in plants at any one time.

Surface storage: includes water in puddles (depression storage), ponds and lakes

Groundwater storage: water stored in the ground either in soil or rocks. 

Channel storage: water held in a river or stream channel.

Flows and Processes (water moving from one place to another)

Surface runoff: water flowing over land - can flow over whole surface or in little channels.

Stemflow: water running down a plant stem or tree trunk

Throughflow: water moving slowly downslope within the soil layer

Infiltration: the downward movement of water into the soil surface

Percolation: water moving down through the soil into the water table due to pull of gravity

Groundwater flow: water flowing slowly below the water table through permeable rock

Baseflow: groundwater flow that feeds into rivers through river banks and beds

Outputs (water leaving the system)

Evaporation: transformation of water droplets into water vapour by heating

Evapotranspiration: the loss of water from a drainage basin into the atmosphere from the leaves of plants

River flow/Channel flow: where water runs from river channel into sea

Water balance is worked out from inputs (precipitation) and outputs (evapotranspiration).It affects how much water is stored in the basin. The general water balance in the UK shows seasonal patterns.

In wet seasons, precipitation exceeds evapotranspiration:

• Creates a water surplus
• Ground stores fill with water so there's more surface runoff and higher discharge, so river levels rise.

In dry seasons, precipitation is lower than evapotranspiration:

• Ground stores are depleted as some water is used by plants and humans and some flows into the river channel but isn't replaced by precipitation
• At the end of a dry season, there's a deficit of water in the ground.