The Hydrological Cycle
This describes the process whereby water in its various forms is continually cycled between the land, sea and atmosphere. It also makes its way into the biosphere to influence animal and plant ecosystems around the globe.
This is a common approach in geography and the two main examples in this topic are:
The hydrological cycle: a closed system.
The drainage basin system: an open system.
Both consist of transfers, stores, inputs of water but the hydrological cycle is a closed system as no gains or losses from outside are added to the system.
The drainage basin system is said to be open as both inputs and outputs of energy and material occur. All systems in their natural state aim to be in a state of balance (dynamic equilibrium) as this is when they function best. Heavy rainfall, drought and human activity such as deforestation can easily upset the balance.
Within the hydrological cycle, four main processes operate:
This is when plants prevent some rainfall from directly reaching the ground, for example, water on leaves or foliage. It may later reach the floor via stem flow (water flows down the stem to the ground) or through-fall, where water drips to the ground. Secondary interception occurs at ground level where water hits undergrowth. Some water returns to the atmosphere via evapotranspiration.
Water lost from vegetation via both evaporation and transpiration. Click on the boxes below to reveal definitions of both terms:
The amount of water that could be lost by evapotranspiration. For example, this is potentially high in deserts, but the amount that can take place is limited due to the minimal moisture available. Actual evapotranspiration is what actually occurs. In the UK there is more water available for evapotranspiration than takes place.
Where water slowly soaks into the soil from the ground. The maximum rate at which this can occur is known as infiltration capacity (mm/hour) and it is dependent on the amount of water already present in soil structure and vegetation.
The most important input into the system forms includes snow, hail, rain, and fog.
Water in the soil does not remain there but moves down slowly into the lower layers of soil and rock. It creates groundwater storage found in rocks and this may later be moved sideways through the rock via groundwater flow.
Water flows through the hydrological cycle in various ways:
Throughflow: where water moves downwards through layers of soil.
Channel flow: downhill movement of water in rivers.
Groundwater flow: Lateral movement of water from the water table.