Drainage Basin Hydrological Cycle

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Hydrological Cycle in a Drainage Basin

The hydrological cycle doesn't just work at a global level - this cycle also works in smaller, open systems such as in drainage basins. The main features of the drainage basin hydrological cycle are:

Inputs into drainage basins

There are three types of precipitation that input into a drainage basin:
- Orographic - when air masses rise over mountains causing it to condense and rain.
- Frontal - when two air masses meet at an area of low pressure creating rain.
- Convectional - rainfall caused by water turning to water vapour due to solar radiation.

Outputs from drainage basins

The water processes we identify as outputs in a drainage basin are:
- Evaporation - when water turns to water vapour and leaves the drainage basin.
- Transpiration - when water leaves plants through holes in their leaves.
- Channel flow - the volume of water that is flowing in a river channel.

Flows

The drainage basin hydrological cycle includes the following water flows:
- Interception - when plants capture precipitation in their leaves.
- Infiltration - when precipitation enters into the soil.
- Direct runoff - when water from precipitation or snowmelt flows across the ground’s surface because the surface is impermeable i.e. tarmac.

More flows

The following are also flows that happen in a drainage basin:
- Saturated overland flow - when the ground is full of water, resulting in runoff.
- Throughflow - the horizontal flow of water through soil/rock layers.
- Percolation - the vertical flow of water between soil and rock layers.
- Groundwater flow - the flow of water horizontality once it has reached the water table.

Physical Factors Affecting Drainage Basins

There are physical components that can impact the relative inputs, flows and outputs of a drainage basin.

Climate

Where cold climates allow for precipitation to fall as snow, the water can be stored and held back until it is thawed (melted).
This may reduce the channel flow during the winter but lead to an increase during the spring and summer months due to glacial melt water.

Soil type

Impermeable soils can stop infiltration and lead to surface saturation resulting in increased surface runoff.
Where there are permeable soils, infiltration and percolation can happen. This can cause groundwater to recharge.

Vegetation cover

When an area has a high coverage of vegetation, the interception and evapotranspiration increases but the surface runoff decreases.

Relief of the land

Steeper slopes mean faster surface runoff and shorter times for water storage.

Rock type

Some rocks are impermeable and can stop the infiltration of water into the ground. Similarly to impermeable soils, these types of rocks can cause the ground to saturate and lead to surface runoff and increase flows in rivers.
In contrast, permeable rocks allow for infiltration and percolation to happen.

Human Factors Affecting Drainage Basins

Humans can impact the inputs, outputs, flows and stores of water in a drainage basin.

Over-abstraction

Abstraction is the process by which humans remove water from underground water stores (e.g. aquifers).
Over-abstraction is when the volume of water being removed is greater than the volume of groundwater being replenished.
Over-abstraction can lead to rivers drying up during periods of low rainfall.

Deforestation

Deforestation is the term used to describe the removal of trees. The impact of deforestation reduces interception and consequently, rainfall strikes soil directly, leading to soil compaction.
Soil compaction and the removal of tree roots can reduce infiltration.
Ultimately deforestation causes an increase in surface runoff, resulting in more soil erosion and flooding.

Urbanisation

Urbanisation is a change of land use that results in moving away from the natural environment to towns and cities.
Urbanisation leads to a large number of impermeable surfaces i.e. tarmac, slate, concrete. These impermeable surfaces reduce infiltration, whilst increasing surface runoff.

Reservoirs

Reservoirs are man-made stores that disrupt the natural water flow, by delaying the flow and increasing water lost from evaporation.
Dams reduce the water flow further downstream resulting in loss of vegetation.
When vegetation grows on the reservoir’s surface, an increase in evapotranspiration happens and in turn increase in salinity of the water.