Hydrological Processes

Hydrological processes in a drainage basin

When precipitation occurs, the water can follow three pathways:

• Reaching the land surface and then infiltrating the topsoil.

• Running off the surface as overland flow, also known as surface runoff.

• Evaporating back into the atmosphere.

Any pathway taken by the water can be delayed by:

• Interception by plants and buildings.

• The percolation of surface water through the rocks underneath to become groundwater, stored in aquifers.

• Surface runoff can flow over impermeable and saturated surfaces until it eventually reaches river channels and becomes streamflow.

Main hydrological processes in a drainage basin

The impact of drainage basin factors on hydrological processes

Drainage basin factors like shape, relief, geology, vegetation, climate and land use determine what happens to the precipitation when it falls. Steeper slopes promote faster movement and shorter storage times compared to gentler slopes. Higher vegetation coverage will increase interception and evapotranspiration and decrease surface runoff.

The geology of the basin is also important. Some rocks are impermeable, preventing infiltration and causing surface saturation.

Cold climates such as snow-capped peaks can hold water until it thaws which can lead to delayed flow. Rural areas permit more natural processes compared to urban areas. Grasslands have higher infiltration, percolation, throughflow and evaporation than arable land. Urban areas have impermeability and increase rapid surface runoff, evaporation and interception.

Precipitation input and hydrological processes

When the warm air rises and cools to become condensation, this leads to clouds which then eventually form rain and snow. Looking at the UK as a case study, we can see three different types of rainfall on the west and the east.

In the west, there are drainage basins which are exposed to the air masses from the Atlantic. They are particularly prone to orographic and frontal rainfall. it is much drier on the east yet the summer months can experience heavy bursts of rain from conventional air instability. As the ground warms up, evaporation means that the air above heats and rises. The heavy rain from summer thunderstorms can lead to flash flooding as the dry soil surface becomes waterlogged quickly, causing rapid surface runoff.

Do human impacts affect hydrological processes?

The inputs, outputs, flows and stores of water of a hydrological cycle can also be affected by human impact.

Over abstraction

Over abstraction is defined as when too much water is abstracted from groundwater reserves. This can lead to rivers drying up in times of low rainfall.

Deforestation

The Tropics have a fragile natural environment with complex biodiversity. Their forests flourish on relatively thin soils. Removing trees can lead to a reduction of interception. Rainfall strikes the soil directly and compacts it, and the runoff of the rainfall that stays on the surface moves quickly to rivers causing floods.

Urbanisation

The physical character of urban areas can affect the local hydrological cycle. One example is the increase in impermeable surfaces due to built-up areas, which alter the natural flow of water.

Reservoirs

Man-made reservoirs can delay the flow through the drainage basin and increase the amounts lost through evaporation.