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Hydrological Processes

Hydrological Processes

Hydrological processes are the major processes within the system of the hydrological cycle. In the Water Cycle, we learn that the global hydrological cycle is a closed system, however, a local hydrological cycle has hydrological processes that operate within areas drained by rivers and their tributaries. These are known as drainage basins which are open systems.

Open systems mean that their inputs are not governed by outputs and more water can be lost than received.

Water is lost by:

  • Evaporation and evapotranspiration to the atmosphere.
  • Surface runoff to the sea.
  • Percolation into groundwater stores.

Drainage basins are also known as catchment areas as they catch the precipitation falling within the watershed.

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

Hydrological processDefinition
InputsPrecipitation Moisture in any form.
StorageInterception Temporary storage of water before reaching soil. It is water captured by plants, buildings and hard surfaces.
Vegetation storage Moisture that is taken up by vegetation.
Surface storageAny water in surface water such as lakes, ponds and puddles.
Soil moisture Water in soil.
Groundwater storage Water held in rocks (also known as aquifer).
Channel storage Water held in streams and rivers.
Flows and ProcessesInfiltration Water that enters topsoil. Common when there is slow or steady rainfall.
Throughflow Water that seeps laterally through soil below the surface but above the water table.
Percolation The vertical flow of water between soil and rock layers.
Base flowSlow-moving water that flows into the river channel.
Channel flow The water flowing in the river channel, also known as discharge and runoff.
Surface runoffFlow over the surface when the ground is frozen, saturated or impermeable clay or during an intense storm. Also known as overland flow.
OutputsEvaporation Water into vapour
TranspirationWater that has been taken up by plants and transpired onto the leaf surface.
Evapotranspiration Both the effect of evaporation and transpiration.
River discharge A volume of water passing a certain point in the channel over a certain amount of time.

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.

Impacts of man-made reservoirs include:

  • Dams reduce the river flow below them, leading to vegetation loss.
  • The mats of floating plants on the reservoir's surface in the Tropics lead to higher evapotranspiration rates than in open water.
  • Salinity levels in the reservoir can rise as water evaporates.
  • Reservoir water takes water from the drainage basin.

Hydrological Processes - Key takeaways

  • Local hydrological cycles have hydrological processes that operate within areas drained by rivers and its tributaries, known as drainage basins.
  • Water follows three pathways, reaching the land surface and then infiltrating the topsoil, running off the surface as overland flow, and evaporating back into the atmosphere.
  • These pathways can be delayed.
  • There are basin-wide factors such as the shape, relief, geology, vegetation, climate and land use of each basin which determine what happens to the precipitation when it falls.
  • There are many factors that affect the hydrological processes in the drainage basins, such as the variety of precipitation and human impact.

Frequently Asked Questions about Hydrological Processes

The hydrological cycle is the continuous circulation of water within the Earth's hydrosphere.

Evaporation, condensation and precipitation are the major hydrologic processes.

The hydrological cycle is the continuous circulation of water within the Earth's hydrosphere. Its importance is connected to the importance of water to all living beings to survive.

Hydrological characteristics are defined by a particular location along the course of a stream or river.

Final Hydrological Processes Quiz

Question

What is a drainage basin?

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Answer

Drainage basins are hydrological processes that operate within areas drained by rivers and their tributaries.

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Question

What is an open system?

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Answer

Open systems mean that their inputs are not governed by outputs and more water can be lost than received.

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Question

How is water lost in an open system?

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Answer

Water is lost by:

-evaporation and evapotranspiration to the atmosphere

-surface runoff to the sea

-percolation into groundwater stores

Show question

Question

What is a catchment area?


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Answer

Catchment area is another name for drainage system, as they catch the precipitation falling within the watershed.

Show question

Question

What are the three pathways that water follows when precipitation occurs in the drainage basin?

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Answer

When precipitation occurs, the water follows three pathways.

-reaching the land surface and then infiltrating the topsoil.

-running off the surface as overland flow, also known as surface runoff.

-be evaporated back into the atmosphere.

Show question

Question

What can delay the water on a pathway?

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Answer

Any pathway taken can be delayed by:

-Water intercepted by plants and buildings, before evaporating or infiltrating the surface.

-Surface water infiltrating through the surface and eventually percolating through the rocks underneath to become groundwater, to then be stored in aquifers.

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Question

What does interception mean as a hydrological process?

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Answer

Interception means temporary storage of water before reaching soil. It is water captured by plants, buildings and hard surfaces.

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What does infiltration mean as a hydrological process?

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Answer

Infiltration means water that enters topsoil. Common when there is slow or steady rainfall.

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What does percolation mean?

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Answer

Percolation is the vertical flow of water between soil and rock layers.

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What is channel flow?

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Answer

Channel flow is the water flowing in the river channel, also known as discharge and runoff.

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Question

What are the four outputs of the hydrological cycle within a drainage basin?

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Answer

-Evaporation = Water into vapour.

-Transpiration = Water that has been taken up by plants and transpired onto the leaf surface.

-Evapotranpiration  = Both the effect of evaporation and transpiration.

-River discharge = A volume of water passing a certain point in the channel over a certain amount of time.

Show question

Question

What kind of precipitation is the western side of the UK prone to?

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Answer

The western side of the UK is particularly prone to orographic and frontal rainfall.

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What kind of precipitation is the eastern side of the UK prone to?

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Answer

The eastern side of the UK is much drier yet the summer months can experience heavy bursts of convectional rainfall.

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What human impact affects the hydrological processes?


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Answer

Over Abstraction, deforestation, urbanisation and reservoirs.

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Question

What is over abstraction?

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Answer

Over abstraction is when abstracting too much water from groundwater reserves leads to rivers drying up in times of low rainfall.

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What is the global water budget?

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Answer

The balance of inputs, outputs and stores of water in the global system.

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What are the main inputs to the water budget?

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Answer

Precipitation and water introduced by humans

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What are the main outputs in the water budget?

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Evapotranspiration, river discharge and water abstraction

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What is it called when more water enters than leaves a system?

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Answer

Water surplus

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What is it called when more water leaves a system than enters it?

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Answer

Water deficit

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What is the equation to calculate the water budget?

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Answer

precipitation = evapotranspiration + discharge +/- changes in storage.

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In the UK, in what months is there more likely to be a water deficit?

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Answer

Summer

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In the UK, in what months is there more likely to be a water surplus?

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Answer

Winter

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Why is there more likely to be a water deficit in the UK summer?

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Answer

Because higher temperatures results in more evapotranspiration (and so more water leaving the system than entering it).

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Why is there more likely to be a water surplus in the UK winter?

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Answer

Because there tends to be more precipitation than evapotranspiration, so more water enters the system than leaves it.

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Question

What are the different subsystems of Earth?

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Answer

Atmosphere; biosphere; cryosphere; hydrosphere; and lithosphere

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How does water flow between the different subsystems of Earth? (Name 6 examples)

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Answer

Evapotranspiration; condensation; cloud formation; precipitation; convection; glacial processes

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Why is it important that water storage is included in the water budget equation?

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Answer

To make sure the equation accounts for all the water in the system.

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Why do we bother to calculate the water budget?

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To work out how much water is in a system

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What is a river regime?

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Answer

Changes in the river discharge over the course of a year

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What is river discharge?

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A measurement of the amount (volume) of water passing through the river channel at any one time

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How do you calculate the river discharge?

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Answer

area of flow x average velocity of flow

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True or false: a circular drainage basin means water takes longer to reach the river channel.

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Answer

False

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Which drainage basin properties affect the river regime?

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Answer

Size and shape

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Question

Name 2 ways that humans can alter the river regime

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Answer

Land use changes; water abstraction and vegetation changes

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Question

What is the main property of a river that alters the river regime?

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Answer

Cross-sectional area

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How can the cross-sectional area of a river be altered?

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Answer

Erosion; weathering; deposition

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Question

What climatic controls affect the river regime?

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Answer

Balance between precipitation and evapotranspiration

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Question

True or false: the river regime often demonstrates seasonal patterns due to changes in the balance between precipitation and evapotranspiration.

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Answer

True

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Question

How does climate change affect the river regime?


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Answer

By influencing the amount of water in the river.

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True or false: rivers are very static systems.

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Answer

False

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True or false: the river regime does not change much between years

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Answer

False (it depends)

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Question

What is a drainage basin?

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Answer

The area drained by a river.

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How does steeper sides of a drainage basin alter the river regime?

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Answer

It means water can reach the river more quickly.

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Question

Why are storm hydrographs used?

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Answer

To measure the response of a river to a storm event

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In a hydrograph, what is it called when the amount of rainfall received is at its highest?

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Answer

Peak rainfall

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What is the base flow?

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Answer

the base flow refers to the amount of water that would be in the river ordinarily (i.e. even without the storm event)

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What are the characteristics of a flashy hydrograph?

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Answer

Steep limbs; high peaks and short lag time

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What is the lag time?

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the amount of time that passes between the time of peak rainfall and of peak discharge

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What does analysis of a storm hydrograph reveal?

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Answer

information about the severity of a storm, the impact on the river and the time that it takes for it to respond to the storm event

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