Rivers: Hydrological Cycle

What is the Hydrological Cycle?

• global circulation of water, where water is constantly being recycled

• closed system, fixed amount of water - water neither enters nor leaves the Earth and its atmosphere.

• includes stores and transfers.

What are stores?

- where water is held during the hydrological cycle

- include the atmosphere, land, and sea

Atmosphere

water exists here as water vapour and minute droplets in clouds

Land

- water is stored in rivers, lakes, vegetation and reservoirs, taken in by plants, stored in the groundwater store

- It mostly exists in the liquid form, but also in the solid form (snow and ice).

Sea

over 95% of Earth's water is stored here. It is mostly held in the liquid form, but also in the solid form (snow and ice)

What are the proportions of water between these stores?

- The proportion held in different stores can change, as a result of changes in the Sun's energy.

- For example, an increase in the Sun's energy will lead to more evaporation and possibly to the melting of ice sheets and glaciers.

What are transfers?

• series of flows between stores

• evaporation, transpiration, evapotranspiration, condensation, precipitation, through flow, overland flow, groundwater flow, and infiltration and percolation.

Evaporation

- The hydrological cycle starts with evaporation due to the heat of the Sun.

- Water is converted from a liquid into a gas (called water vapour).

- This takes place from the surface of the sea and from water surfaces (ponds and lakes) on land.

- Evaporation is particularly important in the transfer of water from the sea store into the atmosphere.

Transpiration

- Plants take up liquid water from the soil, which evaporates from aerial parts, such as leaves, stems and flowers into atmosphere as water vapour.

Evapotranspiration

This is the loss of moisture from the ground by direct evaporation from water bodies and the soil, plus transpiration from plants.

Condensation

- This is the change in the atmosphere when water vapour cools and becomes liquid.

- The liquid takes the form of water droplets that appear in the atmosphere as clouds.

Precipitation

This is the transfer of water in any form (rain, hail or snow) from the atmosphere to the land or sea surface.

Overland Flow

- Most precipitation that hits the ground moves due to gravity and eventually enters a stream, river or lake.

- This is known as run off.

Infiltration and Percolation

This is the transfer of water downwards through the soil and rock into the aquifer or groundwater store.

Throughflow

- This takes place between the ground surface and the top of the groundwater store.

- As a result of gravity, water moves slowly through the soil until it reaches a stream or river.

Groundwater Flow

This happens in the rocks of the aquifer and is the underground transfer of water to rivers, lakes and the sea.

What is a drainage basin?

- an area of land drained by a major river and its tributaries

- involves stores, flows, external inputs, and outputs

- open systen, amount of water varies over time

System of Drainage Basin

- Inputs: energy from the Sun, precipitation formed from moisture picked up outside the basin, water from tributary drainage basin

- Watershed: line dividing different drainage basins, following the tops of hills

- The main river starts (its source) in higher areas near the watershed, where most of precipitation falls.

- Tributaries, smaller streams, join the main river at confluences.

- The river ends at its mouth or estuary, where it flows into the sea.

What are the outputs of a drainage basin?

- Outputs: river's discharge, evaporated or transpired water

- River discharge is the amount of water moving past a particular point along the river's course at any one time, measured in cumecs (cubic metres of water per second)

What is a channel network?

- system of surface and underground channels that collect and transport precipitation falling on the drainage basin.

- can change over short periods of time → during flooding, there are more and longer channels

Features of a drainage basin

- size

- shape

- rock type

- relief

- land use

What and how do the features of a drainage basin affect?

- can affect run off.

- Woodland holds water and slows overland flow. Once cleared for cultivation, run off will speed up.

- built-up areas of towns and cities → rainwater hits impermeable surfaces (roofs, pavements, roads) → speeds up run off → quickly channelled into drains → quick delivery into a stream or river

What is a river regime?

- varying river discharges throughout the year

- reflects local climatic conditions, particularly the rainfall.

What is a hydro-graph?

- shows the average monthly discharge of a river as it passes through a place.

- peaks indicate the impact of passing showers and short periods of heavy rainfall

What is lag time?

- delay between peak rainfall and peak discharge

- shorter lag time → quicker river level rise (steep rise in river discharge) → higher risk of flooding

What is flood risk?

- Storm hydrographs show how discharge levels change after rainfall, meaning you can predict when and where flooding might happen.

- The graph can show the safe discharge level, above which flooding occurs.

What is base and storm flow on storm hydrograph?

- Base Flow: This is the normal level of water in the river.

- Storm Flow: This is the extra water in the river caused by the rainstorm.

What are the six factors that affect river regimes?

- precipitation

- temperature

- vegetation and land use

- human intervention

- steepness of slopes

- rock type

How does precipitation affect river regimes?

- amount and the intensity of the rain

- heavy rain will not infiltrate → becomes run off → quickly reach the river.

How does temperature affect river regimes?

- affect the form of precipitation.

- below freezing temperatures → precipitation as snow → takes weeks to melt → frozen ground → melting snow reaches river quickly

How do dams affect river regimes?

- human intervention

- holds back discharge → reduces risk of flooding downstream

- flow of water out of reservoirs controlled by opening and closing sluice gates

- increasing abstraction of water from rivers impacts river regimes and storm hydrographs

How does the steepness of slopes affect river regimes?

- Steep slopes → rapid surface run off → quick river level rise

- flat and gently sloping land → infiltration → slows delivery of water to river

How does rock type affect river regimes?

- Impermeable rocks → no infiltration → fast run off

- Permeable rock → infiltration and percolation → slows delivery of water to river

How does vegetation and land use affect river regimes?

- Trees and plants intercept and delay rain reaching ground.

- Bare soil, rock, and urban areas under tarmac and concrete → speed up run off, reduce the time lag