River Environments - 1.1 (Hydrological Cycle, Drainage Basin, River Regimes and Hydrographs)

The Hydrological Cycle

• During the hydrological cycle,, water is held in a number of stores and then moves between them by means of a series of flows, also called transfers.

Stores

• The atmosphere: where the water exits either as water vapour or as minute droplets in clouds.
• The land: where water is stored on the surface in rivers, lakes and reservoirs. Water is taken in by plants and stored in vegetation for short periods of time. It is also stored below the ground in bedrock, known as the groundwater store. Water mostly exists in a liquid form, but it can exist in a solid form, like in glaciers.
• The sea: it is estimated that over 95% of Earth's water is stored in the sea, mostly in liquid form, but also ice like in icebergs.

Transfers

• Evaporation: the hydrological cycle starts with evaporation due to the Sun. Water is converted from a liquid into a gas (called water vapour). This takes place from the surface of the sea and water surfaces 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 and 'breathe' it into the atmosphere as water vapour.
• Evapotranspiration: the loss of moisture from the ground by direct evaporation from water bodies and the soil, plus transpiration from plants.
• Condensation: the change in the atmosphere when water vapour cools and becomes liquid, taking the form of water droplets that appear in the atmosphere as clouds.
• Precipitation: 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 runoff.
• Infiltration + Percolation: 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, the 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.

Drainage Basins and its Features

Inputs:

• Energy from the Sun
• Precipitation formed from moisture picked up outside the basin
• Possibly water from tributary drainage basins

Outputs:

• The river's discharge
• The water in its basin from which evaporation and transpiration take place

✩ Drainage basin - An open system (the amount of water varies) in a river

✩ Source - The beginning or start of a river

✩ Watershed - The edge of highland surrounding a drainage basin

✩ Mouth - The point where the river comes to an end, usually when entering the sea

✩ Catchment area - The area within the drainage basin

✩ Confluence - The point at which two rivers or streams join

✩ Tributary - A stream or smaller river which joins a larger stream or river

River Regimes and Hydrographs

✩ River regime - Variations in river discharge

✩ Storm hydrograph - Records the changing discharge of a river. The bar chart showing rainfall and the line graph showing discharge from the rainfall. 

Features of a storm hydrograph

✩ Peak discharge - The highest point on the line graph

✩ Rising limb - The line graph going up

✩ Falling limb - The line graph going down

✩ Time lag - The difference in time between peak rainfall and peak discharge

✩ Baseflow - The normal flow of the river that starts to rise when run-off, ground and soil river reaches the soil

Factors Affecting River Regimes

• Precipitation: Heavy rain will not be absorbed by the ground, so it becomes overland flow or run-off and quickly reach the river.
• Temperature: If temperatures are below freezing, precipitation will be in the form of snow, taking weeks to melt. If the ground remains frozen, melting snow on the surface can reach the river quickly.
• Steep Slopes: Cause rapid surface run-off, so water will reach the river more quickly, while flatter sloped may lead to water sinking in them.
• Rock type: Impermeable rocks will not allow rainwater to sink into it, so will spread up run-off. Permeable rocks absorb water, thus slowing down the delivery of water to the river.
• Land Use: Plants intercept and delay rain reaching the ground, but bare soil and rock speed up run-off.
• Dams: By holding back discharge, dams reduce the risk of flooding downstream.