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Hydrological Cycle (Water Cycle): The continuous movement of water on the land, in the atmosphere and in the oceans.
Inputs: When water is added to a drainage basin.
Inputs: When water is added to a drainage basin.
Precipitation: Any moisture that falls from the atmosphere. The main types of precipitation are rain, snow, sleet, hail, fog and dew.
Interbasin transfer: Water that either naturally (due to the alignment of the rock) or with human involvement (pumps and pipes) moves from one drainage basin to another.
Outputs: When water leaves a drainage system.
Outputs: When water leaves a drainage system.
Evaporation: The process of water turning from a liquid into a vapour. Evaporation only takes place from a body of water e.g. a lake, puddle or the sea.
Transpiration: The evaporation of water from vegetation.
Evapotranspiration: The combined action of evaporation and transpiration
Interbasin transfer: Water that either naturally (due to the alignment of the rock) or with human involvement (pumps and pipes) moves from one drainage basin to another.
River discharge via channel flow: Water entering the sea and leaving a drainage basin. A very small amount of water also enters the sea via throughflow and groundwater flow (baseflow).
Stores: When water is stationary and not moving in a drainage basin.
Stores: When water is stationary and not moving in a drainage basin.
Interception: When water is caught and held by vegetation or man-made structures like buildings.
Surface store: When water is held in the surface of the earth. This may be a puddle, a lake or a garden pond.
Soil moisture store: When water is held in unsaturated soil.
Groundwater store: When water is held in saturated ground.
Transfers (flows): When water is moving within a drainage basin.
Transfers (flows): When water is moving within a drainage basin.
Stem flow: When intercepted water runs down the trunks and stems of vegetation.
Canopy drip: When intercepted water drips off the leaves of vegetation (drip tip leaves in rainforests are actually designed to allow this to happen).
Throughfall: Precipitation that falls directly through vegetation.
Infiltration: Water that moves from the surface of the earth into the soil below.
Throughflow: Water that travels through unsaturated ground.
Pipeflow: Water that travels through holes left by root systems and animals burrows.
Percolation: Water that travels from unsaturated into saturated ground.
Groundwater flow (baseflow): Water that travels through saturated ground.
Capillary action (or rise): Water that may move upwards towards the surface.
Channel flow: Water that travels in a river.
Surface run-off (overland flow): When water travels across the surface of the earth e.g. down a hill.
Great Lakes Drainage Basin
Mississippi River Drainage Basin
System Diagram: Because two dimensional and three dimensional diagrams of drainage basins can be complicated to understand, drainage basin systems are often shown in a systems diagram. Below is an example of a systems diagram, it basically shows all the processes taking place using a series of boxes and arrows. The boxes are usually coloured so that you can clearly see if they are process are an input, output, transfer or store.
Bed: The bottom of the river channel
Bank: The sides of the river channel.
Channel: The confines of the river, encompassing the bed and two banks.
Wetted Perimeter: The total length of the bed and the banks in contact with the river.
Cross-sectional area: The width of the river multiplied by the depth of the river. Because the depth of the river will vary across its width, an average depth reading is normally taken. The cross sectional area is normally given in m2.
Velocity: This is the speed that the water in a river is travelling at. The unit of measurement is normally metres a second (m/s). River velocity can be measured using a flowmeter (pictured right), or more commonly by timing a floating object over a set distance (pictured left). Velocity is then calculated by dividing the time (seconds) by the distance (metres).
Discharge: This is the amount of water in a river at a given point. Discharge is normally measured in cumecs (cubic metres a second). It is calculated by multiplying the cross-sectional area by the velocity.
As you move from the source to the mouth, both the discharge and velocity of a river increases.
Just because a river has a big cross-sectional area, it does not necessarily mean it is efficient. A rivers efficiency is gauged by calculating it hydraulic radius. Hydraulic radius is calculated by dividing a rivers cross-sectional area by its wetted perimeter. If a river has a lower hydraulic radius it means more water is in contact with the bed, banks and surface, making it less efficient. If it has a high hydraulic radius, it means less water is contact with bed, banks and surface, making it more efficient. A river is most efficient just before it reaches bankfull discharge. If it is below bankfull discharge it means a greater proportion of its flow is in contact with the bed, banks and the surface. If it is above bankfull discharge then it means the river is in flood and therefore in contact with the floodplain increasing friction.
Long profile: The long profile looks at how a rivers' gradient changes from the source to the mouth. A rivers' gradient is much steeper near the source and a lot more gentle near the mouth. Later in the topic we will look at how a rivers' features change as you move from the source to the mouth. We will also look at changes in erosion, transportation and deposition (Floodplain management).
Two simple models which look at changes in velocity, cross-section, bed roughness, etc. are the Bradshaw model and the Schumm model (see models below).
Base level: This is the lowest level that a river can erode its bed to. It is basically sea level. A river can not erode its bed below sea level because rivers are unable to travel upwards.
Bradshaw Model
Schumm Model
Erosion
Erosion
Erosion involves the wearing away of rock and soil found along the river bed and banks. Erosion also involves the breaking down of the rock particles being carried downstream by the river.
The four main forms of river erosion
- Hydraulic action - the force of the river against the banks can cause air to be trapped in cracks and crevices. The pressure weakens the banks and gradually wears it away.
- Abrasion - rocks carried along by the river wear down the river bed and banks.
- Attrition - rocks being carried by the river smash together and break into smaller, smoother and rounder particles.
- Solution - soluble particles are dissolved into the river.
Transport
Transport
Rivers pick up and carry material as they flow downstream.
The four different river transport processes
- Solution - minerals are dissolved in the water and carried along in solution.
- Suspension - fine light material is carried along in the water.
- Saltation - small pebbles and stones are bounced along the river bed.
- Traction - large boulders and rocks are rolled along the river bed.
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