Freshwater - Drainage basin hydrology + geomorphology Flashcards
What is the global hydrological cycle?
A closed system - the Earth’s water never changes in quanitity, but constantly changes location and form as it moves through the cycle.
What is a drainage basin?
The area of land from which a river recieves water (area of land drained by a river and its tributaries) -> its catchment area.
What is the name of the higher land that separates river basins?
the watershed (basin boundary).
What is the largest drainage basin in the world?
The Amazon Basin - 7.5 million km^2 (The size of Europe)
What is North America’s largest drainage basin?
The Mississippi Basin (spans almost 50% of the US).
Why is any drainage basin system an open system within the closed system of the global hydrological cycle?
- Water can cross the boundaries of a drainage basin
- The amount of water in a drainage basin varies over time
What are 4 important things a drainage system has?
- Inputs = additions of water
- Stores = storage of water
- Flows (transfers/processes) = movements of water
- Outputs = losses of water
Flow diagram of drainage system
What are factors that affect the store + transfer of water in a drainage basin?
- Climate - temperature
- Soil type
- Geology
- Climate- rainfall
- Relief
- Land use
Why and how does ‘Climate - temperature’ - affect the store + transfer of water in a drainage basin?
- Climate - Temperature –> >Evapotranspiration increases with temperature
>Ice - water locked in ice becomes a store not a flow. Maybe a flow later.
>Frozen ground - prevents infiltration (transfer to groundwater)
>Snow-capped peaks hold water back until thaw (delayed flow)
Why and how does ‘Soil type’ affect the store + transfer of water in a drainage basin?
- Permeability
- Clay/sand
- Impermeable soils prevent infiltration + cause surface saturation
Why and how does ‘Geology’ affect the store + transfer of water in a drainage basin?
- Permeability of the surface
- Granite/sandstone
- Impermeable rocks prevent infiltration + cause surface saturation
Why and how does ‘Climate - rainfall’ - affect the store + transfer of water in a drainage basin?
- Intensity + duration –> impact on ground saturation
Why and how does ‘Relief’ affect the store + transfer of water in a drainage basin?
- Steeper gradients reduce infiltration + increases surface runoff
Why and how does ‘Land use’ affect the store + transfer of water in a drainage basin?
- Permeability –> urban vs rural land surfaces will reduce infiltration (if impermeable)
> Man-made drainage can be used to transfer water
> Forested slopes intercept more precipitation, increased levels of evapotranspiration + reduce surface runoff
> Urban surfaces are impermeable and increase rapid surface runoff (cement), evaporation and interception
What does discharge mean?
The volume of water passing a given point on the river over a set time
How is discharge measured?
m^3/secs
(cumecs)
How is discharge calculated?
CSA (m^2) x Velocity (m/s)
(CSA = Cross-sectional area = width x mean depth)
By what two opposing forces are discharge and stream flow influenced? And how?
- Gravity: causes water to flow downstream to sea level
- Friction: creates resistance between the water and the beds + banks, and opposes flow downstream
What does the Bradshaw model show?
It suggests how river channel characteristics change moving downstream
What is expected to be found in a river’s characteristics if it follows the Bradshaw model?
Discharge is expected to increase downstream despite the decrease in gradient
How is the width and depth (CSA) expected to change according to the Bradshaw model?
They are expected to increase as more water enters the river channel (from tributaries + run-off) eroding a wider, deeper channel = a more efficient channel shape
What does a more efficient channel shape result in?
It results in proportionally less water being in contact with the bed and banks = less friction = faster flow
What does increased erosion do?
It reduces the size of bedload material + decreases channel roughness = less friction = faster flow
How is the efficiency of a stream’s shape measured?
By its hydraulic radius
What is the hydraulic radius?
The CSA (cross-sectional area) divided by wetted perimeter (the total length of beds + banks in contact with the water)
What does a higher hydraulic radius mean?
A more efficient flow
The smaller the frictional loss the higher the velocity
Therefore what is the ideal form for the hydraulic radius?
Semi-circular
