WCC: Water Cycle - Drainage Basins as Open Systems Flashcards
Define groundwater
Water stored in permeable rocks below the surface of the ground.
Define runoff
The total discharge from the drainage basin.
Define transpiration
The evaporation of moisture from vegetation into the atmosphere.
Define groundwater flow
Water flowing through the rocks towards the river.
Define precipitation
Water and ice that fall from clouds into the drainage basin.
Define throughflow
What flowing through the soil towards rivers.
Define evaporation
The change of water from liquid to gas, returning water to the atmosphere.
Define percolation
Water draining through rock towards the water table.
Define throughfall
Precipitation that drips through vegetation to the ground.
Define channel flow
Water flowing in a river.
Define overland flow
The movement of water over the surface of the ground to rivers.
Define surface store
Water lying in the ground.
Define channel store
Water stored in a river.
Define vegetation store
Precipitation that is trapped if stored temporarily on the vegetation.
Define stemflow
Precipitation that runs down plant stems and tree trunks to the ground.
Define base flow
That part of a river’s discharge high is produced by groundwater seeping the bed of the river.
Define infiltration
The process by which after enters the soil.
Define soil water
Water stored in the soil.
What is a drainage basin?
A catchment area from which a river system gets its water, defined as ‘the area of land drained by a river and its tributaries’.
What is the watershed?
An boundary that separates one drainage basin from another, typically following a ridge of high land
Is a drainage basin an open or closed system and why?
An open system, as it exchanges matter with its surroundings, it’s input being precipitation from other drainage basins and its outputs (evaporation and runoff) travelling outside the drainage basin, to other drainage basins or the ocean.
GIve 4 facts about the Mississippi Drainage Basin
World’s 4th largest drainage basin
Empties into the Gulf of Mexico, part of the Atlantic Ocean
Covers nearly 40% of the landmass of the continental United States
Covers over 3,220,000km^2
The highest point within the watershed is at 4,400m
What type of system are drainage basins collectively?
Cascading systems
What are cascading systems?
A series of open systems that link together so that the output of one is the input of the next
How does water enter a drainage basin system?
As precipitation
What are the outputs in a drainage basin system?
Evapotranspiration
Runoff
What is evapotranspiration in a drainage basin system?
The total output of water from the drainage basin directly back into the atmosphere
What is runoff in a drainage basin system?
All the water that enters a river channel and eventually flows out of the drainage basin
What are the stores in a drainage basin?
Interception storage Surface water Soil water Groundwater Channel storage
What are the transfers in a drainage basin?
Stemflow
Infiltration
Overland flow
Channel flow
Define interception storage
The precipitation that falls on vegetation surfaces and is temporarily stored here.
What factors influence interception?
Type of vegetation
Density of vegetation
Which type of trees intercept more precipitation?
Deciduous broad-leaved trees intercept more precipitation than the needles of coniferous trees.
How can tree density affect interception? Give evidence.
Denser vegetation increases interception storage.
E.g a studies show that coniferous forests captured 22% of rainfall and deciduous forests intercepted 19%. This is likely due to density rather than tree type.
What is the direct transfer of water between precipitation and surface storage called?
Unimpeded fall
What is channel fall?
Precipitation falling directly into river channels
What 4 things control infiltration rate?
Gravity, capillary action, soil porosity, and antecedent conditions
Which factor has the greatest impact of infiltration rate?
Soil porosity
What factors control soil porosity?
Texture, structure and organic content
Why do coarse-grained soils have a higher infiltration rate and fine-grained soils?
Coarse-grained soils have larger pores and fissures than fine-grained soils due to having larger soil particles.
How can pores and fissures in soils be made larger?
The burrowing of organisms such as worms and penetration of plant roots can increase the size and number of macro- and micrco-channels within the soil
What normally happens to infiltration rate during a rainstorm event?
Infiltration rate declines rapidly during the early stages, and reaches a constant value after several hours
Explain why infiltration rate decreases then plateaus during a rainstorm event. (3 reasons)
- Water filling small pores on the soil surface reduces the ability of capillary forces to actively move water into the soil.
- As the soil moistens, clay particles absorb water causing them to expand, reducing the size of soil pores.
- Raindrop impact breaks large soil clumps into smaller particles, which clog soil surface pores.
Give evidence of how soil porosity varies between different soil types.
The pores in clay soil account for 40-60% of the volume, whereas in sand this can be 20-45%
What is soil porosity?
The number and size of pores between soil particles
What is permeability?
How well pore spaces are connected, so how well fluid can move through the soil or rock.
Give an example of a fine-grained soil
Clay soil
Give an example of a coarse-grained soil
Sand
What is field capacity?
The maximum amount of water that a given soil can retain
In soil storage, water is in what state?
Gas or liquid
Why do clay soils have a higher field capacity (and therefore tend to be wetter) than sandy soils? (2)
Water molecules hold more tightly to the fine particles of a clay soil than to coarser particles of a sandy soil.
Lesser permeability of clay soils reduces infiltration rate meaning water remains in pore spaces for longer.
Are sandy soils or clay soils more permeable?
Sandy soils
Why do sandy soils have a higher infiltration rate than clay soils?
Sandy soils are coarse-grained so have larger pore spaces and therefore more macropores. These are too large to have any significant capillary force, so water will drain from these pores. The pores of clay soils are so small that water finds it much harder to move through them. Water molecules hold more tightly to the fine particles of a clay soil than to coarser particles of a sandy soil due to the larger surface area.
When is infiltration capacity reached?
When rainfall intensity is greater than the infiltration rate
What happens to surface water when infiltration capacity is reached?
Water will build up on the surface as surface storage in the form of puddles
Why are puddles relatively rare in natural environments?
Infiltration rate is usually greater than precipitation rate
What starts to occur when surface stores are full?
Overland flow
What is sheetflow?
A type of overland flow where water flows across a large surface area
What are rills?
Small channels eroded into unprotected soil by overland flow
What are 2 types of overland flow?
Sheetflow
Flow concentrated into rills
When is overland flow on agricultural land in the UK more likely to occur?
In winter when the soil is bare and unprotected
what problems can overland flow cause in urban areas?
Exacerbated flooding
How does vegetation impact rates of throughflow?
The more vegetated an area, the faster the rate of throughflow because it is aided by root channels in the soil
Why can rivers continue to flow during long periods of drought?
Groundwater flow is a very slow, so can carry on supplying water to a river long after an individual rainfall event.
What is the fastest transfer in a drainage basin system?
Overland flow
What is the slowest transfer in a drainage basin system?
Groundwater flow
What indicates that there is recirculation of water over land?
There is a net transport of 38 units from ocean to land with the same returning by the rivers to the ocean.
The amount of precipitation over continents is almost 3 times as high, indicating a considerable recirculation of water over land.
How does the recirculation of water over land vary?
It is larger during summer and for tropical land areas
In which ocean is there little net transport of water towards land?
Pacific Ocean
What proportion of the total net transport of water towards the continents comes from the Atlantic Ocean?
2/3rds
Which ocean does nearly a third of the total net transport of water towards the continents come from?
Indian Ocean
What are the advantages of viewing a drainage basin as a system?
The consequences of human activity can be better understood as so better managed to reduce negative or unintended consequences
What does the magnitude of groundwater stores and flow depend on?
The lithology and structure of the underlying bedrock
Give an example of how permeability varies between rock types.
Water will move more slowly through older, porous rocks such as sandstone, but it will move very rapidly through widely jointed limestones
Does higher porosity mean higher permeability?
No, as the pores in more porous soils tend to be smaller, which can make it harder for water to travel through them.
