5.1 - The operation and importance of the hydrological cycle Flashcards
what type of system is the hydrological cycle
closed
What is the global hydrological cycle?
A closed system of never ending water circulation driven by solar energy and gravitational potential energy
What is a store?
Reservoirs where water is held e.g. the ocean
What is a flux?
The rate of flow between stores
What is a process?
The physical mechanisms which drive the fluxes of water between the storesy
What is the cryosphere?
Areas of the Earth where water is frozen into snow or ice
What is blue water?
Water stored in rivers, streams, lakes and groundwater in liquid form (visible part of the hydrological cycle)
What is green water?
Water stored in the soil and vegetation (invisible part of hydrological cycle)
What are the main global water stores?
Oceans, icecaps, groundwater, rivers/lakes, soil moisture and atmospheric moisture
Name the 2 sources that power the global hydrological cycle
Solar energy: in the form of heat
Gravitational potential energy: causes rivers to flow downhill and precipitation to fall to the ground.
Name the 4 main stores
the oceans (largest by far)
glaciers and ice sheets (cryosphere, second largest)
surface runoff- land-based stores, including rivers, lakes, groundwater and the moisture held in soils and vegetation.
the atmosphere
Of the freshwater stores:
The cryosphere is the largest, holding 69% of global freshwater
Groundwater holds 30%
Less than 1% is stored in the biosphere (vegetation and soil moisture)
What are flows
transfers of water from one store to another.
What is residence time?
The average time a water molecule spends in a reservoir or store
What is fossil water?
Ancient groundwater formed as the result of former pluvial periods. Not renewable or reachable for human use
What is precipitation?
The movement of water in any form from the atmosphere to the ground
What is evaporation?
The change in state of water from a liquid to a gas
What is transpiration?
The diffusion of water from vegetation to the atmosphere, involving a change from a liquid to a gas
What is groundwater flow?
The slow transfer of percolated water underground through pervious or porous rocks
How much water is available to humans?
Only 2.5% of Earth’s water is freshwater, and 69% of this is locked up in the cryosphere, whilst 30% occurs as inaccessible groundwater. This leaves only 1% of fresh water easily accessible for human use
What store has the longest/shortest residence time?
Ice sheets (800,000 to 1 million years) and groundwater (10,000 years) have the longest. Atmospheric moisture is the shortest (about ten days)
Why is the hydrological cycle a closed system?
There are no external inputs or outputs
What is a drainage basin
Subsystem within global hydrological cycle, the area of land drained by a river and its tributaries
Why is a drainage basin an open system?
Has external inputs and outputs that cause the amount of water in the basin to vary over time
What is a watershed?
The highland which divides and separates waters flowing to different rivers
What 3 conditions are needed to form precipitation?
air cooled to a saturation point with relative humidity of 100%
condensation nuclei
temperature below dew point
What is condensation?
The change from a gas to a liquid
What is dew point?
The temperature at which dew forms, measure of atmospheric moisture
What are the 6 key influencing factors on drainage basin inputs?
amount of precipitation
type of precipitation
seasonality
intensity of precipitation
variability
distribution of precipitation within a basin
What is interception?
The process by which water is stored in the vegetation
How does convectional rainfall form/where is it found?
When the land becomes hot due to the sun,
-> air above it becomes warmer, expands and rises.
->As it rises, the air cools, condenses -> clouds &condensation
- Associated with intense thunderstorms - areas such as Tropics and continental interiors
How does orographic rainfall form and where is it found?
when air is forced to rise over a geographical barrier (mountain)
-> it cools, condenses -> forms rain
Happens in areas where there are mountains e.g. Himalayas
How does cyclonic rainfall form and where is it found?
when warm air ( lighter & less dense) is forced to rise over a cold denser air
As it rises, air cools and its ability to hold water vapour decreases -> condensation and clouds.
Happens in UK
What is infiltration?
The process by which water soaks into or is absorbed by soil
What is infiltration capacity?
The maximum rate at which rain can be absorbed by a soil at a given condition
Expressed in mm/hr
What is surface run off?
The movement of water that is unconfined by a channel across the surface of the ground - also known as overland flow
What factors affect infiltration?
amount of water already in soil (antecedent soil moisture)
soil texture
type, amount and seasonal changes in vegetation cover
slope angle
What are key flows and transfers in the drainage basin?
overland flow
throughflow
percolation
saturated overland flow
groundwater flow
channel flow
What is throughflow?
The lateral transfer of water downslope through the soil via natural pipes and percolines
What is percolation?
The transfer of water from the surface or from the soil into the bedrock beneath
What is saturated overland flow?
The upward movement of water into the evaporation zone
What is evaporation?
The physical process by which moisture is lost directly into the atmosphere from water surfaces and soil
What factors influence evaporation rates?
Climatic: temperature, hours of sunshine, humidity, wind speed
Other: size of water body, depth of water, water quality, type of vegetation cover, colour of surface
What is EVT and PEVT?
Evapotranspiration is the combined effect evaporation and transpiration - most important aspect of water loss to atmosphere
PEVT = potential EVT, water loss that would occur if there was an unlimited supply of water in the soil for use by vegetation
What are the physical factors affecting the drainage basin cycle?
climate
soils
geology
relief
vegetation
How can humans impact on precipitation?
Cloud seeding - introduction of silver iodide pellets or ammonium nitrate to act as condensation nuclei to attract water droplets
-> aim; to ^ rainfall in drought-stricken areas
How can humans impact on evaporation and EVT?
-through changes in land use
-deforestation
-afforestation
->increases surface runoff, increasing flooding
How can humans impact on infiltration and soil water
How can humans impact on interception
-deforestation
-afforestation -> ^ in runoff, sediment loss due to soil compaction when planting the young trees
as interception is largely determined by vegetation type
What is deforestation
cutting down & removal of trees in a forested area
What is an aquifer
underground reservoir
What is afforestation
planting of trees in an area that has not been forested in recent times
How can humans impact on infiltration and soil water?
-change in land use from forest -> farmland -> ^ soil compaction -> rain cant soak into ground -> stays on surface -> runoff -> erosion
-grazing cows -> soil compaction -> trampling of animals -> aeriating soil
How can humans impact on groundwater?
Human use of irrigation -> declining water table levels in aquifers. Too much groundwater abstraction
Name the 4 impacts humans have on the drainage basin
-overabstraction -> abstracting ^ water from groundwater reserves -> rivers drying during low rainfall
-deforestation -> reduced trees -> soil compaction -> rain stays on surface -> runoff -> erosion -> flooding
-> reduced evapotranspiration, interception
-changing land use - urbanisation -> impermeable surfaces -> alter flow of water -> reduces infiltration, interception, evapotranspiration, ^ runoff
-reservoirs -> man-made storage reservoirs -> delay flows
What is a water budget?
The balance between precipitation, evaporation and run off
What formula can be used to express drainage basin water budgets?
P = Q + E ± S
P = precipitation
Q = discharge
E = EVT
S = changes in storage
How does a water budget change throughout the year (seasons)?
winter months -> precipitation exceeds PEVT -> soil moisture surplus for plants
spring -> PEVT exceeds P -> soil moisture utilisation occurs, precipitation is used up.
-> soil moisture is now used up
-> summer months -> soil water deficiency as water is used up, and PEVT exceeds P still
-> winter months -> P exceeds PEVT -> soil water starting to fill again
-> end of december -> soil water store is now full -> field capacity has been reached
What is a river regime?
The annual variation in discharge or flow of a river at a particular point or gauging station
usually measured in cumecs.
What 6 factors affect river regime character?
-size of river, where measurements are taken
amount, pattern and intensity of precipitation
temperature
geology and soils
amount and type of vegetation
human activities such as dam building
What is a storm hydrograph?
A graph to show the variation of discharge within a short period of time, normally an individual storm or group of storms not more than a few days in length
What is the rising limb?
The part of a storm hydrograph in which the discharge starts to rise
What is peak discharge?
The time when the river reaches its highest flow
What is lag time?
The time interval between peak rainfall and peak discharge
What is the falling limb?
The part of the storm hydrograph in which the discharge starts to decrease
What is base flow?
The normal day to day discharge of the river
What does a ‘flashy’ river storm hydrograph look like?
Short lag time, high peak, steep rising limb
What factors create a ‘flashy’ storm hydrograph?
Intense storm which exceeds infiltration of soil, impermeable rocks, low infiltration rate, high steep slopes, circular basin, urbanisation, high drainage density, bare vegetation
What does a ‘flat river’ storm hydrograph look like?
Long lag time, low peak, gently sloping rising limb
What factors create a ‘flat river’ storm hydrograph?
Steady rainfall less than the infiltration capacity of the soil, permeable rocks, low gentle slopes, low drainage density, elongated basin, low population density, dense vegetation
What are the 4 impacts of urbanisation on hydrological processes?
building exposes soil and increases overland flow
tarmac surfaces are impermeable
drains and sewers reduce distance storm water must travel before reaching a channel
bridges restrain free discharge of floodwaters, act as local dams for upstream floods
Give 3 examples of sustainable drainage systems introduced to reduce runoff produced from rainfall
-permeable pavements -> delay runoff by using gaps in pavement slabs
-wetlands -> retention areas with marsh vegetation
-green roofs
