Water EQ1 - Hydrological cycle Flashcards
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 stores
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, atmosphere, biosphere, cryosphere, groundwater, surface water
Which are the largest stores?
Oceans and icecaps
What is residence time?
The average time a water molecule spends in a reservoir or store
Which stores have the longest/shortest residence times?
Longer = cryosphere (up to 800,000 years), groundwater (10,000 years)
Shorter = surface water, atmospheric water (about ten days)
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 (permeable) 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 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, the air above it becomes warmer, expands and rises. As it rises, the air cools, condenses and forms clouds and condensation occurs. 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 such as a mountain, meaning it cools and condenses to form rain - leeward slope receives relatively little rain (rain shadow). Happens in areas where there are mountains e.g. Himalayas
How does cyclonic rainfall form and where is it found?
When warm air mass which is lighter and less dense, is forced to rise over a cold dense air mass. As it rises, the 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
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?
Where soil becomes saturated and there is no longer any space for water to infiltrate so the water table is raised to the surface
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
How can humans impact on evaporation and EVT?
- deforestation
- afforestation
How can humans impact on infiltration and soil water?
Changes in land use - farmland vs forest
Farmland = reduced interception and more overland flow
How can humans impact on groundwater?
Human use of irrigation can lead to declining water table levels in aquifers. Too much groundwater abstraction
What is a water budget?
The annual balance between precipitation, evapotranspiration and run off
What formula can be used to express drainage basin water budgets?
P = Q + E ± S
P = precip
Q = discharge
E = EVT
S = changes in storage
How does a water budget change throughout the year (seasons)?
Water surplus in winter months as precipitation exceeds EVT. Then in spring soil moisture utilisation occurs and eventually precipitation is used up. Then there is deficiency as water is used up, before soil moisture recharge as the soil water soil store starts to fill again. Field capacity then reached and groundwater stores recharged
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 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 capacity 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 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
How can decision makers and planners consider effects on the hydrological cycle?
By increasingly looking at the catchment management upstream (e.g. afforestation) and in the lower course (flood defences) in order to manage impacts of urbanisation on flood risk
