5.1 The Global Water Cycle Flashcards
5.1a The global hydrological cycle
closed system
The global hydrological cycle is the circulation of water around the earth. It is a closed system of linked processes so there are no external inputs or outputs.
For this reason the amount of global water is finite and constant.
The only thing that does change is the state in which the water exists (liquid, vapour, ice).
The proportions of global water held in each stat vary over time with changes in climate
5.1a The global hydrological cycle
drivers
what are the main drivers of the global hydrological cycle?
The global hydrological cycle is driven by two sources
- solar energy: in the form of heat
- gravitation potential energy: causes rivers to flow downhill and precipitation to fall to the ground
5.1b Stores and fluxes
oceans
Oceans, 96.9%, 0%, 3,600 years
5.1b Stores and fluxes
atmosphere
Atmosphere, 0.001%, 0.04%, 10 days,
5.1b Stores and fluxes
biosphere
Biosphere (vegetation and soil moisture) 0.001% total water, 0.05% freshwater, 2-50 weeks
5.1b Stores and fluxes
cryosphere
Cryosphere (icecaps) 1.9% total water, 68.7% freshwater, 15,000 years depending on size
5.1b Stores and fluxes
groundwater
Groundwater, 1.1% total water, 30.1% freshwater, up to 10,000 years for deep groundwater, 100 to 200 years for shallow groundwater
5.1b Stores and fluxes
surface water
Surface water (rivers and lakes), 0.01% total water, 1.2% freshwater, 2 weeks to 10 years
5.1b Stores and fluxes
fluxes
what are fluxes?
Fluxes are the rates of flow between stores
The greater fluxes occur over the oceans
5.1b Stores and fluxes
flows
Flows are the transfers of water from one store to another (in km cubed per year)
5.1b Stores and fluxes
oceans and atmosphere fluxes
evaporation 400,000
precipitation 370,000
5.1b Stores and fluxes
atmosphere and land masses
evaporation 60,000
precipitation 90,000
5.1b Stores and fluxes
landmasses and oceans
surface runoff 30,000
5.1c The global water budget
non-renewable stores
Fossil water is ancient, deep groundwater made from pluvial (wetter) periods in the geological past
The crysophere is areas of the Earth where water is frozen into snow or ice. The cryosphere is non-renewable but during another glacial period more water will once again be locked in glaciers and ice sheets.
5.1c The global water budget
key figures
All water
- 5% in oceans
- 5% freshwater
Freshwater
69% in ice caps and glaciers
30% in groundwater
1% as easily accessible surface water
5.2a The hydrological cycle
inputs
what is the main input of the hydrological cycle and its key features?
The main input is precipitation
- amount: this has a direct impact on drainage discharge and the fluxes within it
- intensity: greater intensity means greater likelihood of flooding
- seasonality: the drainage basin system operated at different flow levels at different times of the year
- form: affects entry of water into the drainage system
- distribution: significant for very large basins where tributaries start in different climate zones
5.2a The hydrological cycle
precipitation types
Orographic:
when air is forced to rise over a barrier, such as a mountain, it cools and condensation takes place forming rain. The leeward (downwind) slope receives relatively little rain, which is known as the rain shadow effect.
Frontal:
This happens when warm air, which is lighter and less dense, is forced to rise over cold, denser air. As it rises, the air cools and its ability to hold water vapour decreases. condensation occurs and forms clouds and rain form
Convectional:
This type of rainfall is common in tropical areas and in the UK during summer. When the land becomes hot, the ir above it becomes warmer, expands and rises. As it rises, the air cools adits ability to hold water vapour decreases. Condensation occurs and clouds develop. If the air continues to rise, rain will fall.
5.2a The hydrological cycle
flows
Interception: the retention of water by plants and soils which is subsequently evaporated or absorbed by the vegetation.
Infiltration: the process by which water soaks into, or is absorbed by, the soil.
Percolation: similar to infiltration, but a deeper transfer of water into permeable rocks.
Throughflow: the lateral transfer of water downslope through the soil
Groundwater flow: the very slow transfer of percolated water through pervious (permeable) or porous rocks.
Surface runoff: the movement of water that is unconfined by a channel across the surface of the ground. A.k.a. overland flow.
River or channel flow: takes over as soon as the water enters a river or stream; the flow is confined within a channel.
5.2a The hydrological cycle
variability
what are the different types of variability?
secular variability happens long term, for example as a result of climate change trends
periodic variability happens in an annual, seasonal, monthly or diurnal(daily) context
stochastic variability results from random factors for example in the localisation of a thunderstorm within a basin
5.2a The hydrological cycle
outputs
Evaporation: the process by which moisture is lost directly into the atmosphere from water surfaces, soil and rock
Transpiration: the biological processes by which water is lost from plants through minute pores and transferred to the atmosphere
Discharge (channel flow): into another, larger drainage basin, a lake or the sea
5.2b Physical factors within drainage basins
drainage basins
A drainage basin is the area of land drained by a river and its tributaries, sometimes referred to as a river catchment. The boundary of a drainage basin is defined by the watershed
The drainage basin is a subsystem within the global hydrological cycle. It is an open system with external inputs and outputs that vary causing changes to the amount of water in the drainage basin.
5.2b Physical factors within drainage basins
climate
climate has a role in influencing the type and amount of precipitation overall and the amount of evaporation ( the major inputs and outputs)
climate also has an impact on the vegetation type
5.2b Physical factors within drainage basins
soils
soils determine the amount of infiltration and through flow and indirectly, the type of vegetation
5.2b Physical factors within drainage basins
geology
geology can impact on subsurface processes such as percolation and groundwater flow (and, therefore on aquifers).
Indirectly geology alters soil formation
5.2b Physical factors within drainage basins
relief
altitude can impact on precipitation totals
slopes can affect the amount of run-off
5.2b Physical factors within drainage basins
vegetation
the presence or absence of vegetation has a major impact on the amount of interception, infiltration and occurrence of overland flow, as well as on transpiration rates
5.2c Humans disrupting the drainage basin by accelerating processes
deforestation
Deforestation reduces evapotranspiration, but increases infiltration and surface runoff
5.2c Humans disrupting the drainage basin by accelerating processes
changing land use
agriculture:
arable to pastoral farming leads to the compaction of soil by livestock, increasing overland flow
pastoral to arable increases infiltration by loosening and aerating the soil through ploughing
urbanisation is covered in later section
5.2c Humans disrupting the drainage basin by accelerating processes
new water reservoirs and abstracting water
river management:
construction of storage reservoirs holds back river flows
abstraction of water for domestic flow and industrial use reduces river flows
abstraction of groundwater for irrigation lowers water tables
5.3a Water budgets
the water budget is the annual balance between precipitation, evapotranspiration and runoff (inputs and outputs)
5.3a Water budgets
P = E + R +/- S
P= precipitation E= evapotranspiration R= runoff S= discharge
5.3a Water budgets
impact on soil water availability
At a local scale, water budgets can inform about available soil water (the amount of water that can be stored and air available for growing crops). this is valuable to users, such as farmers, who can use it identify when irrigation might be required, and how much
5.3a Water budgets
water budget graphs
water budget graph in textbook (Hodder page 14)
stages: water surplus soil moisture utilisation soil moisture deficiency soil moisture recharge (field capacity reached)
5.3b River regimes
river regime
river regimes indicate the annual variation of discharge of a river at a particular point (measured in cumbers)
river regimes result from the impact of climate, geology and soils
5.3b River regimes
influences on river regimes
what influences river regimes?
the size of the river and where the discharge measurements are taken along its course
the amount, seasonality and intensity of the precipitation
the temperatures, with possible meltwater and high rates of evaporation in the summer
the geology and soils particularly their permeability and porosity; groundwater noted in permeable rocks is gradually released into the river as base flow
the type of vegetation cover: wetlands can hold water and release it slowly into the river
human activities aimed at regulating a river’s discharge
5.3c Storm hydrographs
key features
what are the key features of a storm hydrograph?
Once the rainfall starts, the discharge begins to rise - rising limb
Peak discharge is reached some time after the peak rainfall because the water takes time to move over and through the ground to reach the river.
The time interval between peak rainfall and peak discharge is known as lag time.
Once the input of rainwater into the river starts to decrease, so does the discharge; this is shown by the falling or recessional limb
Eventually the river’s discharge returns to its normal level, or base flow.
5.3c Storm hydrographs
shapes of storm hydrographs
Storm hydrographs show discharge changes over a short period of time, often no more than a few days. The storm hydrographic plots two things: the occurrence of a short period of rain (heavy shower or storm) over a drainage basin and the subsequent discharge of a river
‘Flashy’ hydrographs have very steep limbs, especially rising limbs, a high peak discharge and a short lag time.
‘Subdued’/ ‘delayed’/ ‘flat’ hydrographs have gently inclined limbs, a low peak discharge and a long lag time.
5.3c Storm hydrographs
flashy hydrographs
Short lag time, high peak, steep rising limb
Weather/climate:
Intense storm that exceeds the infiltration capacity of the soil. Rapid snowmelt as temperatures suddenly rise above zero. Low evaporation rates due to low temperatures.
Rock type:
Impermeable rocks, such as granite, which restrict percolation and encourage rapid surface runoff.
Soils:
Low infiltration rate, such as clay soils (0-4mm/h)
Relief:
High, steep slopes that promote surface runoff
Basin size:
Small basins tend to have more flashy hydrographs
Shape
Circular basins have shorter lag times
Drainage density:
High drainage density means more streams and rivers per unit area, so water will move more quickly to the measuring point
Antecedent conditions:
Basin already wet from previous rain, water table high, soil saturated so low infiltration/ percolation
Vegetation:
Bare/low density, deciduous in winter, means low levels of interception and more rapid movement through the system
Human activity:
Urbanisation producing impermeable concrete and tarmac surfaces. Deforestation reduces interception. Arable land, downslope ploughing
5.3c Storm hydrographs
subdued hydrographs
Long lag time, low peak, gently sloping rising limb
Weather/climate:
steady rainfall is less than the infiltration capacity of the soil. Slow snowmelt as temperatures gradually rise above zero. High evaporation rates due to high temperatures.
Rock type:
permeable rocks such as limestone, which allow percolation and so limit rapid surface runoff.
Soils:
high infiltration rate, such as sandy soils (3-12mm/h)
Relief:
low, gentle slopes that allow infiltration and percolation.
Basin size:
Larger basins ten to have more delayed hydrographs; it takes time for water to reach gauging stations.
Shape:
Elongated basins tend to have more delayed hydrographs.
Drainage density:
Low drainage density means few streams and rivers per unit area, so water is more likely to enter the ground and move slowly through the basin.
Antecedent conditions:
Basin dry, low water table, unsaturated soils, so high infiltration/percolation.
Vegetation:
Dense, deciduous in summer, means high levels of interception and a slower passage through the system; more water lost to evaporation from vegetation surfaces.
Human activity:
Low population density, few artificial impermeable surfaces. Reforestation increases interception. Pastoral, moorland and forested land.
