Define Hydrological Cycle
- A closed system where no water is added to the global budget and none is removed
What are the key components of the Hydrological Cycle
- Stores = Places where water is held e.g oceans
- Fluxes = Measuring rate of flow of stores
- Processes = Physical factors driving the fluxes of water between stores
- The system is driven by solar energy and gpe
Explain the different states of water in the Global Hydrological Cycle
- In oceans 96.9% of water stored in liquid form with zero fresh water.
- In the cryosphere (cold) 68.7% of fresh water is found in solid state of 1.9% total water.
- Water largely exists as vapour in atmosphere with carrying capacity linked to temperature.
What is residence time?
- The average times a water molecule will spend in the store
- Longer residence time e.g 10,000 years for deep groundwater = unreachable for human use
Explain the difference between blue water and green water
- Blue water is freshwater stored in rivers, streams and lakes whereas green water is freshwater stored in soil and vegetation.
- Blue water is the visible part of the hydrological cycle but green is the invisible part
Explain why a large proportion of the world’s freshwater is unavailable for human use
- 68.7% of the world’s freshwater is stored in icecaps found in high latitude and high altitude locations which have high residence times of around 15,000 years making freshwater inaccessible for human use.
- 30% of the world’s freshwater is groundwater which is very deep seated as fossil water and therefore inaccessible and surface water like permafrost is difficult to access.
Explain how the global water budget limits the availability of fresh water for human use
- Most of the world’s freshwater consisting of 68.7% is stored as icecaps which has long residence times of average 15,000 years making freshwater inaccessible.
- 96.5% of the global water budget is stored in the ocean as salty water which is unfit for human use as it requires desalination which can be costly.
- Rivers are the main source of freshwater for human use but only store 1% of accessible surface freshwater
What is the drainage basin cycle
- A subsystem within the GHC & an open system with eternal inputs & outputs causing the water in the basin to vary over time
- Often referred to as catchment which is the area of land drained by a river and its tributaries
- DB can be any size from a small stream to a major river
What are the physical factors affecting drainage basins?
- Interception: Affects flows where water is prevented from reaching the ground by vegetation & the water later reaches the soil through through-flow where water drips to the ground. Interception occurs more when rain is light and short but as rain gets heavier more will flow to the ground.
- Geology: Affects the importance of the different flows within the system like surface runoff where permeable rocks allows water to accumulate in the soil until water reaches the surface forcing further rainfall to run off the surface.
- Vegetation: Affects the drainage basin cycle where the presence or absence of precipitation has a major impact on the amount of infiltration, interception and transpiration rates
Definitions keywords
Precipitation = Any form of rainfall from the sky
Percolation = The vertical movement of water through rock
Interception = When leaves, branches and plants prevent precipitation reaching soil.
Direct runoff = Rate of rainfall exceeds the infiltration capacity leading to surface runoff
Throughflow = Water moving horizontally through soil
Infiltration = Water moving vertically through soil
Evaporation = Liquid water is transformed into water vapour by heat
Transpiration = Water drawn upwards by plants from soil and evaporated through stomata
Evapotranspiration = Total amount of water removed from a drainage basin
Describe the different types of rainfall
- Orographic rainfall is when warm hot air rises due to height of mountains and cools down forming clouds then dry air descends and warm air causes little rainfall on one side of the mountain
- Conventional rainfall is when the sun heats the land and air above then warm air rises, cools and condenses forming clouds then rain occurs
- Frontal rainfall is when warm air meets cool air then warm air rises above cool air. When warm air is cooled it condenses and clouds start forming allowing rainfall to occur.
What are the human factors affecting drainage basin cycles?
- Ground water abstraction is where water is taken from aquifers at a rate > replacement level causing reduced groundwater flow and a lower water tale.
- Increased deforestation also causes groundwater storage increasing risk of flooding if water reaches surface. E.g in China groundwater irrigated 40% of farmland + provides 70% potable water in NW & groundwater dropped by a m per year between 1974-2000.
- Dam construction which is when building dams increases surface water stores and evaporation reducing downstream river flow and discharge. E.g Lake Nasser in Egypt estimated to have evaporation losses of 10-16billion cubic metres p/a leading to a loss of 20-30% of Egyptian water volume from River Nile.
- Urbanisation where building created impermeable surfaces reducing infiltration and increases surface runoff and throughflow through artificial drains increasing river discharge e.g increasing risk across UK in Manchester 2015
What is the water budget?
- The annual balance between inputs (precipitation) and outputs (channel flow)
- Precipitation = streamflow + evapotranspiration +/- changes in storage
- P = Q + E +/- S
What does the water budget show?
- The water budget shows the times when water naturally enters and leaves the system.
- We can see when there’s more than enough water (+) or not enough (-)
- It’s useful as it shows when there could be drought and challenge to human consumption, agriculture, health etc.
What is a river regime and the main factors affecting the regime of the river?
- A river regime is the variation in discharge of a river (annually)
- Main factors are: Drainage basin area, max altitude, main land use, variation in altitude, geology, mean annual precipitation, mean discharge
Simple river regime vs a complex river regime
Simple regimes
- River has one peak discharge in the year
- Seasonally high discharge followed by low discharge.
- inputs depend on glacial meltwater, snowmelt or seasonal storms.
- Found in rivers with temperate climates rising in mountainous regions where snowmelt occurs
Complex regimes
- larger rivers cross many different climatic zones leading to effects of different seasonal climatic events.
- Usually have multiple peaks of discharge not very high e.g Ganges & human factors contribute to complexity like damming rivers for energy.
A river’s regime reflects differences in rainfall, temperature, evapotranspiration & land use through river’s catchment during the year.
- Longer river = more complex variable tend to be like shape, geology, soil type, land cover
Case studies of complex river regimes
Amazon river:
- located in South America
- 6,308km long
- Drains a 6million km sq basin
- Has a humid tropical climate based by ancient shield rock
- The regime is linked to snowmelt from Andes Mountain Range.
River Nile:
- Located in Northern Africa
- A warm arid climate with a huge drainage basin
- In 1970, the construction of the Aswan Dam altered regime & flow reduced by around 65% and seasonal flow was changed.
Yukon River:
- Located in North America
- 3540km long with a drainage basin of 850k km sq.
- In winter temp declines so water freezes and in summer meltwater is an input into the system
Understanding Storm Hydrographs
- A hydrograph shows how a river’s discharge varies over time by showing rainfall and discharge
- Rises is discharge usually link to rainfall of melting or snow and ice.
- Types of flow carrying rainwater from where it fell to the river and the more this is by surface runoff = shorter lag time
- Intensity of rainfall means heavy rainfall will not sink into the ground but will runoff the surface and quickly reach the river = shorter lag time.
- Geology where hard impermeable rocks lead to more surface runoff but porous rocks lead to water moving slowly via throughflow and groundwater flow.
- Steep slopes mean more runoff and a fast delivery of water into the river.
- Weather conditions where soil is frozen due to cold temp & cannot absorb rainfall resulting in shorter lag time as water enters the river quickly
Land use is where a dense vegetation cover will intercept and delay rain reaching ground but tarmac and paved surfaces, drains and building material speed up runoff
What are the impacts of human activities on storm hydrographs?
- Urbanisation alters the nature of the ground where in heavy rainfall, impermeable surfaces like roof and pavements encourage surface runoff and allow little infiltration. Drains also speed up the delivery of rainwater to the river. Leads to hydrograph becoming a quick responsive one
- Deforestation removes trees leading to less interception and more rainfall reaching the ground so more rainfall reaches river quicker than before. Rising limb becomes steeper and there’s soil erosion in valley sides. Eroded soil adds to river’s load and leads to some deposition = slow response hydrograph.
- Dams/reservoirs regulate flows downstream by storing water and abstraction lowers groundwater levels and increases percolation & infiltration when rain falls.
