EQ1- What are the processes operating within the hydrological cycle from global to local scale?

Hydrological cycle

The circulation of water around the Earth.

• closed system: no external inputs + outputs, but stores and flows.

  • Stores: places where water is held e.g. oceans

  • Fluxes: the measurement of rate of flow between the stores

  • Processes: the physical factors which drive the fluxes of water between stores.

Driven by:

Solar energy: more solar energy in the form of heat melts ice→water or evaporates water→water vapour, while less heat condenses water vapour→water or ice→water.

Gravitational potential energy: Causes rivers to flow downhill + precipitation to fall to the ground.

Description of hydrological cycle

• Atmosphere: water largely exists as vapour with carrying capacity directly linked to temp. Clouds can contain liquid water or ice crystals (at high altitudes).

• Cryosphere (Earth’s ice in all its forms) water: largely found in a solid state, some liquid as melt water + lakes.

• Oceans: majority of water is stored in liquid form, only a small fraction as icebergs.

• Land:

  • blue water: stored in rivers, streams, lakes + groundwater in liquid form.

  • green water: water stored in vegetation or the soil

Relative size + importance of stores

Residence time: the average length of time that a water molecule remains in a store.

-stores with high RT are more easily polluted as water is in situ for long RT’s.

-Soil mositure RT is low as spread thinly across Earth, so water is easily lost by evaporation, percolation, etc.

-Atmosphere RT is low as water evaporated, condenses + falls as rain.

Global water budget

Why is water availability limited?

Global water budget: takes into account all water held in stores + flows of the global hydrological cycle.

• 3% of global water budget is fresh water (safe for human use). 69% of this is trapped in ice caps, leaving 1% to be easily accessible by humans.

2 non-renewable water stores

Fossil water: Ancient, deep groundwater made form pluvial (wetter) periods in the geological past. e.g. Sahara Desert

Cryosphere losses: Made up of areas where water is frozen into snow/ ice. Water is locked up in ice sheets + glaciers; non-renewable if another ice glacial period comes.

Drainage basin

An area of land around the river drained by the river and its tributaries. The boundary of a drainage basin is defined by the watershed (area of high land).

-subsystem within the hydrological cycle.

Inputs of drainage basins

Orographic (relief) rainfall: Warm wet air forced to rise over high land→air cools+condenses as it rises→clouds form+ precipitation occurs

Convection rainfall: Sun heats ground + warm air rises→air cools + condenses as it rises to form clouds→large cumulonimbus clouds are formed→heavy rain storms occur.

Frontal rainfall: An area of warm air meets area of cold air→warm air forced over cold air→ warm air is cooled + water vapour condenses→clouds form + precipitation occurs.

Flows of drainage basins

Interception: water is retained by plants/ soil. May be evaporated/ absorbed by vegetation.

Infiltration: water soaks into/absorbed by soil.

Direct runoff: Water flows across an impermeable ground’s surface.

Saturated overland flow: The flow of water over the surface of land when the ground is saturated with water.

Throughflow: The lateral(sidewards) transfer of water through soil towards rivers.

Percolation: Deeper transfer of water from soil to permeable rocks.

Groundwater flow: The slow transfer of percolated water through permeable or porous rock.

Outputs of drainage basin

Evaporation: Diffusion of water from vegetation into atmosphere.

Transpiration: Water moved through vegetation + evaporates into water vapour.

Channel flow: Water flows into another larger drainage basin e.g. a lake, the sea

Stores of drainage basins

Aquifer: Rock formations or layers of unconsolidated deposits which have hold substantial volumes of water.

-are permeable + high porosity (many pores)

• Confined: enclosed by impermeable rocks

• Unconfined: exposed to water

• Perched: above water table, sitting above impermeable rock

Aquitard: Rock formations which are impermeable + transmit water more slowly.

Physical factors affecting flows of the drainage basin

Climate: High rainfall→saturates soils→soil becomes less permeable→infiltration decreases+surface runoff increases.

Soils: More porous soil→bigger space for water to infiltrate + less surface runoff.

Vegetation: High vegetation cover→more interception+infiltration→less surface runoff

Geology: Permeable rock→water percolates through pores→less surface runoff

Relief: Higher slope angle→water slips off due to lower SA→less infiltration+more surface runoff.

How humans disrupt drainage basin cycle

Abstraction of water

Accelerating processes: deforestation + land use changes

Abstraction

Water abstracted from groundwater for domestic + industrial use.

Problems of fall in groundwater

Drinking water supplies ↓: wells + springs run dry, causing social problems + water insecurity

Lowers water table: Increases costs as longer pumps are needed to extract groundwater from deeper underground.

Reduced river flow: may cause droughts downstream, river volume decreases which has a negative impact on aquatic ecosystems.

Problems of deforestation e.g. on Amazon basin

More erosion of soil: soil more exposed to precipitation→may affect its properties such as volume of water it can store (less mineral content)→less infiltration + more runoff.

Less interception: Less vegetation→less interception→more flooding.

20% vegetation deforested in past 50 years, making flooding 5 times more common in last century.

Less evaporation+transpiration: as there is less vegetation to carry this out. More droughts, vegetation need moisture to thrive.

Contributes to global warming: more GHG→higher temp + less humidity→less cloud formation + precipitation→more evaporation.

Extreme droughts becoming more frequent- once every 13 years in the Amazon.

Problems of land use changes

Agriculture

Intensive farming- ploughing (application of fertiliser) increases infiltration by loosening and aerating the soil. Impact of machines can compact soil, increasing overland flow.

Urbanisation

Impermeable surfaces: tarmac, tiles, speed surface runoff by reducing percolation + infiltration.

Drainage systems: deliver rainfall more quickly to streams + rivers, increasing chances of flooding.

Water budgets

Relationship between input (precipitation) and output (evapotranspiration) over the year.

Impact of climate (zones) on water budget- named examples

Desert climates: low precipitation + high potential evapotranspiration all throughout year. e.g. Atacama Desert, Peru

Tropical rainforest climate: high precipitation October→May, then potential evapotranspiration overtakes during summer months. e.g. Bolivia

River regimes

Indicate the annual variation of discharge (volume of water flowing through) of a river throughout the year.

Factors affecting river regimes

Climate: Higher temp in summer- high rates of evaporation + also meltwater. e.g. Yukon river, Alaska. Intense rainfall- heavy rain becomes overland flow (doesn’t sink) + runs off quickly to river.

Geology: Permeable rocks allow groundwater to gradually release into the river through infiltration + percolation.

Soils: High porosity soil allows for water through infiltration to be released into the river gradually.

Contrasting river basins

Indus, India

• Meltwater from Himalayas increases discharge in April creates initial peak discharge.

• 4 month monsoon brings discharge to its peak in July, then river levels drop from dry winter season.

Amazon, S. America

• Daily rainfall from convection in Feb+March as sun is directly overhead (270mm mean)

• 4 month delay for rainfall in Feb to cause peak discharge in June before dropping again due to large basin size

• Earth tilts away from sun, reducing rainfall. 5 month delay again until river reaches lowest level in Nov (19m)

Yukon, Alaska + Canada

• Snowmelt + melting of permafrost + soils releasing water in May+June cause peak discharge (16 000m³)

• Short summer ends in July when water refreezes during winter, arctic soils freeze, reducing river level as no throughflow + runoff occurs.

Storm hydrographs

Graph that shows how a river changes as a result of rainfall.

-shows rainfall + discharge

Flashy graph: steep limbs, high peak discharge + short lag time.

Subdued graph: flat limbs, low peak discharge, long lag time.

Physical factors affecting shape of storm graphs (flashy/ subdued)

Size: Larger catchment area of the river basin→higher peak discharge + longer lag time.

Shape: More circular the catchment area→ shorter lag time.

Drainage density: dense drainage network→transport water more efficiently→increased peak discharge.

Rock type: impermeable rocks e.g granite→increase surface runoff→ reduced lag time + increased peak discharge.

Soil: soil is saturated→increased surface runoff→increases peak discharge

Relief: Steeper catchment area of river basin→water reaches river quicker→long lag time + increased peak discharge.

Vegetation: More vegetation→higher infiltration→reduced peak discharge

Human factors affecting shape of storm hydrographs

Urbanisation: increased impermeable surfaces e.g. tarmac→more surface runoff→shorter lag time + greater peak discharge

Land use: ploughing agricultural land→increased surface runoff→ shorter lag time + increased peak discharge.