Water cycle

• Changes to the hydrological cycle

  ~ Increased evaporation rates --> more moisture circulating the hydrological cycle

  ~ Changes in precipitation type --> e.g. earlier springtime means more snowmelt occurring earlier in the year

  ~ Increased surface permafrost temperatures

  ~ Reduction in: sea ice, glacier storage, ice caps

  ~ Changes in the capacity of terrestrial ecosystems to sequester carbon and store water
• Hydrological cycle is powered by the sun:


  ~ The sun heats the water in oceans or on land - where evaporation occurs

  ~ Water vapour rises into the atmosphere and condenses - forms clouds - drives the atmospheric circulation

  ~ Cloud droplets fall back to the earth as precipitation
• Energy exchange within the hydrological cycle 


  ~ Energy exchange in the hydrological cycle leads to local temperature fluctuations

  ~ As water evaporates it uses energy from its surrounding for this process to happen

  ~ When water condense the opposite happens - heat is released

  ~ This heat exchange influences the local climate

  ~ Water droplets have high GPE (gravitational potential energy) when they fall on a mountain or slope

  ~ The water droplet's GPE turns into kinetic energy as it is in motion - allows water to move over the surface
• Types of water in the world


  ~ Only 2.5% of all Earth's water is freshwater

  ~ Around 1.6% of this freshwater is locked away as:

  ~ Ice within the cyrosphere - 70%

  ~ Remaining 30% - groundwater

  ~ Only 1% of the earth's freshwater is accessible to humans
• Major stores of water

  ~ Oceans

  ~ Glaciers and ice sheets

  ~ Aquifers (groundwater)

  ~ Rivers

  ~ Lakes

  ~ The moisture held in soil and vegetation
• Why water is stored unevenly around the globe


  ~ It is due to the uneven spread of land to sea

  ~ Different geology types - e.g. porous and permeable rock enabling aquifers to form compared to impermeable rock, where water can't percolate

  ~ Climate - Different global rainfall patterns --> water deficit in areas with low rainfall patterns and water surplus in areas with high rainfall patterns
• What is annual flux

  • Annual flux are the variations in flows due to temperature, seasons and location
  • Flows such as evaporation will be greatest in warmer areas due to increased heating from the sun e.g. at the equator
  • This then leads to high rates of precipitation at the equator too
  • Stores such as ice caps will be getting smaller as a result of climate change whereas ocean stores will be increasing for the same reason
• Global water budget 


  ~ This is the total amount of inputs, outputs and stores of water in the global system

  ~ It examines how water moves within the different subsystems of the earth (atmosphere, biosphere, cryosphere, hydrosphere & lithosphere)

  ~ The global water budget is going to be under lots of change due to climate change:

  > e.g. as global warming continues to warm the earth and melt the ice, the cryosphere will lose significant amounts of water --> therefore shifting the water budget
• Annual water budget equation 


  ~ P=Q+E +/- S

  ~ Preco
• Annual water budget equation

  ~ P=Q+E +/- S

  ~ Precipitation = Discharge + Evaporation +/- Storage
• Residence time


  ~ this is the amount of time that water is held in a store

  ~ water residence time and the size of the stores of water is dictated by:

  > flows/transfers e.g. evaporation

  > global factors e.g. climate change

  > local factors e.g. human activity on a hill slope
• ITCZ (inter-tropical convergence zone


  ~ This is a low pressure zone also known as the 'thermal equator'

  ~ In other words it is a band of low pressure around the earth lying near the equator

  ~ It tracks with the seasonal movement of the sun (north and south of the equator), bringing intense low-pressure rain/monsoon conditions with the movement
• Cryospheric processes - positive feedback loop


  ~ Climate change causes the ice sheets and glaciers (ice if the second largest store of water) to melt

  ~ The melting of these ice sheets adds to the hydrosphere store, and can cause a sea level rise of up to 60m

  ~ It is a positive feedback loop - as the ice sheets / shelves are destabilised, ice calving occurs, which melts the icebergs, adding to the hydrosphere store and rising sea levels --> positive feedback loop
• Drainage basins


  ~ these are subsystems of the global hydrological cycle

  ~ they drain all the water that lands on the earth's surface
• Catchment area of a drainage basin


  ~ also known as the individual drainage basin

  ~ it is the area that gets drained by a river and its tributaries
• Watershed - drainage basin


  ~ this is the boundary of the drainage basin
• Features of a drainage basin


  ~ Watershed

  ~ Source

  ~ Confluence = the place where 2 or more rivers/streams meet

  ~ Tributary

  ~ Mouth
• Atmosphere is a store and transfer:

  ~ clouds store water

  ~ clouds also move water around the planet
• Above ground flows in a drainage basin 1


  ~ Throughfall --> precipitation that makes it to the ground without any interception from plants

  ~ Drip flow --> water that flows off leaves and drips to the ground

  ~ Trunk and stem --> the flow of water down stems of plants and trunks of trees

  ~ Overland surface flow --> when water flows over the land's surface - 2 types:

  > channel flow = water flows in small channels or rills in a defined stream pattern

  > sheet flow = a layer of water on the surface (due to excess surface flow / water can't infiltrate quick enough)
• Above ground flows in a drainage basin 2

  ~ Interception --> interception from vegetation can prevent up to 40% of precipitation from reaching the ground

  ~ Buildings --> prevent water from reaching the ground (drains into gutters and drains, or evaporates)
• Below ground flows in a drainage basin

  ~ Infiltration --> water enters small openings / pores from the ground's surface

  ~ Throughflow --> lateral (sideways) movement of water through the upper soil along lines of seepage called percaline

  ~ Percolation --> water flows down through soil layers and underlying rock is pulled down by gravity (rate of travel is determined by porosity of soil and and permeability of rock)

  ~ Groundwater flow --> the slow transfer of percolated water permeable / porous rocks
• River discharge

  ~ It is the volume of water flowing through a river channel - (the volume of water that streams past a point in the rivers course every second)

  ~ is measured at any given point in cubic metres per second
• Factors affecting drainage basins


  ~ Soil:

  > infiltration and throughflow

  > permeability / frozen / saturated / compacted (e.g. low infiltration rates and more surface runoff)

  ~ Climate:

  > type and amount of precipitation

  > extent of evaporation

  > amount and type of vegetation

  ~ Vegetation:

  > interception

  > infiltration

  > drip flow, and trunk and stem flow

  ~ Relief:

  > steepness of slopes (steeper slope = more surface runoff)

  ~ Geology:

  > permeability of rocks and if they can percolate through - also affects groundwater flow
• Human impacts on drainage basins 2

  ~ Abstraction:

  > over-abstraction leads to reduced flow in rivers

  > reduces the amount of groundwater

  ~ Reservoirs:

  > construction of dams and reservoirs reduce water flow downstream

  > increases evaporation (greater surface area of water)
• Human impacts on drainage basins 1

  ~ Deforestation

  • The felling and clearance of trees reduces interception and infiltration rates
  • It increases surface run off
  • It reduces evapotranspiration which reduces precipitation

  ~ Changing land use

  • Urban surfaces (e.g. tarmac&concrete) are impermeable --> reduced infiltration & inc surface runoff
  • Drainage systems move water to rivers quicker --> inc risk of flooding
  • Agriculture reduces the amount of large vegetation decreases interception & inc surface runoff
  • Livestock farming --> compaction of the soil, reducing infiltration
• Water balance and its equation

  ~ Water balance is the balance between inputs and outputs of a drainage basin

  ~ It shows how much water is stored in a system

  ~ P=O+E +/- S

  ~ precipitation = total runoff (streamflow) + evapotranspiration +/- changes in storage

  ~ E.G. --> water surplus in wet season is created by precipitation rates being higher than evapotranspiration - positive water balance

  ~ E.G. --> when evapotranspiration exceeds precipitation (as plants absorb water and ground stores get depleted), a negative water balance and water deficit occurs
• Hydrographs 


  ~ 2 types: annual hydrograph (also known as river regime) and storm hydrograph

  > river regime = the changes in a river's discharge over the course of a year in response to a number of factors --> different conditions and seasons affect precipitation rates (temperature, vegetation and geology also affect this), affecting river discharge

  * e.g. the big swings in tropical rivers with wet and dry seasons affect the annual hydrograph

  * e.g. spring indicates snow melt

  *e.g. permeable rocks reduce discharge most of the year
• Storm hydrograph


  ~ They show changes in a rivers discharge during and after a storm

  ~ They compare 2 variables: rainfall received during an event (mm) & river discharge (cubic m/sec)
• Parts of storm hydrographs


  ~ Peak rainfall -> the most amount of rainfall that there will be

  ~ Rising limb -> shows the increase in discharge

  ~ Lag time -> time taken between peak rainfall and peak discharge (surface runoff delays lag time (water enters river quicker)&makes it longer)

  ~ Peak discharge -> maximum amount of water that the river has (the quicker the water gets in the river, the higher the peak discharge - greater risk of flooding)

  ~ Falling limb -> speed of water level decline after its peak (the quicker the water leaves the river, the quicker the falling limb)
• The 3 ways that water is transferred into drainage basins 


  ~ Directly into the channel (not often)

  ~ Surface flow (most often)

  ~ Infiltration (through and base flow)
• 'Flashy' or 'flat' storm hydrographs

  ~ 'Flashy':

  > short lag time, high peak discharge and steep rising limb

  > impermeable rock

  > clay soils - low infiltration rate

  > heavy / prolonged rainfall / rapid snowmelt & low evaporation rates

  > saturated soil

  > smaller basin

  > deforestation & urbanisation

  ~ 'Flat':

  > long lag time, low peak discharge, and gentle rising limb

  > permeable rock

  > sandy soil

  > steady rainfall and high evaporation rates

  > More trees --> more interception

  > Larger basins - longer lag times & more gentle rising limbs

  > afforestation - increases interception
• the 3 types of droughts 


  ~ Meteorological --> occurs when there is rain deficit and is caused by changes in atmospheric circulation - causing lack of precipitation and high temperatures, increasing evaporation

  ~ Agricultural --> when there is a water deficiency in the soil - leads to crop failure and reduced biomass

  ~ Hydrological --> occurs when there is a lack of water stored on the surface underground in lakes, rivers, reservoirs and aquifers
• El nino


  ~ El Niño:

  > occurs when sea temperatures are 0.5°C above average

  > occurs due to the trade winds, atmospheric circulation & ocean currents

  > occurs every 2-7 years in the equatorial Pacific Ocean - peak usually in December

  > thermal expansion and sea level rise off coasts of countries e.g. Peru due to ocean temperature rise

  > droughts in Australia

  > occurs more often than La Niña

  ~ Causes of El Nino:

  > trade winds are reduced / reversed (west to east) - leads to a reversal of conditions and rising air over the eastern pacific, and descending air over western pacific(AUS)
• La nina


  ~ Occurs when sea temps fall below average --> brings cooler and drier than average weather in eastern pacific (South America)

  ~ occurs every 3-5 years

  ~ causes:

  > stronger than usual eastward trade winds & ocean currents

  > these trade winds bring cold water to the surface through upwelling (happens near South America)

  ~ increased flooding and rainfall in Australia and SE Asia

  ~ drought in South America

  ~ increased risk of tropical storms in the Atlantic
• How human activity increases drought risk


  ~ Over-abstraction of surface water resources and groundwater aquifers increase the risk of drought (over abstraction can occur faster than nature can replace them)

  > over-abstraction occurs due to: population growth and overcultivation (excessive use of farmland leads to soil exhaustion)

  ~ deforestation also increases drought risk - it reduces the ability of the soil to retain water
• Dams benefits


  ~ Dams block rivers so that reservoirs can build up behind, rather than drain away

  ~ It provides with drinking water all year round

  ~ It reduces water insecurity - especially where precipitation may be seasonal
• Water budget 


  ~ the water budget aims to work out how much water there is in a system such as a drainage basin - it does this by looking at the balance between the inputs and outputs of of the system per year

  ~ major inputs - precipitation & water from human activity -farming&waste

  ~ major outputs --> evapotranspiration, runoff, river discharge

  * water budget equation: P = Q + E (precipitation (inputs)) = discharge + evapotranspiration (outputs)

  ~ the water budget links to water surpluses and water deficits:

  > surplus - inputs exceed the outputs

  > deficit - outputs exceed the inputs
• UK water budget 


  ~ Generally a water deficit in the summer and a water surplus during the winter:

  > higher evapotranspiration in summer and vice versa in the winter

  > water stores have a chance to recharge in winter months after being somewhat depleted

  ~ However there can be events to disrupt this e.g. extreme weather events - storms / heatwaves can alter river-discharge short term
• Falling limb management strategies


  ~ Governments will use management strategies such as channel straightening to have a steeper falling limb --> they want to get the water out of their city as soon as possible (reduce flood risk)
• Types of water scarcity 


  ~ Economic --> water is available but the infrastructure is not available to access the water

  ~ Physical --> occurs when there is greater demand for water than there is supply