Edexcel A-level Geography: Water Cycle & Insecurity

• Edexcel Geography A-level notes cover the water cycle and water insecurity

Global Water Budget

• Oceans are the largest store of global water, containing 97%.
• Freshwater accounts for only 2.5% of global stores.
• Glaciers, ice caps, and ice sheets comprise 69% of freshwater.
• 30% of freshwater is groundwater.
• Surface and other freshwater, including permafrost, lakes, swamps, marshes, rivers, and living organisms, make up around 1% of global stores.

Hydrology in Polar Regions

• 85% of solar radiation is reflected.
• Permafrost creates impermeable surfaces.
• Lakes and rivers freeze.
• There is rapid runoff in spring.
• Seasonal release of biogenic gases into the atmosphere takes place.
• Orographic and frontal precipitation happens.

Hydrology in Tropical Rainforests

• Dense vegetation consumes 75% of precipitation.
• Limited infiltration transpires.
• Deforestation leads to less evapotranspiration and precipitation.
• Temperatures are high.
• Conditions are humid.
• Rainfall is convectional.

Water Distribution and Residence Time

• Oceans hold 96.9% of total water with a residence time of 3,600 years.
• Icecaps contain 1.9% of total water and 68.7% of total freshwater, with a residence time of 15,000 years.
• Groundwater holds 1.1% of total water and 30.1% of total freshwater, with a residence time of 10,000 years.
• Rivers and Lakes contain 0.01% of total water and 1.2% of total freshwater, with a residence time of 2 weeks to 10 years.
• Soil moisture holds 0.01% of total water and 0.05% of total freshwater, with a residence time of 2-50 weeks.
• Atmospheric moisture holds 0.001% of total water and 0.04% of total freshwater, with a residence time of 10 days.

ITCZ (Inter-tropical Continental Zones)

• Earth has six cells of circulating air that dictate climate.
• The explanation below is for the Northern Hemisphere, the Southern Hemisphere goes in opposite directions.
• Hadley Cell: Air rises at The Doldrums, travels upwards, and sinks upon meeting cooler air of the Ferrel Cell, causing precipitation. Air travels southward, heats up, and rises again at the Doldrums.
• Polar Cell: Cold air sinks near the Arctic Circle, cools, and condenses into precipitation, then travels southward, heats up, rises upon meeting warm air from the Ferrel Cell, and travels northward to cool.
• Ferrel Cell: Middle cell; air circulation is determined by the Hadley and Polar cells.

Drainage Basins

• Drainage basin is an open subsystem within the closed global hydrological cycle.
• An area of land drained by a river and its tributaries with a boundary (watershed) consisting of hills and mountains.

Water Cycle Scale

• On a local scale, the water cycle is an open system.
• On a global scale, the water cycle is a closed system.
• The water cycle contains flows/transfers, inputs, outputs, stores/components.

Inputs to the Drainage Basin

• Precipitation is caused by the cooling and condensation of water moisture in the atmosphere in the form of rain, snow, hail, sleet, etc.
• Seasonality: Climates such as monsoon and Mediterranean have strong seasonal rainfall patterns.
• Variability: can cause sudden or long term changes to the climate.
• Secular: long term changes to the water basins.
• Periodic: Annual, seasonal, or monthly context.
• Stochastic: Random factors like localisation of a thunderstorm.
• Latitude: Location of the drainage basin impacts climate and precipitation type. Higher latitudes typically have colder climates.

Types of Rainfall

• Convectional: Occurs when morning heat warms the ground, heating low-level moisture, which evaporates and rises, cools, and condenses into rainfall. Common in tropical climates
• Frontal/Cyclonic: Occurs where a hot air mass wedges within cold air (depression), causing warm, moist air to rise and condense into cyclonic precipitation.
• Relief/Orographic: Occurs when warm, moist air meets high relief land, rises, cools, and condenses into rainfall.

Fluxes and Flows within the Drainage Basin

• Flows rely on gravity and depend on the relief of the land.
• Flows occur at different speeds.
• Interception: Leaves directly intervene by changing the direction or temporarily stopping precipitation.
• The interception store is greatest at the start of storms.
• Infiltration: Movement of water from the surface into the soil, with infiltration capacity affected by soil composition, previous precipitation, vegetation, compaction, and land relief.
• Surface Runoff: Water flows overland; overland flow is faster on greater gradients, making water transfer to river channels.
• Throughflow: Water moves through soil into streams/rivers, flow speed relies on soil type; slower in clay soils.
• Percolation: Water moves from ground into porous rock or rock fractures, which is dependent on fractures and permeability.
• Groundwater Flow: Gradual water transfer through porous rock.

Outputs of the Drainage Basin

• Three main outputs: water and moisture are transferred across the watershed.
• Evaporation occurs when water moisture is lost from the surface of water, soil, and interception to the atmosphere.
• Increase in evaporation rates are caused by warmer, windier and dryer weather.
• Influence in evaporation rates are due to size of water body, vegetation cover or built environment surrounding water, and the colour of the ground beneath.
• Transpiration is the process where water is lost to the atmosphere through plant pores (stomata).
• Transpiration rates are affected by seasonality, vegetation type, moisture content of the air and time of day.

Stores

• Soil Water: Water stored in the soil used by plants; mid-term storage.
• Groundwater: Water stored in rock pore spaces; long-term storage.
• River Channel: Water stored in a river; short-term storage.
• Interception: Water intercepted by plants; short-term storage.
• Surface Storage: Water stored in puddles, ponds, lakes; variable storage.
• The water table is the upper level at which the pore spaces and fractures in the ground become saturated.

Factors Influencing the Drainage Basin

• Physical Factors:
  • Climate: Influences rainfall and vegetation growth.
  • Soil Composition: Influences infiltration and throughflow rates.
  • Geology: Affects percolation and groundwater flow.
  • Relief: Steeper gradients encourage faster surface runoff rates.
  • Vegetation: Affects interception and overland flow.
  • Size: Larger basins collect more precipitation.
• Anthropogenic (Human) Factors:
  • Cloud seeding: Dispersing substances to induce condensation. Example: used to reduce pollution before 2008 Beijing Olympics.
  • Deforestation: Less vegetation means less interception, and more overland flow.
  • Afforestation: More vegetation means interception, less overland flow, and more evapotranspiration.
  • Dam construction: Dams reduce downstream river flow and discharge, and increase evaporation.
  • Change in land use: Converting land to farmland decreases interception and increases soil compaction and surface runoff.
  • Groundwater abstraction: Water extraction exceeds recharge, lowering groundwater flow and the water table.
  • Irrigation: Results in a drop in water tables due to usage.
  • Urbanisation: Increases impermeable surfaces, reducing infiltration, and increasing surface runoff.

Water Balance and River Regimes

• Over long periods of time (annual) surpluses and deficits can cancel out.
  • Precipitation = Discharge + Evaporation ± changes in stores
• A. Soil moisture increases when precipitation exceeds evaporation, creating a soil moisture surplus.
• B. As temperature increases, moisture gained from its surplus is used.
• C. Maximum evaporation is at the highest risk of drought.
• D. Despite evaporation decrease, a soil moisture deficit is created.
• E & F. As precipitation exceeds evaporation rates, soil moisture is regained, reducing the deficit, through recharge.
• A regime is the annual variation in river discharge, mainly supplied by groundwater.
• There are seasonal variations in rainfall and discharge.
• Factors affecting river regime:
  • Channel capacity of the river
  • Area and relief of the drainage basin
  • Volume, pattern and intensity of precipitation
  • Climate
  • Geology of the soil regarding the input of groundwater
  • Anthropogenic activities

Storm Hydrographs

• Indicates discharge variation within a short time.
• Base flow is groundwater being the main supply of water to the river prior to a storm.
• Infiltration and surface runoff increases throughflow.
• Rising limb: River discharge increases, not immediately.
• Peak flow: Maximum discharge after maximum precipitation.
• Lag time: time delay between rainfall and peak discharge.
• Falling limb: Discharge decreases as storm precipitation levels drop.
• Base flow: Discharge returns to the normal level.
• Physical factors influencing hydrographs:
  • Flashy Storm Hydrograph: Short lag time, high peak, intense storm, rapid snow melt, low evaporation, granite rocks, low infiltration, steep slopes, small basin, low density vegetation, basin already wet with high water table, soil saturated.
  • Subdued Storm Hydrograph: Long lag time, low peak, steady/less steady rainfall, slow snow melt, high evaporation, limestone rocks, high infiltration, low and gentle slopes, large basin, high density vegetation, dry basin, soil unsaturated.

Management of Drainage Basins

• Sustainable drainage basin management can be challenging.
• Some river management schemes reduce runoff in order to reduce risk of flash flooding or storm flow discharge:
  • Growing plants on roofs to increase interception.
  • Creating permeable pavements to increase infiltration.
  • Rainwater harvesting.
  • Creating wetlands for temporary water storage.
• Human development outweighs the need for sustainable management, due to housing pressures:
  • Deforestation, tree felling and slash-and-burn causes soil to become exposed.
  • Impermeable surfaces - less infiltration into soil.
  • Bridges can restrict channel flow, worsening flood effects.
  • Drainage and sewage systems will create a quick water flow back to rivers.

Deficits in the Hydrological Cycle

• Imbalance in water inputs and outputs has implications.
• Input being less than output is a water deficit known as drought.
• Types of droughts and their characteristics:
  • Meteorological Drought: Low rainfall and snow cover, high temps, strong winds and increased radiation causes loss of soil water and irrigation drop.
  • Hydrological Drought: Reduced infiltration, low soil moisture, little percolation and groundwater recharge will create more threats to wetlands and habitats.
  • Agricultural Drought: Low evapotranspiration, reduced biomass, decreased groundwater level.
  • Socio-Economic Drought: Loss of vegetation, desertification, increased wildfire risk.

El Nino Southern Oscillation

• El Nino is the change in water body patterns within the Southern Hemisphere
• This causes unusual weather conditions.
• The causes of El Nino aren't fully understood.
• Normally cool water can be found near Peru, and warm waters are located around Australia, ENSO causes these to switch and happens every 3 to 7 years.
• ENSO causes extremely dry conditions in areas off Australia, South & South-East Asia, and North-East Brazil.
• Wetlands acts as temporary water stores within the hydrological cycle, that can mitigate against river floods from sudden storm discharge.
• Chemically, wetlands act like giant water filters by trapping and recycling nutrients and pollutants that helps maintain water quality of the river.
• Wetlands have very high biological productivity which supports diversity for a food web.

Value of Wetlands

• Supporting Life: Stores and flows of carbon, and nutrient recycling.
• Provision of Resources: Fuelwood, fisheries, and tourism
• Regulating Conditions: Regular supply of groundwater for river base flow, water purification, and reduced flood risk
• Cultural Value: Aesthetic and recreational value.
• Meteorological droughts impact wetlands; reduced interception impacts soil nutrients and food web.

Desertification

• Physical Causes:
  • Reduced Precipitation - Protective vegetation for soil is eliminated, leading to erosion and worsening soil.
  • Global Warming - Increased evaporation in tropical climates reduces convectional rainfall, stunting plant growth.
• Human Cause:
  • Over population causes food demands that increases pressure for cultivation, which results in lack of soil fertility.
  • Clearing forests worsens soil quality.

Surpluses and Hydrological Cycle

• All drainage basins are prone to water surpluses with some more susceptible to flooding.
  • Low-lying land and base of a river valley/estuaries.
  • Urbanised, built environments.
  • Small basins.

Mitigation & Adaptation to Flood Risk

• Afforestation: Increasing vegetation will reduce rapid surface runoff.
• Construction restriction on floodplains.
• Establishing temporary extra flood plains during extreme weather.

Impacts of Climate Change on Hydrological Cycle

• Anthropogenic global warming is due to increased greenhouse gases in the atmosphere.
• This leads different impacts on hydrological cycle.
• Land and sea surface temperatures could decrease every 2-3 years due to El Nino cycles.
• Increasing average global temperatures will increase droughts.
• Temperature increase leads to increased rainfall and increased intensity of cyclone.
• Climate Change: Reduces inputs and stores while increasing outputs:
  • Less precipitation and available water in stores.
  • Reduced snow and glacier mass size
  • Water Table drops and Aquifer stores deplete leading to larger evaporation rates.

Inequality and Insecurity of Water

• Water is unevenly distributed, 66% of people only have 25% of rainfall.
• Demand is risen because of population, middle class populations, and economic growth.
• Supply can't meet demand because:
  • of agriculture.
  • Aquifers and wells are dropping and water is being extracted faster.
• Precipitation varies, however geology, topography and humans also play a role.
• Pollution is a big human risk factor and saltwater encroachment is reducing freshwater stores.

Consequences of Water Insecurity

• Can result with clean water increase depending on water extraction with high agricultural.
• Water is important in high manufacturing and crop productivity.
• Many Solutions depend on low consumptions such as many farmers sorting rainwater or high HEP with a reliance on engineering schemes.
  • Mega dams
  • Can provide water and reduce groundwater
  • High energy and expensive
  • Plants release a lot of CO2

Water Poverty Index (WPI)

• An index used to measure localised water stress and improve provisions.
• Concentrating on the 5 components of:-
  • Access of water
  • Uses
  • Enviornmental indicators
  • Handing capacity
• Total score is measured between 0 and 100, with finland having the best and haiti the lowest.
• Sustainable water management can include automation such as:
  • GM crops
  • Restoring damages lakes
  • Replanting dams

Co-Operation on Water Security

• Integrated Water Resource Management which emphasises the river basin as where strategic planning has water security such as:
  • Freedom
  • Enviromental/Food protections
  • Pricing
• Groundwater include things such as water treatment to monitoring technology with Helinski rules on social aspects and more.
• Natural factors such as economic for water sharing.
• UN economic management aims to protect water resources also.