Edexcel Geography A-level: The Water Cycle and Water Insecurity Comprehensive Study Guide

The Global Hydrological Cycle and Global Water Budget

• Global Water Distribution: The global water cycle consists of numerous stores. The largest store is the oceans, which contain $96.9\%$ (approximately $97\%$) of global water.
• Freshwater Distribution: Freshwater accounts for only $2.5\%$ of all stores. Within freshwater stores, the distribution is as follows:     * Glaciers, Ice Caps, and Ice Sheets: $68.7\%$ (approximately $69\%$).     * Groundwater: $30.1\%$ (approximately $30\%$).     * Surface and Other Freshwater: Only accounts for around $1.2\%$ (approximately $1\%$) of global stores. This category includes permafrost, lakes, swamps, marshes, rivers, and living organisms.
• Stores and Residence Times:     * Oceans: Represents $96.9\%$ of total water and $0\%$ of total freshwater; residence time is $3600\,\text{years}$.     * Icecaps: Represents $1.9\%$ of total water and $68.7\%$ of total freshwater; residence time is $15,000\,\text{years}$.     * Groundwater: Represents $1.1\%$ of total water and $30.1\%$ of total freshwater; residence time is $10,000\,\text{years}$.     * Rivers and Lakes: Represents $0.01\%$ of total water and $1.2\%$ of total freshwater; residence time varies from $2\,\text{weeks}$ to $10\,\text{years}$.     * Soil Moisture: Represents $0.01\%$ of total water and $0.05\%$ of total freshwater; residence time is $2-50\,\text{weeks}$.     * Atmospheric Moisture: Represents $0.001\%$ of total water and $0.04\%$ of total freshwater; residence time is $10\,\text{days}$.

Hydrology in Distinct Climatic Regions

• Polar Regions:     * Reflectivity: $85\%$ of solar radiation is reflected (albedo effect).     * Surface Conditions: Permafrost creates impermeable surfaces.     * Phase Changes: Lakes and rivers freeze periodically.     * Runoff Patterns: Rapid runoff occurs during the spring season.     * Atmospheric Exchange: Seasonal release of biogenic gases into the atmosphere occurs.     * Precipitation Types: Primarily orographic and frontal precipitation.
• Tropical Rainforests:     * Vegetation Interaction: Dense vegetation consumes $75\%$ of precipitation.     * Soil Processes: Limited infiltration occurs.     * Deforestation Effects: Leading to reduced evapotranspiration and decreased precipitation.     * Atmospheric Conditions: Characterized by very high temperatures and high humidity.     * Precipitation Types: Dominately convectional rainfall.

Atmospheric Circulation and the ITCZ

• Global Climate Control: The Earth consists of six cells of circulating air. In the Northern Hemisphere (mirrored in reverse for the Southern Hemisphere):
• Hadley Cell: Air rises at the Doldrums, travels upwards, and then sinks upon meeting the cooler air of the Ferrel Cell. Precipitation tends to occur at this meeting point. The air then travels southwards, heating up until it rises again at the Doldrums, restarting the cycle.
• Polar Cell: Cold air sinks near the Arctic Circle, cooling and condensing to form precipitation over northern latitudes. The air travels southwards, heating until it meets warm air from the Ferrel Cell. The air then rises, creating dry conditions for the land beneath, and travels northwards while cooling.
• Ferrel Cell: Positioned as the middle cell (mid-latitude). Its air circulation is determined by the Hadley and Polar cells on either side, functioning like a "cog system."

The Drainage Basin System

• Definition: A drainage basin is an open subsystem operating within the closed global hydrological cycle. It is the area of land drained by a river and its tributaries, bounded by a watershed (usually hills and mountains).
• System Characterization: On a local scale, the water cycle is an open system involving inputs, outputs, and throughputs. On a global scale, it is a closed system with no external inputs or outputs, only internal throughputs.
• Precipitation (Inputs): Caused by the cooling and condensation of atmospheric water moisture into clouds, released as rain, snow, hail, or sleet.
• Factors Affecting Precipitation:     * Seasonality: Strong seasonal patterns in climates like Monsoon and Mediterranean determine input timing.     * Variability:         * Secular Variability: Long-term changes, such as those resulting from climate change trends.         * Periodic Variability: Regular annual, seasonal, or monthly contexts.         * Stochastic Variability: Random factors, such as the localization of a specific thunderstorm.     * Latitude: Generally, higher latitudes (further from the Equator) have colder climates and more frequent snowfall. Convergence of air cells at specific latitudes also dictates air rise and fall.

Types of Rainfall

• Convectional Rainfall: Often a daily occurrence. Morning heat warms the ground, which heats low-level moisture from surface stores or plant dew. This moisture evaporates, rises, cools, and condenses. In tropical rainforests, this typically occurs mid-morning before temperatures become too high for condensation.
• Frontal/Cyclonic Rainfall: Occurs where two air masses meet, creating a wedge of hot air within cold air known as a depression. Warm moist air is forced over the cold air mass, cooling and condensing. The UK experiences approximately $100\,\text{depressions}$ annually.
• Relief/Orographic Rainfall: Warm moist air from the sea meets high relief land (hills/mountains). The air mass is forced to rise, cooling and condensing into clouds and rain. This depends on the relief and the immediate location of the land relative to the sea; coastal landscapes that are too low-lying or have hills set too far inland may not experience this.

Fluxes and Flows within the Drainage Basin

• Interception: Direct intervention of plant leaves changing the direction or temporarily stopping precipitation. The interception store is greatest at the start of a storm. Interception capacity depends on vegetation type.
• Infiltration: Movement of water from the surface into the soil.     * Infiltration Capacity: The maximum rate of absorption.     * Factors: Sandy soils (higher rate) vs. clay soils. Soil saturation (previous precipitation) reduces the rate. Compaction and steep relief reduce infiltration.
• Surface Runoff (Overland Flow): Water flows over the land when it cannot permeate. Faster on steeper gradients; the primary transfer of water to river channels influencing discharge.
• Throughflow: Water moves through soil into streams.     * Clay Soils: High field capacity and small pore spaces result in slow flow.     * Sandy Soils: Drain quickly due to lower field capacity, larger pore spaces, and animal channels (e.g., worms).
• Percolation: Water moves from soil/ground into porous rock or fractures (bedrock and aquifers). Rate depends on rock permeability and fracture presence.
• Groundwater Flow: Gradual transfer through porous rock under gravity. Can create long-term water stores called aquifers.

Outputs and Stores of the Drainage Basin

• Evaporation: Direct loss from water bodies, soil, and interception storage to the atmosphere. Increased by heat, wind, and dryness. Surface area (larger is faster) and surface color (black tarmac vs. white snow) affect rates.
• Transpiration: Biological process where plants lose water through stomata. Affected by seasonality, vegetation type, and time of day (e.g., morning dew in temperate climates).
• Stores (Timescales):     * Soil Water: Mid-term store utilized by plants.     * Groundwater: Long-term store in rock pore spaces.     * River Channel: Short-term storage.     * Interception: Short-term storage on branches/leaves.     * Surface Storage: Variable-term storage in puddles, ponds, and lakes.
• The Water Table: The upper level where pore spaces and fractures become saturated. Used to assess drought, wetland health, and forest restoration.

Physical and Anthropogenic Factors

• Physical Factors: Climate (rainfall/vegetation), Soil Composition (infiltration), Geology (percolation), Relief (runoff speed), Vegetation (interception), and Basin Size.
• Anthropogenic (Human) Factors:     * Cloud Seeding: Dispersing substances to provide condensation nuclei. Example: Used in China before the $2008\,\text{Beijing Games}$ to reduce pollution.     * Deforestation: Reduces interception/infiltration; increases overland flow and flooding speed.     * Afforestation: Increases interception and evapotranspiration; reduces overland flow.     * Dam Construction: Reduces downstream flow. Example: Lake Nasser behind the Aswan Dam in Egypt causes a loss of $10-16 \times 10^9\,m^3$ of water from the Nile.     * Land Use Change: Infiltration is $5\,\text{times}$ faster under forests than grasslands. Farmland increases soil compaction.     * Groundwater Abstraction: Extraction exceeding recharge. Example: In China, groundwater irrigates $40\%$ of farmland and provides $70\%$ of drinking water.     * Irrigation: Causes drop in water tables. Example: The Aral Sea in Kazakhstan shrank in the $1960\text{s}$ due to cotton farming.     * Urbanisation: Impermeable surfaces increase runoff and river discharge, speeding up the cycle.

Water Balance and River Regimes

• The Water Budget Equation: $\text{Precipitation} = \text{Discharge} + \text{Evaporation} \pm \text{Changes in Stores}$
• Soil Moisture States:     * Surplus: Precipitation > Evaporation; soil moisture increases.     * Utilisation: Temperature/Evaporation increases; plants use stored moisture.     * Maximum Evaporation: Point of highest drought risk.     * Deficit: Precipitation is insufficient for vegetation water loss.     * Recharge: Precipitation exceeds evaporation; soil regains moisture.
• River Regimes: Annual variation in discharge at a location. Influenced by groundwater (base flow), glacial meltwater, snowmelt, and monsoons. Complex regimes occur in large rivers like The Ganges or Mississippi.

Storm Hydrographs

• Definition: Variation in discharge within a short period (days). Base flow is supplied by groundwater before a storm.
• Features:     * Rising Limb: Increase in discharge.     * Peak Flow: Maximum discharge.     * Lag Time: Delay between peak rainfall and peak discharge.     * Falling Limb: Decrease in discharge as precipitation ends.
• Flashy vs. Subdued Hydrographs:     * Flashy: Short lag, high peak, steep rising limb. Caused by intense storms, impermeable rock (granite), steep relief, small basins, low vegetation, saturated soil, or urbanisation.     * Subdued: Long lag, low peak, gentle rising limb. Caused by steady rain, permeable rock (limestone), gentle relief, large basins, high vegetation (afforestation), unsaturated soil, or dams.

Management and Deficits (Droughts)

• Sustainable Management: Vegetation on roofs, permeable pavements, rainwater harvesting, and creating wetlands (natural sponges).
• Negative Impact Management: Avoiding slash-and-burn (prevents erosion), managing bridges (to prevent channel restriction), and regulating drainage/sewage systems to prevent flash flooding.
• Drought Types:     * Rainfall Deficit: Low precipitation, high temps; leads to loss of soil moisture.     * Stream Flow Deficit: Reduced storage in lakes/reservoirs; threats to habitats and urban supply.     * Soil Moisture Deficit: Low soil moisture and groundwater recharge; leads to failing irrigation systems.     * Food Deficit: Widespread agricultural failure; leads to humanitarian crises and rural-to-urban migration.

ENSO, Wetlands, and Desertification

• El Ni\u00f1o Southern Oscillation (ENSO): Occurs every $3\,\text{to}\,7\,\text{years}$, lasting $18\,\text{months}$.     * Normal: Cool water in Peru, warm in Australia.     * El Ni\u00f1o: Warm water in Peru (leads to reduced anchovy harvest), cold in Australia (causes dry conditions).
• Wetland Value:     * Supporting: Carbon storage and nutrient recycling.     * Provisioning: Fuelwood and fisheries.     * Regulating: Flood risk reduction and water purification (filters pollutants).     * Cultural: Recreation and aesthetic value.
• Desertification Causes:     * Physical: Reduced precipitation and global warming (increased evaporation).     * Human: Population growth leading to over-cultivation, overgrazing, and deforestation.

Water Insecurity and Future Impacts

• Insecurity Causes: Uneven spatial distribution ($66\%$ of people have access to only $25\%$ of rainfall), high demand from population/middle-class growth, and pollution.
• Climate Change Impacts:     * Rising temperatures may shorten the interval between ENSO cycles to every $2-3\,\text{years}$.     * Increased evaporation may lead to aquifer depletion and lower water tables.     * Potential for more intense cyclones and convectional rainfall.
• Solutions to Insecurity:     * Mega Dams: Provide volume/HEP but are expensive and flood land.     * Desalination: Clean water source but energy-intensive and produces salt waste.     * Water Transfer Schemes: Provide for arid areas but can dry up source areas.
• Water Poverty Index (WPI): Measured out of $100$. Components: Resources, Access, Capacity, Use, and Environment. Finland score: $78$; Haiti score: $35$.
• International Cooperation:     * IWRM: Integrated Water Resource Management (unit is the river basin).     * Helsinki Rules: Principles for equitable use of international rivers.     * Key Players: UN (UNECE Water Convention), EU (Water Framework Directive - Berlin 2000), National Governments (UK Environment Agency).