CAIE AS Level Geography (9696) - Exhaustive Study Notes

Hydrology and Fluvial Geomorphology: Drainage Basin System Outputs

Evapotranspiration is a key output consisting of evaporation and transpiration. Evaporation occurs when liquid changes into water vapour from sources such as puddles and streams. The rate of evaporation increases in hot, dry, and windy conditions and is higher with a larger soil surface area. Transpiration involves water being drawn from the soil by plants and leaving the plants as water vapour through the stomata. Potential Evapotranspiration is defined as the amount of evaporation that would occur if an unlimited water source were available. River Discharge refers to water flowing into the sea or moving through channels like rivers and streams. This water enters the channel through direct channel precipitation or other flows. Discharge is calculated using the formula $Q = A \times V$, where $Q$ is Discharge, $A$ is the Cross Sectional Area, and $V$ is Velocity. It is measured in cubic metres per second, also known as Cumecs ($m^3/second$).

Hydrology and Fluvial Geomorphology: Stores and Soil Moisture

Interception occurs when precipitation is caught and stored by vegetation before reaching the ground. Surface Water storage happens when infiltration capacity is exceeded, causing water to build up on the surface. Temporary stores include puddles and turloughs, while permanent stores include lakes and wetlands. Ground Water is water that has percolated into the bedrock and serves as a store of freshwater; wells and boreholes can be dug below the water table to access this. Ground Water Recharge is the refilling of rock pores as water moves downwards and occurs when the rate of recharge is greater than the rate of abstraction. Channel Storage refers to all water stored in rivers, streams, and drainage channels. Soil Moisture is water held sub-surface in soil pores. Sandy soils have many large pores and are permeable with quick infiltration rates, whereas clays are hygroscopic, meaning clay minerals swell in contact with water, making the soil impermeable and unstable. Soil Moisture (SM) Deficit occurs when available water is being used up. SM Recharge happens when precipitation is greater than potential evaporation, causing dry pores to refill. SM Surplus occurs when soil is saturated and water cannot enter, leading to surface flow. SM Utilisation happens when evapotranspiration and other water uses exceed precipitation. Field Capacity is the amount of water held once excess has drained away, representing the saturation point. The Wilting Point is the range of soil moisture content at which permanent plant wilting occurs. The Water Balance is calculated as: $Precipitation - (run\,off + evapotranspiration + change\,in\,soil\,moisture)$.

Hydrology and Fluvial Geomorphology: Flows Above and Below Ground

Above ground flows include Throughfall, where leaves and twigs become saturated and water drips from them, or precipitation falls through gaps in vegetation cover. Stemflow occurs when intercepted precipitation runs down branches and the main trunk. Overland Flow happens when soil is saturated or precipitation exceeds the infiltration rate, resulting in surface runoff. Hortonian Flow is shallow, laminar, fast-moving water that causes severe soil erosion when precipitation exceeds both infiltration capacity and depression soil capacity. Channel Flow is the movement of water in channels like streams and rivers, defined by $Q = A \times V$. Below ground flows include Infiltration, where precipitation soaks into the soil. Infiltration Capacity is the maximum rate at which soil can absorb precipitation under given conditions and is inversely proportional to overland runoff. It depends on rainfall duration, antecedent soil moisture, porosity, slope angle, and vegetation. Percolation is the slow downward movement of water through soil into bedrock under gravity; this is fast in Carboniferous Limestone and depends on the permeability and porosity of the bedrock. Chalk and sandstones are porous, allowing water to percolate. Throughflow is water flowing through the soil in natural pipes or percolines above the bedrock. Groundwater move into the phreatic zone and discharges into the channel. The Phreatic zone is the part of an aquifer (permeable rocks/sediments) below the water table where all pores are permanently saturated. Baseflow occurs where groundwater seeps into the river's bed, contributing to discharge via very slow transfer from bedrock and deep throughflow, taking months or years.

Underground Water Systems and Springs

The Water table is the upper layer of the phreatic zone, rising and falling based on rainfall percolation and baseflow from lower rocks. The aeration zone is seasonally wetted and dried. Ground water Recharge occurs through infiltration from precipitation, seepage through river banks or beds, leakage and inflow from adjacent rocks or aquifers, and artificially from irrigation or reservoirs. Loss occurs via evapotranspiration (mainly in low areas), natural discharge like seepage and springs, leakage and outflow into aquicludes from aquifers, and artificial abstraction. Aquifers are permeable rocks like sandstone, limestone, and chalk that contain significant water quantities, moving slowly to maintain streamflow during wet or dry periods. Springs form where water flow reaches the surface, often where percolating water hits an impermeable layer or the saturated zone. The water budget equation is given as $S = P - Q - E$, representing Soil Storage, Precipitation, Channel Flow ($Q$), and Evapotranspiration.

Discharge Relationships and Hydrograph Components

A hydrograph plots river discharge against time to show a river's regime. The Annual Hydrograph (river regime) highlights seasonal characteristics influenced primarily by climate. A Storm Hydrograph shows discharge variations over a short period, with both discharge (in Cumecs) and rainfall on the y-axis. Components include the Approach Segment (discharge prior to storm), Rising Limb (quick rise in discharge), Bank full Discharge (channel full; further increase causes flooding), Peak Discharge (maximum discharge), Lag Time (time between max rainfall and max discharge), Receding Limb (decline after peak, usually less steep than the rising limb), Stormflow (stream discharge after a rainstorm), and Quickflow (surface runoff reaching the channel quickly). The Storm Process follows four steps: 1. Rain falls on the drainage basin in large amounts. 2. Overland flow occurs when precipitation exceeds infiltration, building the rising limb to the peak. 3. After a few hours, overland flow reduces, and throughflow contributes to discharge, preventing floodwaters from receding as quickly as they rose. 4. Baseflow takes over, returning the river to its pre-flood state.

Drainage Basin Characteristics and Response

Size and Shape affect response: small basins and circular basins respond quicker, reducing lag time. Drainage Density also influences response; low density causes a long lag time. A dendritic (tree-like) pattern has higher density, increased discharge response, and reduced lag time. Soil Porosity and Permeability are critical; impermeable surfaces lead to greater peak flows and more overland flow. Chalks and gravels allow infiltration, while clay soils do not. Impermeable Rock Type produces a flashier response with high peak discharge and low lag times, whereas limestone hardly produces a storm hydrograph. Steeper Slopes lead to more overland flow, shorter lag times, and higher peak flows. Vegetation Type matters as dense forest intercepts more, reducing flood response, though this varies in winter. Land Use changes like urbanisation (impermeable surfaces) or deforestation increase overland flow. Artificial drains increase drainage density, carrying water to rivers faster, increasing peak flow and reducing lag time.

River Channel Processes and Erosion

River channel processes follow the Bradshaw Model. Erosion types include Abrasion (mechanical impact of the river's load rubbing on sides, increasing with velocity and energy), Corrasion (erosive action of particles), Solution (dissolving calcium-heavy rocks by removing ions, fastest in undersaturated streams over soluble rocks), Hydraulic Action (force of air and water compressing into bank cracks and cause pieces to break away), Cavitation (the implosion of air bubbles in fluids producing tiny water jets that cut rock), and Attrition (collision of sediments wearing each other down into smaller, rounder particles). The rate of erosion is influenced by the amount and weight of the load, velocity, gradient, geology hardness, pH level, and human impacts.

Load Transport and the Hj00lstrom Curve

Load transport mechanisms include Traction (large particles rolled along the bed), Saltation (gravel and stones hopping via eddies), Suspension (silts and clays held up, creating a cloudy appearance), and Solution (dissolved calcareous rocks). Load varies with discharge and velocity and is calculated at bank full. Capacity is the total amount of load a river can carry, while Competence is the diameter of the largest particle transported. The Hj00lstrom Curve describes these relationships. Critical erosion velocity (entrainment) is the lowest velocity needed to pick up a particle. Mean settling velocity is the velocity at which particles are dropped. Sands are the easiest to pick up; clays have high entrainment values due to cohesion, and gravels due to weight. Clays can remain in suspension even if velocity is $0$. Velocity for transport is always less than the velocity required for entrainment.

River Flow and Velocity

Velocity is affected more by friction than gradient. Friction is measured via Bed roughness ($N$) using the formula $N = \frac{V}{R^{\frac{2}{3}} \times S^{\frac{1}{2}}}$, where $R$ is the hydraulic radius, $S$ is channel gradient, and $V$ is velocity. A higher $N$ value indicates a rougher bed. The Hydraulic Radius is calculated as $\frac{channel\,depth \times width}{(2 \times height) + width}$. The Thalweg is the line of fastest velocity. Flow types include Laminar (sheets parallel to bed, rare, common on smooth surfaces), Turbulent (friction slows water near banks/bed, creating eddies), and Helicoidal (horizontal turbulence creating a corkscrew motion where the thalweg moves laterally and vertically).

Channel Types and Fluvial Landforms

Straight channels have a sinuosity < 1.5, though a perfectly straight channel ($1$) is rare and usually artificial due to helicoidal flow. Braided channels are divided by bars (unstable/unvegetated) or islands (vegetated), occurring with steep gradients, coarse material, and variable discharge. Meandering channels result from lateral erosion caused by helicoidal flow and load, not obstacles. Landforms include Meanders (pronounced bends), River Cliffs (steep outside bends), Point Bars (deposits on inside bends), and Oxbow Lakes (formed when a meander neck is breached during a flood). Pools are deep sections with laminar flow and erosion ($negative\,gradient$), while Riffles are shallow sections with turbulent flow and deposition ($steep\,positive\,relief\,gradient$). Waterfalls occur at gradient changes where resistant rock overlies less resistant rock; they retreat to form steep-sided Gorges. Potholes are created by turbulence swirling pebbles in depressions. Rapids are upper course features with steep, rocky beds. Bluffs are steep slopes at the edge of old floodplains. Floodplains are flat lands of alluvium that rise during floods; fine silt is deposited, sometimes creating Backswamps. Levees are coarse deposits on banks following a flood. Deltas form at standing bodies of water due to energy loss and clay flocculation; they consist of Bottomset beds (fine), Foreset beds (coarse), and Topset beds (fine). Delta types include Arcuate (fan-shaped, like where longshore drift exists), Cuspate (pointed, from opposing currents), and Bird's Foot (still sea allowed distributaries to build in any direction).

Human Impact on Catchment Flow

Deforestation reduces evapotranspiration and increases surface runoff and peak discharge while reducing lag time. Afforestation has the opposite effect once established. Urbanization creates impermeable surfaces, using drains to move water quickly, increasing flood peaks and reducing lag times. Building on floodplains reduces available space, causing water to rise higher. Grazing/farming impacts include ploughing (increasing infiltration) or heavy machinery causing soil compaction (reducing infiltration). Waterlogging and salination can occur with poor drainage. Abstraction can dry up rivers and cause saltwater intrusion. Channelisation increases the hydraulic radius, shortening lag times. Reductions in industrial activity can cause old springs to re-emerge, leading to flooding in basements and tunnels. Water storage via dams helps with flood/drought control but can lead to water loss and salinization.

Causes and Impacts of River Floods

Physical causes of flooding include heavy rainfall, melting snow/ice, impermeable bedrock, coastal storm surges, lack of vegetation, or natural disasters like dam failure. Human causes include urbanisation, floodplain development, engineering obstructions, and mechanised farming. Impacts include deaths, damage, and disruption; death tolls are often higher in LICs, while costs are higher in HICs. Recurring Interval estimates how often a flood of a specific size occurs on average (e.g., a 100-year flood). Flood Risk Maps categorize risk as severe ($1-75\,years$) or moderate ($76-200\,years$). High-risk areas include active floodplains, small basins prone to flash floods, and areas below unsafe dams.

Flood Prevention and Management

Strategies include hard engineering (Dams, levees, straightening channels, reservoirs, spillways) which often shifts problems downstream. Soft engineering works with nature (Afforestation, contour ploughing, flood abatement, flood diversion). Forecasting and Warning utilizes satellites, rain/discharge gauges, and computer models. Loss Sharing involves aid and insurance. Hazard Resistant Design includes sandbags, sealing windows/doors, and moving belongings to higher floors, as seen in Yarm on the River Tees. Land Use Zoning prevents building on prone areas, prioritizing appropriate use: wetlands and grazing nearest the channel, then leisure areas, then houses, and finally critical infrastructure like hospitals furthest away.

Diurnal Energy Budgets: Radiation and Heat

Incoming Solar Radiation (insolation) is shortwave UV energy. Approximately 5% is scattered by the atmosphere, 24% is reflected into space, 23% is absorbed by gases, and 48% is absorbed by the Earth's surface. Reflected Solar Radiation depends on Albedo; lighter materials have higher values. Planetary albedo is the proportion returned to space by the Earth. Absorption at the surface transfers heat to lower layers via conduction, especially when moisture is present. The Earth emits Longwave radiation; the greenhouse effect occurs as water vapour and $CO_2$ absorb and return this heat. Daytime budget is calculated as: $insolation - (reflected\,insolation + surface\,absorption + sensible\,heat\,transfers + latent\,heat\,transfers + longwave\,radiation)$. The night-time budget is: $stored\,energy - (latent\,heat\,transfers + sensible\,heat\,transfers + longwave\,radiation)$.

Cloud Effects and Heat Transfers

During the day, clouds have a net cooling effect due to albedo. Low, thick stratus clouds reflect 80% of insolation. Cirrus clouds allow insolation to pass but trap longwave radiation. At night, thick clouds act as insulation, keeping temperatures warm. Sensible Heat Transfer includes Convection (heated air rises, cools, and falls) and Conduction (transfer between ground and air). Latent Heat Transfer occurs during state changes: evaporation absorbs heat from air to cool the surface, while condensation releases heat. Dew forms when water vapour condenses on objects cooler than the dew point, releasing latent heat. At night, the ground cools as heat rises from soil and rocks.

Global Energy Budgets and Atmospheric Transfers

Latitudinal radiation shows an excess (positive budget) in the tropics and a deficit (negative budget) at higher latitudes. Horizontal transfer compensates for these differences. Temperature patterns show little variation at the equator but great variation in high latitudes. A lag time exists between overhead sun and max insolation because the atmosphere is heated from below; oceans have a greater lag time due to high specific heat capacity. Atmospheric transfers involve air moving from high to low pressure. Surface pressure is low at the equator (warm air rising) and high at the poles (cool air descending). The ocean conveyor belt moves cold, salty water from poles to the equator, where it warms and rises.

Seasonal Variations and Global Circulation

Temperature is affected by latitude (concentration of insolation) and land/sea distribution. Land has lower reflectivity, heat confined to the surface, and low specific heat capacity. Sea has higher reflectivity, rays penetrate deep, and high specific heat capacity. Ocean currents rotate clockwise in the N. Hemisphere and anticlockwise in the S. Hemisphere. Altitude influences temperature as thinner air is less able to absorb longwave radiation. Pressure gradients drive wind, which is deflected by the Coriolis force (right in N. Hemisphere, left in S. Hemisphere). Geostrophic wind results from the balance of Coriolis and pressure gradients. The 3 Cell Model includes the Hadley Cell (rising air at ITCZ, trade winds), Ferrel Cell (forced rising at polar front), and Polar Cell (sinking cold air). Rossby waves and Jet streams (polar and subtropical) mix air and flow eastward at high altitudes.

Atmospheric Moisture and Precipitation

Humidity is measured as Absolute (volume of vapour) or Relative (formula: $\frac{actual\,moisture}{saturation\,moisture} \times 100$). Moisture processes include evaporation, condensation (requires hygroscopic nuclei), freezing, melting, deposition, and sublimation. Precipitation theories include Collision Theory (droplets collide to form rain/Coalescence), Aggregation (crystals form snow), and Accretion (crystals collect droplets for hail). The Bergeron-Findeisen Theory explains how ice crystals grow at the expense of water droplets between $-12^\circ C$ and $-30^\circ C$. Rainfall types include Convectional (land heating), Frontal (warm air forced over cold air), and Orographic (air forced over barriers, creating rain slopes and rain shadows). Temperature inversions (Radiation, Frontal, Subsidence) trap air layers. Forms of precipitation include Rain ($0.5\,mm$ to $5\,mm$), Drizzle (< 0.5\,mm), Hail, Snow, Dew, and various Fogs (Radiation, Steam, Advection).

Human Impact on Climate and Global Warming

The Enhanced Greenhouse Effect is driven by increasing $CO_2$ (from $315\,ppm$ in 1950 to $400+ppm$ in 2020), Methane ($0.5-2\%$ annual increase), CFCs ($6\%$ increase, $10,000\times$ more efficient than $CO_2$ at trapping heat), and Nitrous oxides ($8\%$ increase). Evidence includes rising sea levels ($3.1\,mm$ per year), warming oceans ($0.11^\circ C$ per decade), and shrinking Arctic ice ($65\%$ since 1975). Impacts include storm activity, threats to agriculture, heatwaves killing coral, and disease spread. El Ni01o and La Ni01a events alter global cycles. Urban Climates are characterized by higher temperatures due to low albedo (tarmac $10\%$, concrete 20\%\), lower humidity, and slower winds. Mitigation includes the London Plane tree and urban forests to reduce temperatures by $3-5^\circ C$.

Rocks, Weathering, and Plate Tectonics

Plate Tectonics evidence includes coastline fits (Africa and South America), orogenic belts, fossil remains, and glacial deposits. Continental crust is $35-70\,km$ thick, granite, and less dense ($2.6\,kgm^{-3}$). Oceanic crust is $6-10\,km$ thick, basalt, and denser ($2.9\,kgm^{-3}$). Movement is driven by Convection Theory, Hotspots, Ridge push, and Slab pull. Boundary types include Constructive (Mid-Atlantic ridge $0.7-1.4\,cm/yr$; East-African rift $2\,cm/yr$), Destructive (Oceanic/Continental like Juan de Fuca; Oceanic/Oceanic like Japanese Arcs), Collision (Himalayas $5-6\,cm/yr$), and Conservative (San Andreas Fault $3-5\,cm/yr$). Features include sea floor spreading (palaeomagnetism), subduction in the Benioff zone (dipping $30^\circ$ to $70^\circ$), ocean ridges, and volcanic island arcs.

Weathering and Slope Processes

Physical Weathering includes Freeze-thaw (pressure of $14\,kg/cm^2$), Exfoliation (diurnal range stresses), Salt crystal growth (effective at $27^\circ C$, expansion up to $300\%$), and Dilation (unloading). Chemical processes include Hydrolysis (forming kaolin), Hydration (expansion up to $1600\%$, e.g., Anhydrite to Gypsum), and Carbonation ($CaCO_3 + H_2CO_3 \rightarrow Ca(HCO_3)_2$). Factors include climate (Van't Hoff's Law states rates increase $2-3\times$ for every $10^\circ C$ rise), rock type, and relief. Mass movements on slopes include Heaves (soil creep $1-3\,mm/yr$ in UK, $10\,mm/yr$ in rainforest), Slumps (rotational), Flows (mudflows), Falls, and Slides (rockslides). Water movement includes Sheetwash (Hortonian), Surface wash (resulting in Rills), and Rain splash (most effective on $33^\circ-45^\circ$ slopes). Human instability is caused by excavations, loading, and vegetation removal. Management includes Pinning, Netting, Grading, Gabions, Drainage, Grouting, and Shotcrete.