Paper 1 Core Geography: Hydrology, Atmosphere, and Rocks

THE DRAINAGE BASIN SYSTEM

• Definition of a Drainage Basin: A drainage basin is a natural system consisting of inputs, flows, and stores of water and sediment. It refers to the area drained by a river and its tributaries.

• Unique Characteristics: Every drainage basin is distinct based on its climate, geology, vegetation, soil types, size, shape, and the extent of human activities within its area.

• System Classification: The drainage basin is an open system because energy and matter move across its boundaries.

• Hydrology: The study of water as it moves on, under, and through the Earth’s surface.

• The Hydrological Cycle: Also known as the water cycle, it describes the movement of water between the atmosphere, lithosphere, and biosphere (air, land, and sea). Water is stored at various stages including vegetation, surfaces, soil moisture, groundwater, and water channels.

• System Components and Formulaic Symbols (Figure 1.1):     * Precipitation: $p$     * Evapotranspiration: $e$     * Interception storage: $D$     * Surface storage: $R$     * Infiltration: $f$     * Soil moisture storage: $M$     * Aeration zone storage: $L$     * Groundwater recharge: $d$     * Groundwater storage: $G$     * Seepage: $s$     * Baseflow: $g$     * Channel storage: $S$     * Channel runoff: $q$     * Overland flow: $q_o$     * Throughflow: $m$     * Interflow: $i$

• Inputs:     * Precipitation: The main input, involving moisture transfer from atmosphere to land (rain, snow, frost, hail, dew). Key characteristics affecting hydrology include total amount, intensity ($mm/hour$), type, geographical distribution, and temporal variability (seasonality).

• Stores and Processes:     * Interception: Precipitation collected and stored by vegetation. Components include:         * Interception loss: Water retained by plant surfaces that later evaporates or is absorbed.         * Throughfall: Water falling through vegetation gaps or dropping from leaves/stems.         * Stemflow: Water trickling down branches and the main trunk.     * Evaporation: Transformation of liquid water into water vapour gas.     * Transpiration: Water loss from vegetation to the atmosphere.     * Evapotranspiration (EVT): Total loss from both processes. Affected by temperature (most important), humidity, windspeed, water availability, vegetation cover, and albedo (reflectivity). In arid areas, EVT can account for near $100\%$ of annual precipitation; in humid areas, it is approximately $75\%$.     * Potential Evapotranspiration (PEVT): The water loss that would occur with an unlimited soil water supply. Example: Egypt’s actual EVT is < 250\,mm due to low rainfall, but its PEVT is $2000\,mm$ due to high heat.     * Infiltration: Water soaking into or being absorbed by soil. Infiltration Capacity is the maximum rate of absorption. It is inversely related to runoff and influenced by rainfall duration, antecedent soil moisture, soil porosity, vegetation, raindrop size, and slope angle.     * Ground Cover Influence on Infiltration (Table 1.1):         * Old permanent pasture: $57\,mm/hour$         * Permanent pasture (moderately grazed): $19\,mm/hour$         * Permanent pasture (heavily grazed): $13\,mm/hour$         * Strip-cropped: $10\,mm/hour$         * Weeds or grain: $9\,mm/hour$         * Clean tilled: $7\,mm/hour$         * Bare, crusted ground: $6\,mm/hour$     * Soil Moisture: Subsurface water. Field Capacity is the water remaining after excess drainage (saturation). Wilting Points represent the moisture range where plants permanently wilt, limiting growth.     * Flows:         * Throughflow: Water moving through soil via natural pipes and percolines.         * Groundwater: Subsurface water accounting for $96.5\%$ of Earth's freshwater. The Phreatic Zone is the permanently saturated zone in rocks; its upper layer is the Water Table.         * Baseflow: Constant river discharge from groundwater seepage.         * Recharge: Refilling water pores after drying or human extraction. Groundwater can be non-renewable if recharge does not occur.         * Aquifers: Rocks containing significant water. Springs occur where the water table meets the surface.

RAINFALL-DISCHARGE RELATIONSHIPS

• Hydrographs: Graphs showing water level changes in a river over time.

• Annual Hydrographs (River Regimes): Show flow variation over a year. Influenced by climate (primary factor), rocks (porosity/permeability), basin morphology (shape, area, slope), vegetation, and soil cover.     * Guadalquivir (Spain): Mediterranean regime with peak flow of $20\,litres/second/km^2$ in March and summer drought (nearly $0$ flow in August) due to high-pressure systems.     * Shannon (Ireland) and Gloma (Norway): Comparative regimes included in Figures 1.3 and 1.4.

• Storm (Flood) Hydrographs: Variation over $1$ to $7$ days for specific events.     * Quickflow: Water reaching the river rapidly via overland flow.     * Rising Limb: Speed of floodwater rise.     * Recessional Limb: Speed of water level decline after peak.     * Peak Flow: Maximum storm discharge.     * Lag Time: Time between peak storm rainfall and peak river discharge.

• Urbanization Effects: Increases peak flow and decreases lag time due to impermeable surfaces and high drainage density (sewers/drains).

• Factors Affecting Storm Hydrographs (Table 1.2):     * Precipitation: High intensity causes overland flow; snow causes delayed flooding upon melting.     * Temperature: Higher temperature increases EVT (less water) but warm air holds more water (higher peak potential).     * Antecedent Moisture: Saturated ground leads to rapid overland flow and shorter lag times.     * Basin Size/Shape: Small basins (e.g., Boscastle < 15\,km^2) respond quickly. Circular basins have faster responses than linear ones (e.g., Mississippi over $3700\,km$).     * Drainage Density: High density (urban sewers) leads to faster response.     * Rock/Soil Porosity: Chalk and gravel (permeable) increase lag; clay (impermeable) increases peak flow.     * Slopes: Steeper slopes increase overland flow and peaks.     * Vegetation: Broad-leafed trees increase interception (especially in summer), reducing peaks and increasing lag.

RIVER CHANNEL PROCESSES AND LANDFORMS

• Transport Mechanisms:     * Suspension: Silts and clays carried as suspended load.     * Saltation: Sands, gravels, and small stones transported in 'hops'.     * Traction: Pebbles shunted along the bed (bed load).     * Solution: Dissolved load (calcareous materials).

• Load Metrics:     * Stream Capacity: Largest amount of debris a stream can carry.     * Stream Competence: Diameter of the largest particle a stream can carry.

• Deposition (Hjulstrom Curves): Work depends on velocity and particle size (Figure 1.7).     * Small and large particles require high velocities to move. Particles $0.1\,mm$ to $1.0\,mm$ need $100\,mm/s$; clay (< 0.01\,mm) needs > 500\,mm/s due to cohesion; gravel (> 2\,mm) needs high velocity due to weight.     * Entrainment requires higher velocity than transport.     * Particles drop at their Settling Velocity when velocity falls.

• Eroding Processes:     * Abrasion (Corrasion): Wearing of bed/banks by load.     * Attrition: Wear between particles within the load (rounding them).     * Hydraulic Action: Force of air/water in cracks.     * Corrosion (Solution): Chemical removal of ions (e.g., calcium).

• Velocity and Discharge:     * Discharge ($Q$): $Q = Cross-sectional\,Area \times Velocity$. Measured in cusecs ($cubic\,feet\,per\,second$) or cumecs ($m^3/s$).     * Hydraulic Radius: Measure of efficiency. $HR = \frac{Cross-sectional\,Area}{Wetted\,Perimeter}$.

• Patterns of Flow:     * Laminar: Smooth, straight channel; water flows in parallel layers (laminae).     * Turbulent: Higher velocity and bed roughness; creates hollows via vertical turbulence.     * Helicoidal: 'Corkscrewing' horizontal turbulence; forms meanders by creating alternating pools and riffles.

• Channel Landforms:     * Sinuosity: Ratio of channel length to valley length. Straight (< 1.5); Meandering (> 1.5; high values > 4.4).     * Thalweg: Line of maximum velocity.     * Meanders: Asymmetric cross-sections. Erosion on outer bank (river cliff/bluff); deposition on inner bank (point bar/slip-off slope). Oxbow lakes form from meander isolation.     * Braiding: Channels divided by islands (vegetated/stable) or bars (unvegetated/temporary). Caused by coarse material and variable discharge.     * Pools/Riffles: Pools are deep (erosion); riffles are sediment ridges in straight sections (coarse gravel).     * Waterfalls: Formed by sudden gradient changes and resistant rock bands; retreat creates Gorges.     * Rapids: Mini-waterfalls over hard rock bands (e.g., Nile Cataracts).     * Levées: Elevated embankments from repeated flooding and coarse sediment deposition at channel edges.     * Alluvial Fans: Semi-arid mountain streams enter plains; velocity drops. Fans (slope < 1^{\circ}) or steep cones (slope up to $15^{\circ}$).     * Deltas: Formed by heavy sediment load entering standing water; saline water causes clay flocculation.

HUMAN IMPACT ON HYDROLOGY AND CLIMATE

• Urbanization (Table 1.3): Removal of trees (decreased EVT); construction (decreased infiltration/lower water table); storm drains (increased peak discharge downstream).

• Precipitation Modification: Cloud Seeding using silver iodide, solid $CO_2$ (dry ice), or ammonium nitrate to attract droplets.

• Evaporation/Transpiration Modification:     * Dams: Increase evaporation (Lake Nasser loses up to $1/3$ of its water).     * Deforestation: Reduces EVT, increases runoff, declines lag time.

• Abstraction: Texas High Plains water level declined $30-50\,m$ in $50$ years; aquifer reduced by > 50\%.

• Recharge Management: Providing water-spreading through coastal dunes or glacial deposits to "top up" aquifers.

• Aswan Dam Case Study:     * Salinisation affects $1/3$ of irrigated area.     * Population displacement ($100,000$ Nubians).     * Seismic stress (earthquake of November $1981$).     * Delta erosion at $2.5\,cm$ per year.     * $95\%$ decline in sardine yields.     * Spread of schistosomiasis (bilharzia).

• Drought Definitions:     * Absolute Drought: $15$ consecutive days with < 0.2\,mm rainfall.     * Partial Drought: $29$ consecutive days averaging < 0.2\,mm daily.

• Floods: Recurrence intervals define the regularity (e.g., $100$-year flood).

ATMOSPHERE AND WEATHER

• Daytime Energy Budget:     $Energy_{surface} = Incoming\,Solar\,Radiation - (Reflected\,Solar + Surface\,Absorption + Sensible\,Heat\,Transfer + Long-wave\,Radiation + Latent\,Heat\,Transfer)$     * Albedo (Table 2.1): Fresh snow ($75-90\%$); Grass ($20-30\%$); Black road surface ($5-10\%$).     * Sensible Heat Transfer: Convection air movement.     * Latent Heat Transfer: Energy used in evaporation or released in condensation.

• Night-time Energy Budget: Components include long-wave radiation loss, latent heat (condensation), sub-surface supply (heat from soil/bedrock), and sensible heat.

• Mist and Fog: Cloud at ground level.     * Mist: Visibility $1000-5000\,m$; relative humidity > 93\%.     * Fog: Visibility < 1000\,m (Dense fog < 200\,m).     * Advection Fog: Warm moist air passes over a cold surface (e.g., Grand Banks near Newfoundland, dense fog $70-100$ days/year).     * Radiation Fog: Ground loses heat at night via long-wave radiation under high pressure/clear skies.

• Temperature Inversions: Increase in temperature with height. Acts as a lid on pollutants and forms frost hollows.

• Atmospheric Energy Variations:     * Insolation imbalance: Positive in tropics, negative at poles.     * Specific Heat Capacity: Land heats $5\times$ faster than water. Water distributes heat deeper via transparency and currents.

• Winds and Pressure:     * ITCZ: Band where tropical winds converge and rise (low pressure).     * Monsoon: Seasonal reversal of winds (e.g., Asia is $20^{\circ}C$ warmer than sea in summer, causing onshore winds).

• Lapse Rates:     * Environmental Lapse Rate (ELR): Average $6^{\circ}C/km$.     * Dry Adiabatic Lapse Rate (DALR): $10^{\circ}C/km$.     * Saturated Adiabatic Lapse Rate (SALR): $4-9^{\circ}C/km$ (average $5^{\circ}C/km$).

• Fohn Effect: Air cools at SALR on ascent, loses moisture, then warms quickly at DALR on descent as hot/dry wind.

• Global Warming:     * Enhanced Greenhouse Effect: $CO_2$ risen from $270\,ppm$ to $360\,ppm$; expected $600\,ppm$ by $2050$.     * Stern Report (2006): Climate change costs $5-20\%$ of global GDP if ignored, vs $1\%$ to manage.

• Urban Climates: Heat islands (up to $11^{\circ}C$ warmer at night). Canyon effect funneling winds. Higher precipitation ($5-30\%$ more) due to hygroscopic nuclei and convection.\n

ROCKS AND WEATHERING

• Earth Structure: Lithosphere ($70\,km$ deep).     * Continental Crust: $35-70\,km$ thick; older; density $2.6$; granitic.     * Oceanic Crust: $6-10\,km$ thick; younger; density $3.0$; basaltic.

• Plate Tectonics:     * Convection Current Theory: Radioactive decay in the core drives magma rise.     * Dragging Theory: Cold/heavy edges sink and pull plates.     * Hotspots: Vertical lava plumes (e.g., Hawaii).     * Sea-floor Spreading: Paleomagnetism confirms growth. Mid-Atlantic Ridge is slow-spreading; East Pacific Rise is fast-spreading.     * Boundaries: Divergent (Mid-ocean ridges), Convergent/Subduction (Trenches/Island arcs), Convergent/Collision (Fold mountains like Himalayas), Transform (San Andreas Fault).

• Weathering Processes: Decomposition/disintegration in situ.     * Physical: Freeze-thaw (expands $10\%$; pressure $2100\,kg/cm^2$); Exfoliation (diurnal heating); Salt crystallisation ($300\%$ expansion of sodium sulfate); Pressure release.     * Chemical: Hydrolysis (orthoclase feldspar to kaolin); Hydration (anhydrite to gypsum); Carbonation (calcium carbonate to soluble bicarbonate); Oxidation ($FeO$ to $Fe_2O_3$).     * Biological: Chelation (organic acid exchange).

• Weathering Controls:     * Van’t Hoff’s Law: Chemical weathering rate increases $2-3\times$ for every $10^{\circ}C$ rise.     * Goldich (1938): Resistant minerals (quartz) vs weak minerals.     * Granite Landforms: Tors (Linton’s corestone theory vs periglacial frost shattering).     * Limestone (Karst): Clints/Grikes, Dolines, Stalactites, Stalagmites.

• Mass Movements:     * Heave: Slow movement forming terracettes.     * Slides: Material moves along a slip plane.     * Slumps: Rotational movement on clay with high water content.     * Flows: Continuous, fluid movement (mudflows or earthflows).     * Avalanches: Rapid snow/rock movement on slopes > 22^{\circ}.

• Human Impact on Rocks:     * Mining: Opencast (habitat destruction) vs Underground (subsidence).     * Acidification: Dry/Wet deposition of $H_2SO_4$ and $HNO_3$. Corrodes stone; acidifies Swedish lakes ($9000$ lakes affected).

QUESTIONS & DISCUSSION

• Hydrological Definitions: What are interception, evaporation, and infiltration? (Answer: Interception is vegetation storage; evaporation is liquid-to-gas; infiltration is soil absorption.)

• Flow Differences: Comparison of overland flow, throughflow, and baseflow. (Answer: Overland is rapid surface flow; throughflow is soil movement; baseflow is slow groundwater seepage.)

• Infiltration Influence: Ground cover effect. (Answer: Pasture allows $57\,mm/hr$, while bare ground only $6\,mm/hr$.)

• River Regimes: Comparison of Guadalquivir, Shannon, and Gloma. (Answer: Guadalquivir has summer drought; Gloma peak flow is affected by spring melt; Shannon is more consistent/temperate.)

• Hydrograph Mistakes: Lag time must be measured from peak storm intensity to peak discharge, not from the storm start.

• Urban Climates: Why are they best observed in anticyclonic weather? (Answer: Calm conditions allow heat islands to develop without wind-mixing.)

• Plate Tectonics: Name the six major plates. (Answer: African, North American, South American, Eurasian, Australian-Indian, Antarctic.)

• Weathering Theories: Linton vs Periglacial Tor formation. (Answer: Linton suggests deep chemical weathering; periglacial suggests frost action removing weak rock.)

• Drought Comparisons: What is the difference between absolute and partial drought? (Answer: Absolute involves $15$ days; partial involves $29$ days with specific avg thresholds.)

• Mining Environments: Impacts of opencast vs dredging. (Answer: Opencast causes visual intrusion and dump failure; dredging causes habitat destruction and water pollution.)