20 markers

With reference to a river catchment that you have studied, assess the potential factors that could impact the flood hydrograph (20)


intro:

  • a flood hydrograph shows changes in river discharge over time following a rainfall event, including lag time, discharge and rising limb

  • the shape of a hydropgraph is influenced by physical factors (rainfall, geology, soils, relief) and human factors (urbanisation, land use, river management)

  • using the river exe catchment, this essay assesses how these factors interact to influence flood risk and hydropgraph response


P1: rainfall, relief and basin characteristics

Point: Rainfall and relief strongly influence the steepness of the rising limb and lag time.

Evidence (River Exe)

  • Source in Exmoor (514 m) with high rainfall (~1900 mm/year).

  • Mouth at Exmouth (26 m) with lower rainfall (~700 mm).

  • Large, elongated drainage basin draining SW England.

Explanation

  • High relief increases gravitational flow, speeding up overland flow.

  • Intense rainfall at the source produces rapid runoff into tributaries.

  • This leads to a short lag time and high peak discharge.

Assessment

  • Rainfall and relief are fundamental controls, but their impact is amplified by soil and geology, making them necessary but not sufficient alone.


P2: geology, soils and water balance

Point: Geology and soils are major factors controlling infiltration and runoff.

Evidence

  • 85% of the catchment underlain by impermeable rock.

  • Peaty moorland soils on Exmoor store carbon but saturate easily.

  • Around 65% of the water balance is runoff.

Explanation

  • Impermeable geology limits percolation, increasing surface runoff.

  • Saturated peat soils reduce infiltration capacity during heavy rainfall.

  • Results in a flashy hydrograph with rapid rise in discharge.

Assessment

  • These factors are highly significant as they directly control how much rainfall becomes runoff, arguably more important than rainfall alone.


P3: Urbanisation in Exeter (Human factor increasing flood risk)

Point: Urbanisation significantly alters the flood hydrograph.

Evidence

  • Exeter (~50 m above sea level) is highly urbanised.

  • Impermeable surfaces (roads, roofs) reduce infiltration.

  • Increased use of drains and sewers speeds water transfer to the river.

Explanation

  • Urban areas increase surface runoff and reduce lag time.

  • Storm water reaches the river quickly, increasing peak discharge.

Assessment

  • Urbanisation is a key human factor, especially downstream, but its impact is localised compared to catchment-wide physical controls.


Conclusion;

  • The flood hydrograph of the River Exe is influenced by a complex interaction of physical and human factors.

  • Geology and soils are the most significant controls, as they determine infiltration and runoff across the whole catchment.

  • Rainfall and relief amplify these effects, creating a short lag time and high peak discharge.

  • Human factors, including urbanisation and land management, can both increase and reduce flood risk.

  • Overall, physical factors set the baseline hydrograph response, but human management increasingly determines flood severity, making both critically important.



assess the potential causes and impacts of changes to the water balance within a tropical rainforest that you have studied (20)


intro:

  • the water balance refers to the relationship between inputs (precipitation), stores (interception, soil moisture, groundwater) and outputs (evapotranspiration, runoff)

  • tropical forests such as the Amazon have a naturally balanced water cycle due to high rainfall, dense vegetation and high evapotranspiration

  • however, human activity particularly deforestation and land-use change, has disrupted this balance, leading to both local and regional impacts


P1: Natural characteristics maintaining the Amazon water balance (baseline)

Point: The Amazon’s natural structure supports a stable water balance.

Evidence

  • Covers ~7 million km² in equatorial South America.

  • Receives 1500–3000 mm rainfall annually through convectional storms.

  • Dense canopy → high interception and high evapotranspiration.

  • Complex root systems absorb surface water rapidly.

Explanation

  • High evapotranspiration recycles moisture back into the atmosphere.

  • Water is transferred gradually through stemflow and drip flow, reducing runoff.

Assessment

  • This natural balance makes the Amazon resilient, but highly dependent on vegetation cover, meaning disruption has severe consequences.


P2: Deforestation as the main cause of change to the water balance

Point: Large-scale deforestation is the most significant driver of change.

Evidence

  • 20% of the Amazon destroyed in the last 50 years.

  • Cleared for cattle ranching, logging and mineral extraction.

  • Downwind rainfall reduced by up to 20%.

Explanation

  • Reduced canopy → less interception and evapotranspiration.

  • Lower atmospheric humidity → drier regional climate.

  • Increased surface runoff due to soil compaction.

Assessment

  • Deforestation fundamentally alters water transfers, making it the most important cause of imbalance.


P3: Agricultural land use and differences in impact (evaluation)

Point: The type of agriculture determines the severity of water balance disruption.

Evidence

  • Pastoral farming removes vegetation entirely → greatest impact.

  • Arable farming (e.g. palm oil) retains some interception and evapotranspiration.

  • Cattle ranching compacts soil, increasing runoff and flood risk.

Explanation

  • Reduced infiltration increases river discharge and erosion.

  • Warmer ground increases convection, causing intense but localised rainfall.

Assessment

  • This shows that not all land-use change has equal impact, but pastoral farming is particularly damaging.


CONC:

  • The Amazon rainforest’s water balance is naturally well-regulated due to high rainfall, dense vegetation and high evapotranspiration.

  • Deforestation is the most significant cause of change, as removing canopy cover directly reduces interception and evapotranspiration, leading to lower regional rainfall and increased runoff.

  • Agricultural land use intensifies this imbalance, with pastoral farming having a greater impact than arable farming due to soil compaction and total vegetation removal.

  • Overall, changes to vegetation cover - particularly large-scale deforestation - are the key drivers of disruption to the Amazon’s water balance, outweighing natural variability.


Assess the impact of farming practices on the carbon budget (20)


intro:

  • The carbon budget refers to the balance between carbon stores, transfers and outputs within an ecosystem.

  • The Amazon rainforest is a major global carbon sink due to its large biomass and high rates of photosynthesis.

  • 30-50% global photosynthesis - produces 20% global O2

  • However, farming practices—particularly pastoral farming and arable agriculture—have disrupted this balance - 20% destroyed in last 50 years


P1:

  • Pastoral farming for cattle ranching requires large-scale deforestation.

  • Trees are often burned, releasing large amounts of CO₂ into the atmosphere.

  • Removal of vegetation reduces photosynthesis, limiting carbon sequestration.

  • Methane emissions from cattle further increase greenhouse gas levels.

  • This practice has the greatest negative impact on the Amazon’s carbon budget.


P2:

  • Arable farming, such as palm oil plantations, also involves forest clearance.

  • However, crops still absorb carbon through photosynthesis, partially offsetting losses.

  • Soil carbon can be temporarily increased by ash from burning vegetation.

  • Overall carbon storage remains lower than in natural rainforest.

  • Arable farming therefore has a less severe but still damaging impact.


P3:

  • Different farming practices have unequal impacts on the carbon budget.

  • Pastoral farming removes almost all biomass, while arable farming retains some vegetation cover.

  • Both reduce long-term carbon storage compared to intact rainforest.

  • The scale and permanence of land clearance determines the severity of impact.

  • This highlights farming as a key driver of carbon budget change in the Amazon.


CONC:

  • Farming practices have significantly disrupted the carbon budget of the Amazon rainforest.

  • Pastoral farming has the most severe impact due to deforestation, carbon release and methane emissions.

  • Arable farming is less damaging but still reduces carbon storage.

  • Overall, farming has shifted the Amazon away from being a stable carbon sink, making agricultural expansion a major threat to the carbon budget.