Drainage Basin Key Concepts L3

Drainage basin is an open system with inputs, stores, flows (transfers) and outputs.
Starter terms: Infiltration, Orographic rainfall, Interception, Watershed.

Inputs: Precipitation (main input).
Stores: Interception; Surface storage; Soil moisture; Groundwater storage; Vegetation storage; Stemflow.
Flows: Infiltration; Throughflow; Percolation; Channel flow; River runoff; Evaporation; Transpiration; Sublimation; Groundwater flow.
Outputs: Evaporation; Transpiration; Evapotranspiration; Overland flow; River discharge.

Evaporation: Moisture loss from water surfaces; largest transfer; driven by sun and air movements.
Transpiration: Water loss from plants; depends on season and vegetation cover.
Evapotranspiration: Total moisture removed by evaporation and transpiration.

Snow-capped peaks delay flow by storing water as snow until thaw.
Large basins collect more precipitation and are affected by basin-wide factors.
Drainage density: low density => slower water movement; high density => faster movement.
Forested slopes intercept more precipitation, increase evapotranspiration and reduce surface runoff.
Impermeable soils and rocks prevent infiltration and cause surface saturation.
Steep slopes promote faster movement and shorter storage times; gentle slopes slow movement.
Permeable soils and rocks allow more infiltration and percolation, recharging groundwater.
Reservoirs store water and create new surface stores.
Urban areas with impermeable surfaces increase rapid surface runoff and alter interception/evaporation.

Air masses influence precipitation and evaporation rates.
Annual rainfall ranges vary by region (data typically presented in regional climate charts).

Human activities can disrupt process speeds, create new stores, or abstract water; hard engineering schemes can also disrupt balance.
Consider impacts on: precipitation, evaporation/transpiration, interception, infiltration/soil moisture, groundwater.

Cloud seeding with substances like silver iodide to induce rainfall; used in some regions (e.g., before events, ski areas); effectiveness is debated.

Interception depends on vegetation density; deforestation reduces interception; afforestation increases interception.
Infiltration rates: forests ~5× higher than farmland; pastoral farming increases soil compaction; ploughing reduces infiltration.
Infiltration depends on soil type: sandy soils are permeable; clay soils are poorly permeable; porosity influences rate.
Other factors: vegetation type and cover, soil moisture, compaction, slope angle. How can humans affect these?

Groundwater use for irrigation affects supplies (examples in Texas/California).
In the UK, groundwater rebound can occur as industrial water use declines; rebound can cause basement flooding and potential groundwater pollution in urban infrastructure (e.g., tunnels).

Deforestation reduces interception; increases surface runoff and erosion; lowers groundwater recharge; can raise flood risk.
Channelling rivers under cities alters flow paths and evaporation dynamics.

Dense canopy leads to high interception and evapotranspiration, sustaining regional rainfall; deforestation reduces ET and rainfall, increasing runoff and river discharge (e.g., Tocantins River case).

Basin-wide controls: shape, relief, geology, vegetation, climate and land use; location affects inputs.
Major threats: over-abstraction, deforestation, urbanisation, reservoirs and dams.