Drainage Basin System (SL IB Geography)

• Drainage basins are areas of land where precipitation (rain or snow melt) drains downhill into a body of water such as a river, lake, wetland or ocean
• Drainage basins are open systems with inputs, transfers and outputs
• Features of a drainage basin include: 
  • Watershed
  • Source
  • Tributary
  • Confluence
  • Floodplain
  • Mouth

Diagram showing the features of a drainage basin

Drainage basin features

• After falling as precipitation, water can take many different routes before it reaches its end point 
• Water can be stored in the system for a few days, years or centuries in aquifers

Diagram of the hydrological cycle

The hydrological cycle

Inputs, Flows, Outputs and Stores in the Drainage Basin

Inputs	Flows	Stores	Outputs
Precipitation	Throughflow Surface run-off (overland flow) Groundwater(base) flow infiltration Percolation	Cryosphere Channel stores Vegetation Soil Aquifers Atmosphere as water vapour	Transpiration Evaporation Sublimation

Inputs

• Precipitation is the primary input into the drainage basin
• Precipitation is rainfall, snow, frost, hail and dew
• Key characteristics of precipitation impacting local hydrology (movement of water) include:
• Total amount of precipitation
• Intensity 
• Type (e.g. snow or rain) 
• Geographic distribution
• Variability

Flows

Infiltration

• Infiltration is the process where water permeates, or is absorbed by the soil
• Infiltration capacity is the maximum rate at which rain can be absorbed in a given condition
• Infiltrated water becomes chemically enriched as it collects minerals and organic acids from vegetation and soil
• Plant roots create fine channels for percolation known as percolines

Surface runoff

• Overland flow (surface runoff) occurs when precipitation exceeds the infiltration rate or when the soil becomes saturated
• High precipitation intensity and low infiltration capacity lead to common surface runoff in areas like semi-arid regions and cultivated fields
• Surface flow happens near streams and river channels
• Throughflow refers to water moving naturally through soil pipes and percolines
• Base flow is the constant part of a river's discharge supplied by groundwater seepage into the riverbed, which slightly increases after wet periods

Stores

Vegetation

• Vegetation interception is when water remains on the surface of the leaves before evaporation
• Interception loss varies based on vegetation type
• Coniferous trees intercept more water in winter
• Deciduous trees intercept more water in summer

Soil

• Soil moisture is subsurface water within the soil
• Field capacity is the retained water level after excess drainage and near saturation
• Wilting point is the moisture range causing permanent plant wilting and setting plant growth limits

Aquifers

• Aquifers serve as significant water reserves
• Water in aquifers moves slowly and absorbs rainfall that would otherwise rush into streams
• Aquifers help maintain stream flow during extended dry spells
• Aquifers can lead to springs, which can become the source of streams or rivers
• Groundwater is subsurface water that percolates slowly into the rock beneath the soil
• Percolation speed depends on rock permeability
• Carboniferous limestone and chalk percolation speed can be relatively fast
• The permanently saturated zone in rocks and sediments is the phreatic zone
• The upper layer is known as the water table 
• Seasonally variability: aquifers are higher in winter due to increased precipitation
• The seasonally wet and dry zone is called the aeration zone
• Groundwater accounts for 96.5% of all freshwater on Earth but it can take up to 20,000 years to recycle

Aquifer

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Aquifer

Groundwater recharge

Type of recharge	How groundwater recharges
Infiltration	Through total precipitation at ground surface
Seepage	Through banks and beds of surface water
Leakage and inflow	From adjacent rocks and aquifers
Artificially	From irrigation and reservoirs

Cryosphere

• The cryosphere includes Earth's snow and ice
• It contains up to 66% of the world's freshwater
• Over 97% of Earth's water is salty and freshwater resources are limited
• High-latitude and high-altitude regions store significant snow and ice
• Seasonal melting plays a key role in altering the basin's hydrological cycle

Outputs

• Evaporation is the conversion of liquid or solid substances into a gas 
• Evaporation:
  • Involves the transformation of precipitation into water vapour in the atmosphere
  • Is most prominent over oceans and seas and is influenced by climatic conditions
  • Increases under warm, dry conditions
  • Decreases under cold, calm conditions
• Factors affecting evaporation include:
  • Temperature
  • Humidity
  • Wind speed
  • Availability of water
  • Vegetation
  • Surface colour

Evapotranspiration

• Transpiration is the release of water vapour from living plants through their leaves and into the atmosphere
• Evapotranspiration (EVT) represents the primary source of water loss 
• EVT accounts for nearly 100% of annual precipitation in arid areas and 75% in humid regions
• Evaporative losses can occur over ice and snow fields, barren rock slopes, desert areas, water surfaces and bare soil
• Potential evapotranspiration is determined by the availability of moisture.

The water budget

• Water budgets are the annual balance between inputs and outputs
• Water budgets can impact soil water availability
• The balances can be calculated at various scales, from global to local
• Water budgets at the regional level tell us the amount of water that is available for human use
• On a local scale, the water budget can tell us how much water is available in the soil

Diagram showing the water budget

Water budget

• The water budget uses the following equation: P + Qin = ET + ∆S + Qout
• P = precipitation (rain, snow, etc.)
• Qin = water flow into the watershed
• ET = quantity of evapotranspiration from soils, surface water, plants, etc.
• ∆S = Change in water storage
• Qout = sum of water flowing out of the watershed

Factors influencing the water budget

Factors that can affect the water budget	How the water budget is affected
Permeable rocks and porous soils	Encourage infiltration and percolation, which means the flow is slowed down and there is an increased storage in the water basin
Dense forests	Intercept rainfall, absorbing water through the canopy
Shape, relief and size of drainage basin	Influence amount of water flowing overland
Vegetation density	Affects patterns of water flow and stores in the basin
Seasonality	There will be more water in wet seasons, which will create a water surplus
Climate	Determines the amount and type of precipitation that transfers through a river basin