Drainage Basin System (DP IB Geography)

Drainage Basin System

• 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

• 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

Inputs, Flows, Outputs and Stores in the Drainage Basin

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

Groundwater recharge

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

• 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