The drainage basin is an open system within the global hydrological cycle

The Drainage Basin Water Cycle

• A subsystem within the global hydrological cycle

• Open system - Has external inputs and outputs

  

• Drainage basin → An area of land drained by a river and its tributaries → Catchment (The area of land drained by a river and its tributaries)

• Boundary around the basin = watershed (high land which divides and separates waters flowing to different rivers)

Drainage Basin System Inputs

Precipitation

For precipitation to form, certain conditions must be met:

• Air cooled to saturation point with a relative humidity of 100%
• Condensation nuclei (eg. dust particles) to facilitate growth of droplets in clouds
• A temp below dew point

Rain Shadow

🔗 A dry area on the leeward side of the mountain. Little rainfall as the mountains shelter it from rain producing weather systems

Types of rainfall

• Convectional rainfall → Often associated with intense thunderstorms
• Cyclonic or frontal rainfall → A period of sustained, moderately intensive rainfall
• Orographic rainfall → Concentrated on the windward slopes and summits of mountains

Types of rainfall

Key Influencing Factors on the Drainage Basin Hydrological System

1. Amount of precipitation - can have a direct impact on drainage discharge
2. Type of precipitation
3. Seasonality
4. Intensity of precipitation
5. Variability (seen in 3 ways):
6. Secular variability happens long term
7. Periodic variability happens in an annual, seasonal etc basis
8. Stochastic variability results from random factors
9. Distribution of precipitation within the basin

Fluxes in the Drainage Basin

Interception → Process by which water is stored in the vegetation Components:

• Interception loss → Water that is retained by plant surfaces and later evaporated or absorbed by the vegetation and transpired ↳ Interception loss from vegetation is usually greatest at start of a storm, especially when following a dry period

• Throughfall → When the rainfall persists or is relatively intense, and the water drops from the leaves, twigs etc

• Stem flow → When water trickles along twigs and branches and then down the trunk → Process by which water soaks into (or is absorbed by) the soil Infiltration

↳ The Infiltration capacity is the max rate water can be absorbed by the soil

• Infiltration capacity ↓ w/ time through a period of rainfall until a relatively constant low value is reached

• Rate of infiltration depends on the amount of water already in the soil (antecedent soil moisture)

• Soil Texture - influences soil porosity

• The type, amount and seasonal changes in vegetation cover

• Nature and structure of the soil surface - more compacted = inhibit infiltration

• Slope angle Infiltration is inversely related to surface run-off

• Surface run-off → The movement of water that is unconfined by a channel across the surface of the ground

Flows and Transfers

Overland Flow (Surface run-off) → Movement of water that is unconfined by a channel across the surface of the ground

↳ Developed by Horton - Saw this flow was the main way rainwater was transferred to the river channel

• For this flow, precipitation intensity must exceed the infiltration rate (eg. Torrential storms)

• Primary cause of soil erosion - sediment removed by a range of erosive processes (rain splash, sheet, rill & gully erosion) Throughflow → Lateral transfer of water down slope through the soil via natural pipes and percolines (Lines of conc water flow between soil horizons to the river channel).

• Slower than direct overland flow but can occur rapidly over porous, sandy soils Percolation → Transfer of water from the surface or from the soil into the bedrock beneath - permeable (joints/pores)

• Much more likely in humid climates w/ vegetated slopes Saturated Overland Flow → The upward movement of the water table into the evaporation zone

• Much slower process

• Common after successive winter storms causing the water table to rise to the surface in depressions and at the base of hill sides ↳ Saturated overland flow ∴ contributing to channel flow (is a component of flooding) Groundwater Flow → Very slow transfer of percolated water through pervious and porous rock

• Vital to maintain a steady level of channel flow in droughts and other varying weather conditions → Flow of water in streams or rivers Channel Flow

• From 3 transfer processes: Overland flow, through flow or groundwater flow

• Direct channel precipitation is added to channel storage (Storage of water in streams or rivers)

Drainage Basin System Outputs

Evaporation → Process by which moisture is lost directly into the atmosphere from water surfaces & soil

• Results from the effects of the Sun's heating & air movement

  • Rates ↑ in warm, windy and dry conditions

• Climatic factors: Temperature, hours of sunshine, humidity and wind speed

• Other factors: Size of water body, depth of water, water quality, type of vegetation cover & colour of surface (determines the albedo → A measure of the proportion of the incoming solar radiation that is reflected by the surface back into the atmosphere and space Transpiration → Water lost from plants through stomata and transferred to the atmosphere

• Depend on: Time of year, type & amount of vegetation cover, degree of availability of moisture in atmosphere & length of growing season

• Evapotranspiration (EVT) → Combined effect of evaporation and transpiration

• Potential evapotranspiration (PEVT) → Water loss that would occur if there was unlimited supply of water in the soil by use by vegetation

Physical Factors That Influence the Drainage Basin Cycle

• Climate → Influences type and amount of precipitation overall and the amount of evaporation. Climate also has an indirect impact and the vegetation type
• Soils → Determine the amount of infiltration and throughflow and, indirectly, the type of vegetation
• Geology → Can impact on subsurface processes such as percolation and groundwater Indirectly, geology alters soil formation
• Relief → Altitude can impact precipitation Slopes can affect the amount of run-off.
• Vegetation → Major impact on the amount of interception, infiltration & occurrence of overland flow alongside transpiration rates

Human Factors that Influence the Drainage Basin Cycle

• Precipitation Impact
  • Introduction of iodine pellets, ammonium nitrate etc to act as condensation nuclei
• Evaporation and Evapotranspiration
• Interception
  • Deforestation → ↓ evapotranspiration & ↑ surface run-off. ∴ flooding potential ↑ → ↓ surface storage
• Infiltration and Soil Water