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Geography Lecture 8 (3.2 Overview of Components of the Drainage Basin…

- - - - Deposition of moisture from the atmosphere
      - Includes rain, hail, snow, sleet
      - Precipitation in a drainage system can vary in type, quantity and intensity
  - - - Refers to the holding of raindrops by plants as water falls onto plants
      - Precipitation - Interception Loss = Water reaching the ground
      - Amount of precipitation is affected by various factors
        
        leaf type
        
        canopy architecture
        
        magnitude and duration of precipitation
        
        antecedent moisture conditions of the canopy
        
        Available heat energy
        
        Humidity
        
        Wind speed
    - - Rainfall that travels down the plant's stem
    - - Precipitation that falls through the canopy without being intercepted is called throughfall
    - - Flow of water through a permeable surface
      - It contributes rainwater to the soil moisture storage
      - Infiltration rate and capacity
        
        Rate = speed
        
        Capacity = amount
      - Factors affecting infiltration
        
        a) Rainfall Intensity
        
        b) Type of soil
        
        c) Vegetation cover
    - - Flow of water downwards through the subsurface soil
      - Rate of percolation slows as it gets deeper
      - Factors affecting percolation
        
        a) Volume of pore spaces
        
        b) Soil Antecedent Moisture
    - - Horizontal flow of water in the soil moisture storage
      - Percolation is impeded, hence throughfow happens
      - In the Zone of Aeration, above the water table
    - - Horizontal flow of water in the groundwater storage
      - Factors affecting baseflow
        
        a) Porosity
        
        b) Permeability
        
        c) Hydraulic gradient
    - - The portion of water which does not infiltrate the soil but flows over the surface to a river channel
      - Often faster than sub-surface flows
      - Hortonian Flow (HF) and Saturation Overland Flow (SOF)
        
        HF
        
        Occurs when infiltration cannot occur, leading to overland flow
        
        Factors
        
        Precipitation intensity exceeds infiltration capacity
        
        Ground is frozen
        
        Slopes are steep
        
        Lack of vegetation cover
        
        Top soil layer is compacted or bare
        
        SOF
        
        Occurs when ground is saturated and rainwater is unable to infiltrate
        
        Factors
        
        High rainfall over a few days, water table height increased
        
        Water table is high due to antecedent moisture
        
        Slope concavities (water table close to ground at low reliefs)
    - - Flow of water within a river channel, out to sea
      - Most water in rivers comes from overland flow, throughflow and baseflow
  - - - Rainwater that is held by plant canopies
      - Can only be removed through evaporation
    - - Storage of moisture in soil
      - Zone of Aeration, where pores are filled with water and air
    - - Zone of Saturation, fully saturated
      - Water is stored in joints, pores, cracks and fissures in the rock strata
    - - Rivers act as flow and as stores
      - Water input from flows and small amounts of direct precipitation
      - Rivers may flood and take the entire breadth of the floodplain
  - - - Water flowing into the sea
    - - Evaporation and Transpiration through plants
- - - - Precipitation is more than PE, so AE = PE and surplus. Streams are thus seldom dry, water table is near surface and soil is moist. All these are reflected by dense natural vegetation
    - - Precipitation is lower than PE, so AE<PE and deficit. Soil is dry, groundwater is deep and rivers are low or non-existent
    - - Seasonal precipitation and PE, so it depends on situation
        
        Wet season, precipitation more than PE, AE = PE, thus recharge and surplus
        
        Dry season, precipitation lower than PE, AE < PE, thus depletion and deficit
- - - - Boundary of a catchment area is marked by a highland beyond which any precipitation will drain into another basin
      - This boundary is called the drainage divide or watershed divide