2.4 What are the hydrological processes associated with drainage basins?

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  • 2.4 What are the hyrdological processes associated with drainage basins?
    • The drainage basin system: inputs, flows, stores and outputs
      • Inputs
        • Precipitation: ice and water that falls from the clouds in the form of rain, snow or hail
      • Flows
        • Throughfall and stemflow: occur through the vegetation
        • Infiltration: movement of water from above to below the surface of the ground
          • The rate of infiltration depends on the amount of water already in the soil, the porosity and structure of the soil, and the type and extent of vegetation
        • Percolation: the movement of water down through the soil and permeable rock
        • Surface runoff: travels across saturated or impermeable land
        • Throughflow: the movement of water through the soil towards the surface, often emerging on valley sides to form springs
        • Groundwater flow: the movement of water through rocks towards the surface
      • Outputs
        • River runoff
          • As water is carried overland to the sea
        • Evaporation
          • As water is changed to water vapour
        • Transpiration
          • The evaporation of moisture through the stomata on leaf surfaces
      • Stores
        • Interception: vegetation and other surfaces catch falling precipitation
          • Interception by vegetation may remove up to 30% of water from the system
        • Surface storage
          • Includes any body of water from a puddle to a lake
        • Soil water storage
          • Essential for plant growth
        • Groundwater stoage
          • In permeable rocks
            • Large stores of groundwater are called aquifers and the surface of this underground water is the water table
      • Water budget
        • The balance between the water inputs and outputs
          • As river runoff and precipitation are easily measurable, it is possible to estimate evaportranspiration and changes in storage
            • precipitation (P) = streamflow (Q) + evapotranspiration (E) +/- changes in storage (S)  P=Q+E +/- S
      • A system
        • Definition: a set of objects or characteristics which are related to one another and operate together
        • Open systems: exchanges both energy and matter (drainage basin)
        • Closed systems: open only to transfer energy. it does not have any inputs/outputs of matter (water cycle)
        • The GLOBAL hydrological cycle- a CLOSED system
          • There is a fixed amount of water ciulating in our atmosphere and on the surface. There are no exchanges outside the earth and it's atmposhere
    • The characteristics of river regimes and the physical and human factors influencing them
      • Climate
        • Humid environments
          • Rivers are fed by constant flow of groundwater and are described as having perennial flow regimes
        • Dry climates
          • Insufficent water to maintain channel flow, so these rtivers are described as intermittent flow rivers
        • Driest regions
          • Channels may be fed by occasional flash flooding and are described as having ephemeral flow regimes
        • Distinct wet and dy seaons e.g. monsoon climates
          • Reflected in the river regime which may flood during the rainy season
        • The regimes of rivers fed by snow and glacier meltwater show distinct peaks and troughs in their annual hyrdographs
        • Evapotranspiration is higher in the summer than in winter if there is a significant seasonal difference in temperature throughout the year
      • Geology
        • Permeable rocks may be:
          • Porous: containing pore spaces (sandstone and chalk)
          • Pervious: they contain joints and cracks through which water may travel (limestone)
          • Rivers running over permeable rocks will be more likely to dry up during droughts as water will be stored in the rock
        • Impermeable rocks
          • Water can only pass through fractures in these rocks (granite and basalt)
          • River systems running over impermeable rocks will be more prone to flooding as there is less groundwater storage
      • Land use
        • Presence or absence of forestry: determines the level of interception
          • Affects the time it takes for input from precipitation to work through the system and reach the river channel
        • Farming practices
          • The development of winter crops that are sown in late autumn means that fields are comparatively bare during the wetter winter months, reducing interception and increasing surface runoff
            • Leads to higher discharge and a greater risk of flooding
          • Traditionally, farmers left their fields after harvest with the remains of the crop plants binding the soil
            • Fields were ploughed in later winter and sown with spring crops
          • Irrigation; in some parts this had led to a significant impact on the volume of water stored in aquifers and affected the level of base flow
        • Ubanisation
          • Increase in impermeable sufaces
            • Increase runoff and discharge
          • Decrease in vegetation
            • Loss of infiltration=faster overland flow=increased risk of flooding
          • Pollution
            • Impervious surfaces collect polluntants
      • River management
        • Dam-building has the most significant impact on the regime of a river
          • Following the construction of a dam, the downstream flow of water in the river is controlled and rearly affected by seasonal variations in precipitation or meltwater
        • People also affect discharge by changing the channel: straightening meanders, deepening the channel and canalising it increases the velocity
    • The characteristics of flood hyrdographs and the human and physical factors influencing them
      • Hydrographs- show the variations in river discharge in response to a single rainfall event
      • Lag time: the difference between peak rainfall and the peak discharge
        • The time taken for the water to reach the river channel

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