Variations in Runoff & The Water Cycle

• river discharge becomes a flow when it goes into a new drainage basin
• river discharge :


  volume of water in river per sec (cumecs)
• High runoff = High river discharge
• hydrography show river discharge over time and how vol of water flowing at certain point changes
• Flood hydrographs :


  show river discharge at time of storm event
• Flashy hydrograph :


  . basin has rapid runoff & low storage capacity

  . short lag time + high peak discharge

  . steep even rising & falling limbs
• Peak discharge :


  where river discharge is highest

  
• Lag time :


  delay between peak rainfall & peak discharge (takes time for rainwater to flow into river)

  shorter = increased peak discharge (more water enters quicker)
• Rising limb :


  part of graph up to peak discharge

  river discharge increases as rainwater flows into river
• Falling limb :


  part of graph after peak discharge

  river discharge is decreasing as less water is flowing into river

  shallow limb : water is flowing in from other stores after rain
• Factors affecting runoff & hydrograph shape :

  1. basin shape
  2. basin size
  3. ground steepness
  4. rock/soil type
• 1)Basin size

  circular = all watershed points same distance from discharge measurement point = lots of water reaches at same time = flashy
• 2)Basin shape

  large = catch more precipitation = higher peak discharge

  small = precipitation has less distance to travel = short lag time
• 3)Ground steepness

  steep sided basin = flows quicker downhill = shorter lag time = less time to infiltrate soil = higher runoff
• 4)Rock/soil type


  impermeable = no infiltration and storage = increased runoff in shorter time = decreased lag time & peak discharge
• Water cycle variations :

  Physical / Natural processes :

  1. Storms⚡️ & Precipitation🌧️
  2. Seasonal changes & Vegetation🌱

  Human activities :

  1. Farming processes 🌾
  2. Land use change
  3. Abstraction
• 1)Storms ⚡️ & Precipitation 🌧️

  . intense storms = higher peak discharge

  . larger input = flows & stores increase in size

  . some flows can't happen fast enough for input size (infiltration)
• 2)Seasonal changes & Vegetation 🌱

  Winter : water freezes = reduces flows through basins = cryosphere grows

  Summer : ice melts = flows & outputs larger

  plants have seasonal variation : lots of deciduous trees = highest interception

  more veg in basin = more lost by evapotranspiration before reaching channel = decreased peak
• 1)Farming practices🌾

  . affects infiltration & runoff

  . ploughing = breaks up surface = ⬆️ infiltration

  . crops = ⬆️ interception & infiltration

  . evapotranspiration = ⬆️ precipitation

  . livestock compact soil

  . irrigation = ⬆️ runoff
• 2)Land use changes

  . deforestation = ⬇️ interception = more water reaches surface

  . dead plant material in forests holds water for infiltration

  . building construction 🏢= impermeable layer = water passes through system quicker = flood risk
• 3)Abstraction

  . high population density = high demand = reduces stores (reservoirs & groundwater)

  . dry seasons = more abstracted for consumption & irrigation = further depleted