Variations In Runoff And The Water Cycle

River Discharge: The volume of water that flows in a river per second – measured in cumecs: Velocity x CS

  • Hydrographs: are graphs of river discharge overtime. They show how the volume of water flowing at certain points in a river changes over a period of time.
    • Storm Hydrograph: Plots the increase of a river’s discharge during and following a storm.
    • Peak discharge: this is the highest point on the graph, when rivers discharge is greatest.
    • Lag time: this is the delay between peak discharge. This delay happens because it takes time for the rainwater to flow into the river.
      • Shorter lag time increases peak discharge as more water reaches river in short time
    • Rising limb: Up to peak discharge. The river discharge increases as rainwater flows into it
    • Falling Limb: part of the graph after peak discharge. Discharge is decreasing because less water is flowing into the river.
    • Approach Segment/Antcendent flow rate: The discharge of the river BEFORE rainfall
    • Flood: When river exceeds Bankful discharge and inundates into floodplain
  • Flashy=short lag time, a steep rising limb and a very high peak discharge. Water reach river quickly
  • Attenuated= long lag time, a gentle rising limb, and a low peak discharge. Water reach river slowly

Permanent Controls: Physical factors that don’t vary much

  • Size of a drainage basin: larger drainage basins catch more precipitation = a higher peak discharge
  • Shape of drainage basin: circular basins are more likely to have a flashy hydrograph as all points on the watershed are roughly the same distance from the point of discharge measurement. This means lots of water will reach the measuring point at the same time.
  • Ground steepness: water flows quickly downhill in steep sided drainage basins, shortening lag time.
  • Rock and soil type: impermeable rocks and soils don’t store water or let water infiltrate. This increases surface runoff


Transient Controls- Physical controls that vary flooding

  • Storms and precipitation; Intense storms generate more precipitation and greater peak discharges
  • Seasonal changes: Winter = water freeze. Reduce size of flows while store of frozen water grows.
  • Vegetation: More vegetation, more water is lost, decreasing runoff and peak discharge


Human Factors:

  • Farming practices:
    • Ploughing breaks up the surface so that more water can infiltrate, reducing runoff.
    • Crops increase infiltration and interception compared to bare ground, reducing runoff.
    • Livestock compact the soil, decreasing infiltration and increasing runoff.
    • Irrigation can increase runoff if some of the water cant infiltrate
  • Land use change: Deforestation
    • Construction of new buildings and roads – creates an impermeable layer over the land, preventing infiltration= increases runoff = water passing through the system rapidly

Water abstraction: Reduces the amount of water in stores such as lakes and rivers