Hydrographs

• What do hydrographs show?


  River discharge over a period of time
• what is river discharge?


  The volume of water (m3) that flows in a river per second
• what is river discharge measured in?

  cubic metres per second aka CUMECS
• What do high levels of runoff do?

  increase the discharge of a river because more water makes it into the river, increasing its volume
• What are hydrographs?


  Graphs of river discharge over time
• Hydrographs show what?

  How the volume of water flowing at a certain point in a river changes over a period of time
• What are storm hydrographs also known as?

  Flood hydrographs
• What do flood/storm hydrographs show?


  River discharge around the time of a storm event

  They only cover a relatively short time period (hours/days nistead of weeks/months)
• What is PEAK DISCHARGE?


  The highest point on the graph, when river discharge is at its greatest
• What is lag time?


  The delay between peak rainfall and peak discharge. This delay happens because it takes time for the rainwater to flow into the river
• What can a shorter lag time do?

  Increase peak discharge because more water reaches the river during a shorter period of time
• What is the rising limb?


  The part of the graph up to peak discharge. The river discharge increases as rainwater flows into the river
• What is the falling limb?


  The part of the graph after peak discharge

  discharge is decreasing because less water is flowing into the river
• What does a shallow falling limb show?


  Water is flowing in from stores long after it's stopped raining
• What does a basin with RAPID RUNOFF and LITTLE STORAGE CAPACITY give a hydrograph with?


  • short lag time
  • high peak discharge
  • called a "flashy" hydrograph
  • steep, roughly symmetrical rising and falling limbs
• What factors are the amount of runoff and the shape of the hydrograph affected by?

  • Size of drainage basin
  • Shape of drainage basin
  • Groundsteepness
  • rock and soil type
• How does SIZE OF DRAINAGE BASIN affect the amount of runoff and the shape of the hydrograph?

  Larger drainage basins catch more precipitation- so higher peak discharge than smaller basins

  Smaller basins have shorter lag times bc precipitation has less distance to travel- so reaches main channel more quickly
• How does SHAPE OF DRAINAGE BASIN affect the amount of runoff and the shape of the hydrograph?

  Circular basins more likely to have a flashy hydrograph than long, narrow basins because all points on the watershed are roughly the same distance from the point of discharge measurement. this measn lots of water will reach the measuring point at the same time
• How does GROUND STEEPNESS affect the amount of runoff and the shape of the hydrograph?

  Water flows more quickly downhill in steep-sided drainage basins, shortening lag time

  This also means water has less time to infiltrate the soil = higher runoff
• How does ROCK AND SOIL TYPE affect runoff and hydrograph shape ?

  Impermeable rocks and soils do not store water or let water infiltrate. This increases surface runoff

  Peak discharge also increases as more water reaches the river in a shorter period
• What is runoff also affected by?


  Precipitation, seasonal changes ,human activity.