Hydrograph = shows how a river’s discharge changes in response to a period of rainfall
○ Vertical axis = measures precipitation in mm , measures discharge in cm 3 /s
○ Horizontal axis = measures time in days/hours
○ Bars = represent rainfall
○ Line graph = represents discharge
● Shape of hydrograph helps to judge likelihood/severity of a flood in that location
Hydrograph terminology:
● Peak rainfall = highest amount of rainfall per time unit
● Rising limb = how quickly the discharge rises after a
rainstorm
● Falling limb = reduced discharge once the main effect
of runoff has passed
● Peak discharge = highest recorded discharge
following a rainfall event
● Lag time = time difference between peak rainfall and
peak discharge
● Base flow = normal flow of a river when it’s water level
is being sustained by groundwater
● Bankfull discharge = level of discharge above which
flooding will occur
Typical hydrographs:
● Rising limb is steeper than falling limb
● Rising limb is fed by surface runoff → reaches river quickly over impermeable surfaces
● Gentler slope of falling limb shows how discharge steadily falls once surface runoff has stopped
● Water reaching river is now mostly through soil as throughflow
Flashy response (storm) hydrograph:
● Flash flooding = sudden flooding
○ Rising limb is steep → rainfall has caused
lots of surface runoff
■ Water gets into channel quicky
■ Short lag time
○ River has higher peak discharge = danger of
flooding
● Several conditions result in a flashy hydrograph:
○ Prolonged rainfall → soil is saturated
○ Long drought → impermeable soil
○ Clay soil → unable to infiltrate
○ Little vegetation to intercept rain
○ Urban area with impermeable concrete
○ Rural area with poor
Slow response hydrograph:
● Due to rainfall but has a less steep rising limb (in comparison to flashy hydrographs)
● Peak discharge is low
● Lag time is longer
● Flood risk is low