Hydrograph

Interpretation

What is a hydrograph?

a graph to show the relationship between rainfall intensity and stream discharge

Features of a hydrograph

it contains a bar graph showing the amount of rainfall through time;

and a line graph showing the discharge of a stream

Features of a hydrograph

after the rain, the discharge of the stream begins to increase.

the rising amount of discharge forms the rising limb.

Features of a hydrograph

after a certain period of time, discharge of the streams comes to the highest, which is called peak discharge.

Features of a hydrograph

when the rain stops, the discharge of the stream will also gradually reduce.

the decreasing amount of discharge forms the falling limb or recession limb.

Features of a hydrograph

the dotted line below is called the base flow

which is the normal discharge of a stream supplied by groundwater.

Features of a hydrograph

the time-lag is the time difference between the rainfall peak and the peak discharge.

the shorter the time lag, the higher the risk of the flood

Factors affecting peak discharge

amount of rainfall duration of precipitation - longer

rainfall --> reduces infiltration, increases overland flow --> high peak discharge

no. of tributaries

Factors affecting time lag

amount of precipitation duration of precipitation rate of infiltration steepness of the slope human activities, e.g. deforestation

Drainage basin A

more infiltration more interception less runoff smaller peak

discharge smaller amount of

discharge longer time lag gentle rising limb

Drainage Basin B

less infiltration no interception by

trees greater surface runoff higher peak discharge larger amount of

discharge shorter time lag steeper rising limb

Drainage Basin C

rain water/surface runoff is trapped in the reservoir

less runoff downstream

water overflows only when the reservoir is full

smaller peak discharge

longer time lag

Which drainage basin has higher risk of flooding? Why?