Construction
And
Analysis of Hydrographs
Hydrograph
Record of River Discharge over a period of time
River Discharge
rivers mean
= cross sectional area X (average) velocity
(at a particular point in its course)
Storm Hydrographs
Show the change in discharge caused by a
period of rainfall
Why
Construct & Analyse
Hydrographs ?
To find out discharge patterns of
a particular drainage basin
Help predict flooding events,
therefore influence implementation
of flood prevention measures
©Microsoft Word clipart
Construction
©Microsoft Word clipart
Of
Storm (flood)
Hydrographs
Discharge (m3/s)
3
2
1
0
12
24
36
48
30
72
Hours from start of rain storm
Rainfall shown
in mm, as a
bar graph
Discharge (m3/s)
3
2
mm
4
1
3
2
0
12
24
36
48
30
72
Hours from start of rain storm
Discharge in
m3/s, as a
line graph
Discharge (m3/s)
3
2
mm
4
1
3
2
0
12
24
36
48
30
72
Hours from start of rain storm
The rising
flood water in
the river
Discharge (m3/s)
3
2
mm
4
1
3
2
0
12
24
36
48
30
72
Hours from start of rain storm
Peak flow
Peak flow
Maximum
discharge in
the river
36
30
Discharge (m3/s)
3
2
mm
4
1
3
2
0
12
24
48
72
Hours from start of rain storm
Peak flow
Discharge (m3/s)
3
Falling flood
water in the
river
2
mm
4
1
3
2
0
12
24
36
48
30
72
Hours from start of rain storm
Basin lag time
Basin lag time
Peak flow
Discharge (m3/s)
3
2
mm
4
1
3
2
0
12
24
36
48
Time
difference
between the
peak of the
rain storm
and the peak
flow of the
river
30
72
Hours from start of rain storm
Basin lag time
Base flow
Peak flow
Discharge (m3/s)
3
Normal
discharge of
the river
2
mm
4
1
3
2
0
Base flow
12
24
36
48
30
72
Hours from start of rain storm
Basin lag time
Overland flow
Through flow
Discharge (m3/s)
3
2
mm
=
Overland
flow
Storm Flow
Through flow
4
1
+
Peak flow
3
2
0
Base flow
12
24
36
48
30
72
Hours from start of rain storm
Basin lag time
Peak flow
Discharge (m3/s)
3
2
mm
Through flow
4
1
Overland
flow
3
2
0
Base flow
12
24
36
48
30
72
Hours from start of rain storm
Basic terms:
Discharge: the amount of water passing a sampling point. It is a
combination of run off, throughflow and base flow. Measured in
cumecs (cubic metres per second).
Overland flow: the sum of all the rainwater that flows over the
surface of the river basin (surface run off and streams)
Through flow: the downslope movement of water through soil
towards streams and rivers.
Base flow (groundwater flow): groundwater movement, which
often lags behind precipitation by weeks, months or even years.
Stormflow: all discharge that is above base flow.
Bankful discharge: the point at which a river is full. If the level
increases any more, the river floods.
Basin lag time
Peak flow
Discharge (m3/s)
3
2
mm
Through flow
4
1
Overland
flow
3
2
0
Base flow
12
24
36
48
30
72
Hours from start of rain storm
Why use hydrographs?
Hydrologists measure water so that
they can predict the discharge of a river.
 Discharge of a river depends on precipitation,
channel shape, speed of river, volume of water.
 River discharge can increase dramatically during
rain storms and during snow melt.
 River flow can be analysed in a storm hydrograph.
 For looking at reservoir storage capacity.
 For flood prediction (rivers are particularly prone to
flooding after rain storms and during snow melt).
 Used to help stabilise river flow throughout the
year.

Analysis
©Microsoft Word clipart
Factors influencing
Storm Hydrographs
• Area
• Land Use
• Shape
• Drainage Density
• Slope
• Precipitation / Temp
• Rock Type
• Tidal Conditions
• Soil
©Microsoft Word clipart
Interpretation of Storm
Hydrographs
Basin lag time
Need to refer to:
Peak flow
3
Discharge (m3/s)
•Rising Limb
•Recession Limb
•Lag time
•Rainfall Intensity
•Peak flow compared to Base flow
•Recovery rate, back to Base flow
2
mm
4
3
2
1
0
Overland
flow
Through flow
Base flow
12
24
36
48
30
Hours from start of rain storm
72
Here are some theoretical
interpretations of influencing
factors
BUT……
When interpreting hydrographs
all factors must be considered
together !
Area
Large basins receive more
precipitation than small
therefore have larger runoff
Larger size means longer lag
time as water has a longer
distance to travel to reach
the trunk river
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Shape
Elongated basin will produce a lower peak flow
and longer lag time than a circular one of the
same size
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Slope
Channel flow can be faster down a steep slope
therefore steeper rising limb and shorter lag
time
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Rock Type
Permeable rocks mean rapid infiltration and
little overland flow therefore shallow rising limb
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Soil
Infiltration is generally greater on thick soil,
although less porous soils eg. clay act as
impermeable layers
The more infiltration
occurs the longer the
lag time and shallower
the rising limb
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Land Use
Urbanisation - concrete and tarmac form
impermeable surfaces, creating a steep rising
limb and shortening the time lag
Afforestation - intercepts the precipitation,
creating a shallow rising limb and lengthening the
time lag
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Drainage Density
A higher density will allow rapid overland flow
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Precipitation & Temperature
Short intense rainstorms can produce rapid
overland flow and steep rising limb
If there have been extreme temperatures,
the ground can be hard (either baked or frozen)
causing rapid surface run off
Snow on the ground can act as a store
producing a long lag time and shallow rising limb.
Once a thaw sets in the rising limb will become
steep
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Tidal Conditions
High spring tides can block the normal exit for
the water, therefore extending the length of
time the river basin takes to return to base
flow
Area
Rock Type
Drainage Density
Shape
Soil
Precipitation / Temp
Slope
Land Use
Tidal Conditions
Remember!
These influencing factors will:
Influence each other
Change throughout the rivers
course
Hydrograph dominated by quickflow processes
 surface runoff
Steep
rising
limb
High peak
discharge
Short lagtime
Steep
falling
limb
Relief
Steep slopes.
No time to infiltrate = surface runoff.
Rock Type
Impermeable rocks.
No infiltration = no throughflow.
Soil
Very thin.
Less infiltration.
Vegetation
Thin grass.
Less interception.
Land use
Urban (tarmac and drains)
No infiltration = surface runoff.
River use
Limited use.
Drainage
density
High. More streams so water gets into
channel quicker.
Precipitation
Very heavy intense rainfall.
No time to infiltrate = surface runoff.
Amount of
water in soil
Saturated/Very dry = difficult for water
to infiltrate.
Hydrograph dominated by slowflow processes
 throughflow
Gentle rising limb
Low peak discharge
Gentle falling limb
Long lagtime
Relief
Gentle slopes. Time to infiltrate = throughflow
Rock Type
Permeable rocks. Infiltration = groundwater flow.
Soil
Deep soil. More infiltration = throughflow.
Vegetation
Forest. More interception + roots/leaves delay throughflow and
absorb moisture, lost through transpiration.
Land use
Rural. Permeable surfaces and many plants = high interception.
River use
Water extracted for industry, agricultural and domestic use. Dams.
Drainage density
Low. Less streams so water gets to channel slower.
Precipitation
Light showers. Infiltration + throughflow.
Amount of water in soil
Moist.