2.4 Hydrograph

A hydrograph is a graph that shows how a river discharge varis over time in a river system. It therefore shows a trend in the river. Often (as in the diagram below), rainfall is plotted on the same graph as a bar chart. The hydrograph has a particular structure and terminology to go with that structure which is explained below using the following diagram. [fig 2.4(a)]

2.4.1 Terminology   [refer to fig 2.4(a)]
A - DISCHARGE (in cumecs). This axis measures the amount of water passing through the river in cubic metres per second

B - TIME LAG (or LAG TIME). The time between the peak discharge and highest point in a river. This is the time it takes for precipitation to run-off from where it has fallen to the river channel. Many factors can affect this, such as permeability of rocks and interception from vegetation.

C - PEAK DISCHARGE. This is the very highest point to which the river discharge reaches.

D - ZONE OF FLOOD RISK. The area above the dotted line within the loop of the graph is the time at which the river is likely to flood.

E - ZONE OF NORMAL DISCHARGE. The dotted line represents the normal discharge of the river when there is no rain. This zone is when the river is not a position to flood.

F - RECESSION LIMB. The discharge at this point is falling.

G - RISING LIMB. The discharge at this point is rising.

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2.4.2 Conditions   [fig 2.4(b-e)]

CONDITION 1: The weather [fig2.4(b)] fig 2.4(b)

When rainfall is heavy and lasts a long time there will be a lot of run-off. Water will reach the river quickly, making the time lag short and discharge high. Rain will not be able to sink into the ground when it is frozen which will also mean that the time lag will be short.

CONDITION 2: Nature of the catchment area [fig2.4(c)]

fig 2.4(c)

Impermeable rock does not allow water to pass through it and so a catchment area made of impermeable rock will have a lot of run-off. A catchment area made from permeable rock will allow infiltration to occur, reducing run-off and increasing the lag time. This will lower discharge in the river.

CONDITION 3: Land use [fig2.4(d)]

fig 2.4(d)

Trees in a river catchment area delays water by intercepting it. This reduces run-off and therefore time lag and river discharge. Where land has been built on by humans, time lag is shortened due to the covering of the ground by impermeable surfaces (tarmac and concrete for example). This will decrease the time lag and increase discharge.



CONDITION 4: River management [fig2.4(e)]

fig 2.4(e)

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