Storm hydrographs and river discharge

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Storm hydrographs and river discharge

Storm hydrographs are graphs that show how a drainage basin responds to a period of rainfall. They are useful in planning for flood situations and times of drought as they show the discharge (amount of water reaching channel via surface run-off, throughflow, and base flow) that originated as precipitation.

A great deal of information can be gleaned from a hydrograph and the interpretation of them is often tested in exam questions. The diagram below shows the main points:

Storm hydrographs and river discharge

Drainage basins all have a variety of characteristics in terms of vegetation, geology, soil type and so on, all of which interact to influence how quickly or slowly river discharge increases after a storm. The table below outlines the major influences on hydrographs and drainage basins:

A Size of basin, shape and relief Size - the smaller the basin the less time it takes for water to drain to the river, resulting in a shorter lag time. Shape - the shape of basin that lends itself to most rapid drainage is circular. In a long, narrow basin water takes longer to reach the river. Relief - the steeper the basin the more quickly it drains.
B Forms of precipitation Heavy Storms - in such a situation, rainfall is often far in excess of the infiltration capacity of the soil leading to much overland flow, and rapid rises in river levels. Lengthy rainfall - leads to the ground being saturated and overland flow. Snowfall - until snow melts, potential discharge for a river is held in storage. Rapid melting can lead to flooding.
C Temperature High rates of evapotranspiration reduce amounts of discharge, and low temperatures can store water in the form of ice and snow.
D Land Use Vegetation - Important in reducing discharge as it intercepts precipitation and adds to rates of evapotranspiration. Roots of plants take up water reducing throughflow. Interception is less in winter in the UK due the shedding of leaves from deciduous trees. Flooding is more likely in deforested areas.
E Geology Rock type varies within drainage basins and can be permeable (allowing water through) or impermeable (not allowing water through). Permeable rocks can be porous such as chalk that store water within them or pervious, such as limestone where water flows along bedding plains. Impermeable rocks encourage grater amounts of surface run-off and a more rapid increase in discharge than permeable rocks.
F Soil A control on the rate of infiltration, amount of soil moisture storage and rate of throughflow. Larger pore spaces as found in sand, allow for greater water storage and limit the risk of flooding.
G Drainage density As stated earlier, the higher the density the greater the risk of flooding.
H Tides and storms High spring tides (illustrated by the Severn Bore) prevent water from entering the sea and increase the risk of flooding.
I Urbanisation A major impact because of its alteration on the hydrological process. The main affects are shown on the diagram below. Click on the magnifying glass to see the graph in more detail.

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