Human Activity & Storm Hydrographs
- There are many human activities which change the river landscape
- This impacts on the storm hydrograph, changing the lag time and discharge
Human Factors Affecting Storm Hydrographs
Human Factors | Impact | Effect on Hydrograph |
Deforestation |
Lack of trees reduces interception and infiltration, increasing overland flow |
|
Afforestation |
An increased amount of trees increases interception and infiltration reducing overland flow |
|
River management |
Dams can be used to control the level of discharge downstream |
|
Urbanisation |
Impermeable concrete and tarmac increase overland flow Water flows into the drains reaching the river rapidly |
|
Agriculture |
Bare soil and ploughing increase overland flow, especially where ploughing is downslope Replacement of pasture (grassland) with arable crops also increases overland flow |
|
Climate change |
Rising global temperatures may increase storm frequency and intensity, increasing precipitation Increasing drought which leads to baked soil and periods of extreme cold which freeze the soil which means that water cannot infiltrate |
|
- Deforestation, urbanisation and agriculture all increase the overland flow
- Overland flow reaches the river more rapidly than through flow or groundwater flow
- This means that the lag time is reduced
- They also increase the amount of water which reaches the river
- This increases the level of discharge leading to a steep rising limb
- The combination of these factors leads to a storm hydrograph which has a rapid response to a rainfall or storm event
- Afforestation increases the time the lag time and increases infiltration so less water will reach the river
Image showing comparison between urban and rural hydrography
Worked example
Explain two ways in which human activities can affect storm hydrographs
(4 marks)
- You will not be awarded marks if you make a point and then for the second part write the opposite. For example, you might refer to deforestation increasing discharge and decreasing the lag time. If you then write about afforestation decreasing discharge and increasing the lag time, you will only be awarded 2 marks
Answer:
- More urbanisation (1) increases rates of run-off (or equivalent idea e.g. reduces infiltration) and/or more water in the river/higher discharge (1)
- Changing agricultural practices/ land use (1) such as ploughing land previously left in grass which will increase runoff (1)
- Deforestation/Afforestation (1) affects interception and infiltration rates so changing run-off and thus lag-times (1)
- Climate change my increase/decrease storminess (1) which will affect lag-times because of changes in infiltration/run off relationships (1)
- Human management of rivers e.g. levees (1) maintains more water in the channel (1)
Exam Tip
In 4 mark answers take care to not simply write the opposite for the second set of two marks. If you do this you are not showing the breadth of knowledge expected and will only be awarded marks for the first part of your answer. See the above worked example which illustrates where this could happen.