Ok, firstly, hydrographs are graphs which show river discharge over a given period of time and show the response of a drainage basin and its river to a period of rainfall.The discharge of the river is calculated in cumecs/cm^3.
Secondly, 2 most important things we must take note of a hydrograph are the lag time and the shape of the graph.
Lag time is the time between the peak rainfall to peak discharge, as stated in the diagram above.
Factors which affect the lag time are:
1. vegetation
2. humidity
3. precipitation
4. relief of drainage basin
5. overland flow [ water flowes fastest ]
6. through flow/ groundwater flow [ water flows slowest ]
These factors determine the speed at which water reaches the river.
A shorter lag time will mean that water had flowed quickly to the river.
A longer lag time will mean that had flowed slowly to the river.
A steep or gentle gradient will result from the duration of the lag time and affect the shape of the graph.
Steep: Short lag time, higher peaks
Gentle : Long lag time, lower peaks
Factors which affect the shape of the graph are:
1. Geology of the drainage basin [ Eg. impermeable or permeable bedrock ]
2. Distance between infiltration and the river
Examples:
Urbanisation
In urbanised areas, vegetation has been replaced by tarmac and cement surfaces. This will prevent infiltration of precipitation into the soil, resulting in significant overland flow. In addition, complex drainage and pipe systems in these urbanised areas will increase interception of precipitation. Hence, precipitation will be transported quickly to the river, decreasing the lag time, resulting in a steep sloped hydrograph.
The same goes for grazing and deforestation. In these 2 other examples, vegetation has also been removed. Hence infiltration and interception of precipitation will decrease and surface run-off will increase. A steep-sloped hydropgraph will result as well.
Afforestation
In afforested areas, vegetation is present. Infiltration will increase as the roots of the plants provide pathways for precipitation to percolate underground. Interception [ stem flow and through fall] will increase as well. In addition, evapotransiration will occur. When these happen, lag time will increase as precipitation takes a longer time to reach the river and thus giving a gentle sloped hydrpgraph.
Our lovely IS2104 hydrosphere booklet
That's all!
The end:):):)
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