A hydrograph or storm hydrograph is a type of graph that shows how a rivers' discharge responds to a period of precipitation. Discharge is measured in one point (using the units cumecs), but precipitation is measured over the whole drainage basin. On the hydrograph, precipitation is shown using a bar graph and discharge is shown using a line graph.
Peak rainfall: The highest rainfall (usually measured in mm) during a storm.
Peak discharge: The highest discharge as a result of a storm event.
Lag time: The period of time between peak rainfall and peak discharge
Response time: The time between the first rain falling and the first change in discharge.
Rising limb: The period of time that the river is experiencing an increase in discharge.
Falling or recessional limb: The period of time that a rivers' discharge is falling after a storm event and returning to its normal flow.
Bankfull discharge: The maximum amount of discharge that a river can hold before it burst its banks and floods.
As the period from the initial storm increases, throughflow and baseflow will become increasingly more important in contributing to a rivers' discharge. Initially surface run-off will be the most important factor in contributing to a rivers' discharge because this is the quickest way for rainfall to enter a river.
Factors Effecting Storm Hydrographs
Permeable and Impermeable Rock: If a drainage basin has a large amount of impermeable rock, then there will be less infiltration and more surface run-off. This will greatly reduce both the response time and lag time. Alternatively if a drainage basin has a lot of permeable surfaces then there will be more infiltration and less surface run-off. This may not significantly change the response time, but will delay the lag time.
Deforestation and Reforestation: If a drainage basin has been deforested, then there will be less interception. Precipitation will fall directly on the ground. This will increase the speed that the ground becomes saturated and also increase surface run-off. This will reduce the response time and lag time. If any area is forested or been reforested then there will be more interception, root uptake and transpiration. These three factors will reduce the peak discharge as well as slowing the lag time.
Saturated and Unsaturated Soil: If there has been previous rainfall or the water table is high, then the ground will become saturated quickly. If the ground is saturated there will be limited infiltration and therefore much more surface run-off. The surface run-off will reduce response time and lag time and increase peak discharge.
Urbanisation: Settlement building can have varying effects. Urban areas usually result in deforestation and an increase in impermeable surfaces. This can increase surface run-off and reduce lag time and increase peak discharge. Urbanisation can, however, increase interception with the large number of buildings - this will slow the lag time. Artificial drains may control the flow of water or increase the flow of water into rivers.
Agriculture: Irrigation canals may increase the speed of surface run-off and therefore reduce the lag time and increase the peak discharge. Alternatively terraces and contour (cross) ploughing may actually reduce surface run-off and therefore increase the lag time and reduce the peak discharge as more water will have time to infiltrate. If fields are bare or fallow, then there will be no interception allowing the ground to become saturated quicker and possibly increasing surface run-off. If fields are full of vegetation then there will be much more interception.
Valley gradient: If the valley sides are steep precipitation is less likely to infiltrate, but instead more like to run-off into streams and rivers. If valley sides are more gentle, then there will be more infiltration (if the ground is not saturated) and slower surface run-off increasing the lag time.
Drainage basin size: In small drainage basins, it is more likely that precipitation will reach the river more quickly. In bigger drainage basins, surface run-off, throughflow and baseflow are all going to take longer.
Dams: Dams can hold water and regulate the discharge of water. Although some water will be released from dams during periods of heavy rain, it is very unlikely that the river will be allowed to exceed bankfull discharge. Therefore, this will reduce the peak discharge.
Drainage density: Looks at the relationship between the total length of all the streams and rivers in a drainage basin and the total area of the drainage basin. If the drainage basin is high, it means there is likely to be a shorter lag time and a bigger peak discharge.
Drainage Density: Drainage density looks at the relationship between the total length of all the rivers and streams in a drainage basin (km) and the total area of a drainage basin (km2). If a drainage basin has a high density, it means that precipitation gets into streams quicker, If a drainage basin has a low drainage density, it means that more precipitation has to travel by surface run-off, throughflow and baseflow.
Some factors effecting drainage density:
Geology and soils - drainage densities are higher on impermeable surfaces because there is less infiltration.
Land use - vegetation increases interception and reduces drainage density.
Time - the number of tributaries and therefore the drainage density tend to reduce overtime.
Precipitation - areas of high precipitation tend to have higher drainage densities.
Relief - drainage densities are usually higher on steep land because there is less infiltration and often less vegetation (depending on aspect).
Stream ordering and Bifurcation Ratio: There are many different ways to order streams, one of the most common is Strahler stream ordering. The ordering system is fairly simple. All streams start off as order one. If two order one streams join (converge), then they become an order two stream. If then two order streams join, then they become and order three stream. The stream order will only increase if two streams of the same order join. If an order two stream and an order three stream join, then the resulting river will remain order three.
The Bifurcation ratio, looks at the relationship between the streams in different orders. The drainage basin to the right has 7 order one streams, 4 order two streams and 2 order three streams. To calculate the bifurcation ratio you divide the number of order 1 streams by the number of order 2 streams (7/4=1.75 ), then divide the number of order two streams by the number of order three streams (4/2=2). You then add together the results (1.75+2=3.75) and divide by the total sets of values (3.75/2=1.875). The drainage basin to the right therefore has a bifurcation ratio of 1.86. The higher the bifurcation ratio, the shorter the lag time is likely to be and the higher the peak discharge is likely to be.
Hydrographs
A hydrograph or storm hydrograph is a type of graph that shows how a rivers' discharge responds to a period of precipitation. Discharge is measured in one point (using the units cumecs), but precipitation is measured over the whole drainage basin. On the hydrograph, precipitation is shown using a bar graph and discharge is shown using a line graph.
Peak discharge: The highest discharge as a result of a storm event.
Lag time: The period of time between peak rainfall and peak discharge
Response time: The time between the first rain falling and the first change in discharge.
Rising limb: The period of time that the river is experiencing an increase in discharge.
Falling or recessional limb: The period of time that a rivers' discharge is falling after a storm event and returning to its normal flow.
Bankfull discharge: The maximum amount of discharge that a river can hold before it burst its banks and floods.
As the period from the initial storm increases, throughflow and baseflow will become increasingly more important in contributing to a rivers' discharge. Initially surface run-off will be the most important factor in contributing to a rivers' discharge because this is the quickest way for rainfall to enter a river.
Factors Effecting Storm Hydrographs
Permeable and Impermeable Rock: If a drainage basin has a large amount of impermeable rock, then there will be less infiltration and more surface run-off. This will greatly reduce both the response time and lag time. Alternatively if a drainage basin has a lot of permeable surfaces then there will be more infiltration and less surface run-off. This may not significantly change the response time, but will delay the lag time.
Deforestation and Reforestation: If a drainage basin has been deforested, then there will be less interception. Precipitation will fall directly on the ground. This will increase the speed that the ground becomes saturated and also increase surface run-off. This will reduce the response time and lag time. If any area is forested or been reforested then there will be more interception, root uptake and transpiration. These three factors will reduce the peak discharge as well as slowing the lag time.
Urbanisation: Settlement building can have varying effects. Urban areas usually result in deforestation and an increase in impermeable surfaces. This can increase surface run-off and reduce lag time and increase peak discharge. Urbanisation can, however, increase interception with the large number of buildings - this will slow the lag time. Artificial drains may control the flow of water or increase the flow of water into rivers.
Agriculture: Irrigation canals may increase the speed of surface run-off and therefore reduce the lag time and increase the peak discharge. Alternatively terraces and contour (cross) ploughing may actually reduce surface run-off and therefore increase the lag time and reduce the peak discharge as more water will have time to infiltrate. If fields are bare or fallow, then there will be no interception allowing the ground to become saturated quicker and possibly increasing surface run-off. If fields are full of vegetation then there will be much more interception.
Drainage basin size: In small drainage basins, it is more likely that precipitation will reach the river more quickly. In bigger drainage basins, surface run-off, throughflow and baseflow are all going to take longer.
Dams: Dams can hold water and regulate the discharge of water. Although some water will be released from dams during periods of heavy rain, it is very unlikely that the river will be allowed to exceed bankfull discharge. Therefore, this will reduce the peak discharge.
Drainage density: Looks at the relationship between the total length of all the streams and rivers in a drainage basin and the total area of the drainage basin. If the drainage basin is high, it means there is likely to be a shorter lag time and a bigger peak discharge.
Some factors effecting drainage density:
The Bifurcation ratio, looks at the relationship between the streams in different orders. The drainage basin to the right has 7 order one streams, 4 order two streams and 2 order three streams. To calculate the bifurcation ratio you divide the number of order 1 streams by the number of order 2 streams (7/4=1.75 ), then divide the number of order two streams by the number of order three streams (4/2=2). You then add together the results (1.75+2=3.75) and divide by the total sets of values (3.75/2=1.875). The drainage basin to the right therefore has a bifurcation ratio of 1.86. The higher the bifurcation ratio, the shorter the lag time is likely to be and the higher the peak discharge is likely to be.