A storm hydrograph/flood hydrograph is a graph showing the relationship between rainwater and discharge in a river. Discharge is the water flowing in a river in a given unit of time. The storm hydrograph is mainly used to observe discharges for given storm events. The storm hydrograph has these properties:
Factors Affecting the Storm Hydrograph
Several factors affects the time water reaches a river and its discharge.
Heavy, sudden bursts of rainfalls lead to steep rising limbs and short lag times. Water is not given time to infiltrate thoroughly. This is why in deserts dried up rivers quickly replenish after a heavy storm (flash floods). Conversely, light rains favour gradual and thorough infiltration, hence less water reaches the river. This results in long lag times. But if rainfall continues for a long period, the ground may become completely saturated and water can flow as saturation overland flow, this is typical of tropical areas.
Nature of Surface
Ice covered surfaces hinder infiltration and encourage high run-off rates leading to higher discharges and short lag times. Baked surfaces in arid regions prevent infiltration and encourages more run-off. The results are high peak discharges and short lag times.
Vegetation can absorb and hinders surface runoff from reaching the river. In addition, densely vegetated areas in tropical regions can intercept rainwater and less water reaches the surface as runoff. This often result in long lag times and gentler hydrographs
Porous soils such as sands or rocks such as limestone are permeable, therefore runoff is reduced leading to long lag times and gentler hydrographs.
Overland flow (run-off) is highest on steep slopes because infiltration is reduced, therefore water reaches the river fast (short lag time) and discharge is high.
Basin Morphology and size
Small circular catchments will have short lag times as runoff water quickly reaches the river. High elevated basins result in high runoff rates which shorten the lag time.
Human Impacts Affecting Storm Hydrographs
Lack of vegetation encourages high runoff rates. In addition, interception is reduced and water readily reaches the surface. This result in short lag times and high peak discharge.
Conversely, dense vegetation cover hinders runoff and intercepts more rainwater. In addition, roots absorb water which reduce runoff.
UrbanisationRoads and pavements impede infiltration and favours high runoff, thereby increasing surface runoff and short lag times and high peak discharges. Storm drains channel water to rivers increasing discharge. Urbanisation produces pollutants which increase rainfall amounts and the overall river discharge.
Dams stores water and blocks off the natural flow of the river. This can create long lag times.
Poor farming methods, tractors and cattle can compact the soil creating a hard layer which hinders infiltration and encourages high runoff which in turn shorten the lag time. The removal of vegetation for farming also means that run-off increases which shorten the time water reaches a river.
Storm Hydrographs by region
|Tropics||Sub-tropics||Deserts (Hot & cold)||Temperate|
|shape||gentle||fairly steep||very steep||fairly steep|
NB: It is important to note that tropical storm hydrographs are gentler for a given storm event, but long term discharge is usually high as water is abundant and flows as saturation overland flow. The regional storm hydrographs only show a broader view of a typical storm hydrograph, but local and specific factors are the most determinants of a storm hydrograph.