The Geo RoomHydrology and Fluvial Geomorphology

Definition of the Day
Kninckpoint
The point where a suddden break in the river's gradient occurs. See Knickpoints

Storm Hydrograph & Factors Affecting It

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:

Baseflow
Water supplied to the river by the slow seepage of water from groundwater. The response of baseslow to rain is slow and can be negligible.
Rising Limb
Water entering the river, steeper rising limbs indicate fast entering of water into the river
Peak Discharge
Flow of water is at its highest.
Lag Time
Lag time is the time between highest rainfall and peak discharge.
Falling/Recession Limb
Shows falling discharge in a river.
Approach Segment
This shows a river's flow before a storm. When a storm begins rain does not readily enter into the river and fills it, instead, most contributions come from overland flow(runoff) or throughflow. Short lag times indicate that rain is heavy and entering the river fast. Long lag times indicate that rain water(runoff) is slowly reaching the river.
Storm Hydrograph
Storm Hydrograph src AlevelGeography.com

Factors Affecting the Storm Hydrograph

Several factors affects the time water reaches a river and its discharge.

Precipitation

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 Cover

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

Soil/Rock Type

Porous soils such as sands or rocks such as limestone are permeable, therefore runoff is reduced leading to long lag times and gentler hydrographs.

Slope

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

Deforestation

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.

Afforestation

Conversely, dense vegetation cover hinders runoff and intercepts more rainwater. In addition, roots absorb water which reduce runoff.

Urbanisation

Roads 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.

Dam/Resevoirs

Dams stores water and blocks off the natural flow of the river. This can create long lag times.

Agriculture

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

TropicsSub-tropicsDeserts (Hot & cold)Temperate
shape gentlefairly steepvery steepfairly steep
lag time longlong-shortshortlong-short

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.

Gentle tropical storm hydrograph
A typical tropical storm hydrograph: Not for scale

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