The Geo RoomHydrology and Fluvial Geomorphology

Definition of the Day
The unconcetrated movement of water along slopes as sheetwash or concetrated in rills

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 has these properties:

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 gap between highest rainfall and peak discharge.
Falling/Recession Limb
Shows falling discarge 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 comes from overlandflow(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

Factors Affecting the Storm Hydrograph


Heavy,sudden bursts of rainfalls leads to steep rising limbs and short lag times.This is particulalar because of the raindrop impact which dislodges soil particles.Water is not given time to infiltrate thoroughly.This is why in deserts dried up rivers quickly replenish after a heavy storm(flash floods).In contrast to light rains,water is given time to infiltrate hence less reaches the river.But if rainfall continues for a long period,the ground may become completely saturated and water can flow as saturated overlandflow,this is typical of tropical areas.

Nature of Surface

Ice covered surfaces hinders infiltration and encourage high runoff rates leading to higher discharges and short lag times.Baked surfaces such as in arid regions prevents infiltraton and encourage more runoff

Vegetation Cover

Vegetation may hinder surface runoff to reach the river .In addition densley vegetated areas can intercept rainwater and less water reaches the surface.

Soil/Rock Type

Porous soils such as sands or rocks such as limestone are easily infiltrated by water,therefore runoff is reduced and lead to long lag times and gentler hydrographs.


Overlandflow is highest on steep slopes because infiltration is reduced on steep slopes. Therefore water reaches the river fast (short lag time) and discharge is high.

Basin Morphology

Small circular catchments wil have short lag times as runoff water quickly reaches the river. High elevation basins result in high runoff rates and decrease the lag time.

Human Impacts on Storm Hydrographs


Lack of vegetation encourages high runoff rates.In addition,interception is reduced and water readily reaches the surface. These factors result in short lag times and high peak discharge


Trees hinders runoff and intercept more rainwater.In addition,roots absorb water and therefore infitration occurs and runoff reduced.


Road and pavement construction impedes infiltration and favours runoff therefore increasing surface runoff and short lag times and high discharge.Storm drains stores water and affect water that is suposed to reach the river.Industrial gases gives condensation nucleis which increase rainfall and can increase river discharge.


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


Tractors,vehicles and cattle compacts the soil creating an impermeable layer which hinders infiltration and encourage high runoff which in turn shorten the lag time.

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