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