runoff variation and the flood hydrograph

base flow

The normal day-to-day discharge of the river and is the consequence of slow moving soil throughflow and groundwater seeping into the river channel.

discharge

The amount of water in a river flowing past a particular point calculated by multiplying the cross-sectional area of the river by the water velocity

lag time

The time between the peak rainfall and the peak discharge

flood hydrograph

A graph of discharge of a river over the time period when the normal flow of the river is affected by a storm event.

river regimes

The variability in its discharge throughout a year in response to precipitation, temperature, evapotranspiration and drainage basin characteristics

flood hydrographs use

they can predict how a river might respond to a rainstorm, this can help in managing the river. 

physical factors affecting the flood hydrograph (6)

• Steep sides lead to faster flowing water – steep hydrograph 

• Drainage basin already saturated, overland flow increases – reduced lag time, high peak discharge. 

• Impermeable rock, high overland flow, low infiltration and throughflow – flashy hydrograph 

• Thick vegetation, interception – increased lag time, reduced peak discharge 

• Amount, type and intensity of precipitation 

• Size of basin – smaller basin has shorter lag time but lower peak discharge. 

Human factors affecting the flood hydrograph (5)

• Deforestation reduces interception rates and infiltration rates – flashy hydrograph (afforestation has opposite affect) 

• Farming: 1. Ploughing breaks up soil and increases infiltration – subduing hydrographs 2. animals can affect soil and increase overland flow 

• Urban sprawl/urbanisation (especially on floodplains) increases number of impermeable surfaces – flashy hydrograph 

• Soft engineering – subdues hydrograph 

• Water abstraction reduces baseflow