Variations in flood hydrographs/ variations in basin runoff

what is runoff

a measure of the amount of total precipitation within a drainage basin, that is transferred into rivers

10 physical factors influencing variations in river discharge/ hydrographs

soil type, rainfall amount and intensity, temperature, antecedent conditions, underlying geology, vegetation type, drainage basin relief, size of drainage basin, drainage basin shape, drainage basin density

how does soil type influence river runoff

sandy soils have high rates of infiltration due to relatively large air spaces between soil particles - less surface runoff so less river discharge

clay soils and silts have small pore spaces which allow every little throughflow - more drainage basin runoff

effect of soil type on hydrograph

sandy soil - flat hydrograph (less discharge)

effect of rainfall amount and intensity on runoff variation

heavy rainfall results in rapid saturation of upper soil level - increased surface runoff - high river discharge - light rainfall - increased interception - greater infiltration rates and slower runoff

effect of rainfall amount and intensity on the flood hydrograph

Heavy rainfall – flashy

Snow – flat hydrograph as increased lag time (rapid melting of snow could cause a flashy hydrograph)

Effect of temperature on runoff variation

higher temperature means more evapotranspiration so less surface runoff and soil storage - less river discharge

Effect of high temperature on flood hydrograph

flat hydrograph

effect of antecedent conditions on river runoff

if drainage basin is already saturated by antecedent rainfall - more surface runoff as infiltration capacity will have been reached. saturated ground acts as impermeable surface - more river discharge

effect of antecedent rainfall on flood hydrograph

flashy hydrograph

effect of underlying geology (impermeable surface)

impermeable surface - higher surface runoff, reduced throughflow and infiltration.

permeable surface e.g. granite (not porous so impermeable

chalk - porous so permeable

if frozen ground permeable or impermeable

impermeable

effect of steeper relief on runoff variation

faster runoff as water flows more quickly as both throughflow and surface runoff