Runoff Hydrology

Runoff

flow of water from runoff flow paths within 24 hours of a storm event

Hortonian overland flow

When rain intensity exceeds infiltration capacity; after depressions fill, water moves across the land

Direct precip on saturated surface

(usually near a stream) occurs bc the soil pores are completely saturated, leaving no space for infiltration

Subsurface storm flow (through flow)

within the soil on soil horizon or in burrow

Imperviousness runoff

connected imperviousness, hydrologically connected to a stream

Groundwater flow during event

groundwater that is discharged into into a stream within 24 hours

interception

prevents rain from hitting the ground

stem flow

rain hits canopy leaves, goes along stems, to trunk, to ground

Depression storage

water stored in the microtopography

Initial abstraction

the initial loss of rainfall before runoff starts

total abstraction

total amount of water that is prevented from becoming surface runoff (interception, depression storage, cumulative infiltration)

Total imperviousness

portion of watershed covered by anthropogenic surfaces

effective imperviousness

portion of total imperviousness surface that is directly connected to a storm drain system

VSA (variable source area)

runoff comes from areas close to the stream networks. They vary in size/shape/extent over the seasons

Curve number

used to estimate potential abstraction in VSA equation

Antecedent moisture conditions

how much moisture is in the soil before the event

TR-55

urban hydrology curve number approach

Topographic slope index

the tendency of an area to accumulate and retain water

Why is runoff calculated only in the first 24h?

biggest floods occur in 24h after midpoint of event

Where is runoff likely to occur?

along the stream corridor; areas of convergent flowpaths; connected imperviousness

How do VSAs change by season?

large in winter (frozen soil and moisture), small in late summer (dry)

Topographic wetness index (TWI)

GIS based approach for identifying VSAs, derived from DEM

Other methods for estimating runoff

Green + Ampt equation, Hortonian infiltration equation, Rational method