Rainfall-Runoff Modeling

WatershedRainfall Runoff

Rainfall-RunoffModel

Rainfall Runoff

Modeling Considerations

• Development of a rainfall-runoff model depends on

(1) Time scale

(2) Basin scale

For larger time scale (month, year, or longer)

Runoff = f(Rainfall); VQ = g(VRain)

For shorter time scale (eg. Storm events)

Runoff Hydrograph = f( Rainfall Hyetograph)

Q(t) = f [ I(1), I(2), …, I(t) ]

Rational Formula• First proposed by Kuichling in 1889

• Qp = k C i A

where k = unit conversion factor,

1.008 for English unit; 0.278 for metric unit

Qp = peak discharge (ft3/s or m3/s);

i = rainfall intensity (in/hr or mm/hr)

A = drainage area (acres or km2)

• Note: i = average intensity of rainfall corresponding to the

duration of time-to-concentration.

Time-of-Concentration• Definition:

• It is an elusive hydrologic variable and it is very difficult to measure.

Time-of-Concentration Methods (1)

Time-of-Concentration Methods (2)

Time-of-Concentration Methods (3)

Comparison of Time-of-Concentration Models

Time-Area Method• Can be considered as an extension of the rational method

in which rainfall intensity in not uniform over the storm duration

Time-Area Method (Eg. 1)

Time-Area Method (Eg. 2)

Time-Area Method (Eg. 3)

Time-Area Method (Eg. 4)