1. Rainfall-Runoff Modeling
Watershed
Rainfall
Runoff
Rainfall-Runoff
Model
Rainfall
Runoff
Rainfall-Runoff

2. 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) ]
Rainfall-Runoff

3. Rational Formula First proposed by Kuichling in 1889 Qp = k C i A
where k = unit conversion factor,
1.008 for English unit; 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.
Rainfall-Runoff

4. Time-of-Concentration
Definition:
It is an elusive hydrologic variable and it is very difficult to measure.
Rainfall-Runoff

5. Time-of-Concentration Methods (1)
Rainfall-Runoff

6. Time-of-Concentration Methods (2)
Rainfall-Runoff

7. Time-of-Concentration Methods (3)
Rainfall-Runoff

8. Comparison of Time-of-Concentration Models
Rainfall-Runoff

9. Time-Area Method
Can be considered as an extension of the rational method in which rainfall intensity in not uniform over the storm duration
Rainfall-Runoff

10. Time-Area Method (Eg. 1)
Rainfall-Runoff

11. Time-Area Method (Eg. 2)
Rainfall-Runoff

12. Time-Area Method (Eg. 3)
Rainfall-Runoff

13. Time-Area Method (Eg. 4)
Rainfall-Runoff