1. Synthetic UH
Definition: Synthetic Hydrograph is a plot of flow versus time and generated based on a minimal use of streamflow data.
Example: A pending land use change and the resulting runoff hydrograph is thus unknown, but nevertheless must be estimated.

2. Synthetic UH Developed for basin that were un- gauged
Based on data from similar gauged basins
Most methods are very similar in nature
Revolutionized ability to predict hydro response

3. Gauged and ungauged watersheds
Watersheds where data on precipitation, streamflow, and other variables are available
Ungauged watersheds
Watersheds with no data on precipitation, streamflow and other variables.

4. Need for synthetic UH
UH is applicable only for gauged watershed and for the point on the stream where data are measured
For other locations on the stream in the same watershed or for nearby (ungauged) watersheds, synthetic procedures are used.

5. Synthetic UH Synthetic hydrographs are derived by
Relating hydrograph characteristics such as peak flow, base time etc. with watershed characteristics such as area and time of concentration.
Using dimensionless unit hydrograph
Based on watershed storage

6. Synthetic UH Methods Snyder’s Method (1938) Clark Method (1945)
Nash (1958)
SCS (1964, 1975)
Espey-Winslow (1968)
Kinematic Wave (1970s)

7. Snyder’s Method
Synder (1938)- based on study of large catchment in eastern US developed a set of empirical equation for synthetic UH.
The equation are use in USA and with some modification in many other countries.

9. Snyder’s Method Methods to characterize ungaged basins - 1938
Use data and relationships developed from gages
Variety of approaches but most based on tp and Qp,
Where tp = lag time (hr) and
Qp = peak flow in cms/cfs

10. Snyder’s Equation tp = Ct (LLca)0.3 tp=basin lag in hour
L= basin length in km
Lca= distance along the main water course from gauging stn. To a point opposite to the watershed centroid in km
Ct = regional constant represent watershed slope and storage effects.

11. Lag time
The most important characteristic of basin due to storm- basin lag (lag time)
Lag time- time difference between the centroid of the input (rainfall excess) and the output (DRH)
Represent time of travel of water from all parts of watershed to the outlet during a given storm.

12. Snyder’s UH Method

13. SCS Method Dimensionless UH Based on a study of large number of UH
Developed by US Soil Conservation Services (SCS)

14. The SCS Unit Hydrograph
For large watersheds, time of concentration tc  duration (D) of constant rainfall intensity
Rainfall cannot last long enough that the peak flow, Qp, will occur at time tc
Instead, the peak flow, Qp, will occur at time tp, which is a function of rainfall duration D and the watershed characteristics represented by tc

15. Triangular UH
SCS Methods
Dimensionless UH

19. SCS dimensionless hydrograph
Synthetic UH in which the discharge is expressed by the ratio of q to qp and time by the ratio of t to Tp
If peak discharge and lag time are known, UH can be estimated.
Tc: time of concentration
C = 2.08 (483.4 in English system)
A: drainage area in km2 (mi2)

20. Ex. 7.7.3 Construct a 10-min SCS UH. A = 3.0 km2 and Tc = 1.25 hq
7.49 m3/s.cm
Multiply y-axis of SCS hydrograph by qp and x-axis by Tp to get the required UH, or construct a triangular UH t
2.22 h

21. Clark Unit Hydrograph (UH) Computation
Based on the use of time-area method.
Concept of instantaneous unit hydrograph (IUH)

22. Clark Unit Hydrograph (UH) Computation
Figure 6.23 shows the shape of different hydrographs for different D values.
As D is reduced, the intensity of rainfall excess being equal to 1/D increases and the UH becomes more skewed.
A finite UH indicated as D 0.
The limiting case of UH of zero duration is known as instantaneous unit hydrograph (IUH).

23. Method of IUH Develop a time area (TA) curve
Route the time area curve through a linear reservoir with a Clark routing parameter

25. Example 6.16

26. Nash Linear model

27. Kinematic Wave
Overland
Collector Channels
Main Stream
Manning’s Eqn

28. Kinematic Wave