1. HEC-HMS
Runoff Computation

2. Modeling Direct Runoff with HEC-HMS
Empirical models
- traditional UH models
- a causal linkage between runoff and excess precipitation without detailed consideration of the internal processes
A conceptual model
- kinematic-wave model of overland flow
- represent possible physical mechanism

3. User-specified Unit Hydrograph
Basic Concepts and Equations
Qn=storm hydrograph ordinate
Pm=rainfall excess depth
Un-m+1=UH ordinate

4. User-specified Unit Hydrograph
Estimating the Model Parameters
1. Collect data for an appropriate observed
storm runoff hydrograph and the causal
precipitation
2. Estimate losses and subtract these from
precipitation. Estimate baseflow and
separate this from the runoff

5. User-specified Unit Hydrograph
Estimating the Model Parameters
3. Calculate the total volume of direct runoff
and convert this to equivalent uniform
depth over the watershed
4. Divide the direct runoff ordinates by the
equivalent uniform depth

6. User-specified Unit Hydrograph
Application of User-specified UH
- In practice, direct runoff computation with
a specified-UH is uncommon.
- The data are seldom available.
- It is difficult to apply.

7. Snyder’s UH Model
Basic Concepts and Equations

8. Snyder’s UH Model Basic Concepts and Equations - standard UH
- If the duration of the desired UH for the watershed of interest is significantly different from the above equation,
tR=duration of desired UH,
tpR=lag of desired UH

9. Snyder’s UH Model Basic Concepts and Equations - standard UH
- for other duration
Up=peak of standard UH, A=watershed drainage area
Cp=UH peaking coefficient,C=conversion constant(2.75 for SI)

10. Snyder’s UH Model Estimating Snyder’s UH Parameters
- Ct typically ranges from 1.8 to 2.0
- Cp ranges from 0.4 to 0.8
- Larger values of Cp are associated with smaller values of Ct

11. SCS UH Model
Basic Concepts and Equations

12. SCS UH Model Basic Concepts and Equations
- SCS suggests the relationship
A=watershed area; C=conversion constant(2.08 in SI)
t=the excess precipitation duration;tlag=the basin lag

13. SCS UH Model
Estimating the SCS UH Model Parameters

14. Clark Unit Hydrograph
Models translation and attenuation of excess precipitation
Translation: movement of excess from origin to outlet
based on synthetic time area curve and time of concentration
Attenuation: reduction of discharge as excess is stored in watershed
modeled with linear reservoir

15. Clark Unit Hydrograph Required Parameters: TC Not
Time of Concentration!!!
Storage coefficient

16. Clark Unit Hydrograph Estimating parameters: Time of Concentration: Tc
Estimated via calibration
SCS equation
Storage coefficient
Flow at inflection point of hydrograph divided by the time derivative of flow

17. ModClark Method
Models translation and attenuation like the Clark model
Attenuation as linear reservoir
Translation as grid-based travel-time model
Accounts for variations in travel time to watershed outlet from all regions of a watershed

18. ModClark Method
Excess precipitation for each cell is lagged in time and then routed through a linear reservoir S = K * So
Lag time computed by:
tcell = tc * dcell / dmax
All cells have the same reservoir coefficient K

19. ModClark Method Required parameters:
Gridded representation of watershed
Gridded cell file
Time of concentration
Storage coefficient

20. ModClark Method Gridded Cell File
Contains the following for each cell in the subbasin:
Coordinate information
Area
Travel time index
Can be created by:
GIS System
HEC’s standard hydrologic grid
GridParm (USACE)
Geo HEC-HMS

21. Kinematic Wave Model Conceptual model
Models watershed as a very wide open channel
Inflow to channel is excess precipitation
Open book:

22. Kinematic Wave Model
HMS solves kinematic wave equation for overland runoff hydrograph
Can also be used for channel flow (later)
Kinematic wave equation is derived from the continuity, momentum, and Manning’s equations

23. Kinematic Wave Model Required parameters for overland flow:
Plane parameters
Typical length
Representative slope
Overland flow roughness coefficient
Table in HMS technical manual (Ch. 5)
% of subbasin area
Loss model parameters
Minimum no. of distance steps
Optional

24. Baseflow Three alternative models for baseflow
Constant, monthly-varying flow
Exponential recession model
Linear-reservoir volume accounting model

25. Baseflow Constant, monthly-varying flow User-specified
Empirically estimated
Often negligible
Represents baseflow as a constant flow
Flow may vary from month to month
Baseflow added to direct runoff for each time step of simulation

26. Baseflow Exponential recession model
Defines relationship of Qt (baseflow at time t) to an initial value of baseflow (Q0) as:
Qt = Q0Kt
K is an exponential decay constant
Defined as ratio of baseflow at time t to baseflow one day earlier
Q0 is the average flow before a storm begins

27. Baseflow
Exponential recession model

28. Baseflow Exponential recession model Typical values of K
0.95 for Groundwater
0.8 – 0.9 for Interflow
0.3 – 0.8 for Surface Runoff
Can also be estimated from gaged flow data

29. Baseflow: Exponential recession model:
Applied at beginning and after peak of direct runoff
User-specified threshold flow defines when recession model governs total flow

30. Baseflow Linear Reservoir Model:
Used with Soil Moisture Accounting loss model (last time)
Outflow linearly related to average storage of each time interval
Similar to Clark’s watershed runoff

31. Applicability and Limitations
Choice of model depends on:
Availability of information
Able to calibrate?
Appropriateness of assumptions inherent in the model
Don’t use SCS UH for multiple peak watersheds
Use preference and experience