2. HEC-HMS Runoff Computation

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

4. User-specified Unit Hydrograph Basic Concepts and Equations Basic Concepts and Equations Q n =storm hydrograph ordinate Q n =storm hydrograph ordinate P m = rainfall excess depth P m = rainfall excess depth U n-m+1 = UH ordinate U n-m+1 = UH ordinate

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

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

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

8. Snyder’s UH Model Basic Concepts and Equations Basic Concepts and Equations

9. Snyder’s UH Model Basic Concepts and Equations Basic Concepts and Equations - standard UH - standard UH - If the duration of the desired UH for the watershed of interest is significantly different from the above equation, - If the duration of the desired UH for the watershed of interest is significantly different from the above equation, t R =duration of desired UH, t R =duration of desired UH, t pR =lag of desired UH t pR =lag of desired UH

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

11. Snyder’s UH Model Estimating Snyder’s UH Parameters Estimating Snyder’s UH Parameters - C t typically ranges from 1.8 to 2.0 - C t typically ranges from 1.8 to 2.0 - C p ranges from 0.4 to 0.8 - C p ranges from 0.4 to 0.8 - Larger values of C p are associated with smaller values of C t - Larger values of C p are associated with smaller values of C t

12. SCS UH Model Basic Concepts and Equations Basic Concepts and Equations

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

14. SCS UH Model Estimating the SCS UH Model Parameters Estimating the SCS UH Model Parameters

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

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

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

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

19. ModClark Method Excess precipitation for each cell is lagged in time and then routed through a linear reservoir S = K * So Excess precipitation for each cell is lagged in time and then routed through a linear reservoir S = K * So Lag time computed by: Lag time computed by: t cell = t c * d cell / d max t cell = t c * d cell / d max All cells have the same reservoir coefficient K All cells have the same reservoir coefficient K

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

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

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

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

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

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

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

27. Baseflow Exponential recession model Exponential recession model Defines relationship of Qt (baseflow at time t) to an initial value of baseflow (Q 0 ) as: Defines relationship of Qt (baseflow at time t) to an initial value of baseflow (Q 0 ) as: Q t = Q 0 K t K is an exponential decay constant K is an exponential decay constant Defined as ratio of baseflow at time t to baseflow one day earlier Defined as ratio of baseflow at time t to baseflow one day earlier Q 0 is the average flow before a storm begins Q 0 is the average flow before a storm begins

28. Baseflow Exponential recession model Exponential recession model

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

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

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

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